The Key to Sleep: Circadian Entrainment

If you endure sleepless nights and groggy mornings, then this article just might change your life. I always thought of myself as a night owl. I believed it was hard-wired into me — a genetic polymorphism. That might be partially true. There is sound scientific evidence to suggest that some people are genetically prone to staying up late, while others are disposed to springing out of bed before the rooster.¹ However, our internal clocks are actually a lot more malleable than we might realise.² An understanding of some recent scientific findings, coupled with a few simple techniques based on those findings, has markedly improved my sleep. We talk a lot about nutrition, exercise, supplements and biohacks for optimizing cognitive performance, but sleep is often overlooked. This article dissects some crucial, low-hanging fruit, which studies have only just revealed as recently as this year. These are the best biohacks for improving every facet of general health, in my opinion.

I’ll focus on free circadian optimizations that require minimal effort. None of these methods involve any supplements or any form of meditation. In fact, my favourite tip requires no effort whatsoever.

The application of this foundational understanding can dramatically improve your sleep quality and help with overcoming jet lag. We all know what jet lag is but not everyone may be aware that it doesn’t take any planes, trains or automobiles to become jet-lagged. If you’re anything like me, someone who regularly stares at screens by moonlight, some of your sleep issues may be partially attributable to a misaligned circadian rhythm. It’s like being jet-lagged without the consolation of a nice holiday!

We’re gradually learning more about the detrimental health effects of jet lag and shift work. In 2008, 38 Danish women in shift work were diagnosed with breast cancer and were granted financial compensation under Danish law.³ The approval of breast cancer as an occupational disease in shift working is currently being discussed.⁴ There’s a strong link between disrupted circadian rhythms and mood disorders such as depression, as well as numerous other diseases.⁵ I’d rather not dwell on those. I’d rather focus on striving for benefits rather than dwelling on fears.

Although my late-night partying days are behind me and I usually rise at dawn, I’m still by nature somewhat of a night owl. I work around the clock, liaising with people in different time zones. Therefore, I’ll spare you my hypocritical waffle about health risks. I’ll try to focus on practical tips for doing the best we can, given the demands of our modern day life and our own unique situations.

Firstly, what exactly determines how well we sleep and how refreshed we feel during waking hours? It’s governed by two factors. The first is a chemical called adenosine. When we wake up after a good sleep, adenosine levels are low. Adenosine gradually accumulates throughout the day, building up “sleep pressure,” like a hunger for sleep. The higher our adenosine, the sleepier we feel.⁶ After a long day, adenosine becomes so high that we can no longer fight the urge to sleep.

The reason a cup of coffee wakes us up is because caffeine is an adenosine antagonist.⁷ It stops adenosine from binding to its receptors. That stranded adenosine is still floating around though. When the caffeine wears off, the adenosine rushes in to bind to its receptor sites, sometimes with even greater affinity. It’s like a Black Friday stampede of people who’ve been waiting to clammer into a store. That’s why we get the dreaded caffeine crash. I think we’re all fairly well aware of how caffeine affects us individually. Some of us can tolerate caffeine quite late, some not at all. This can be due to genetic variations causing mutant forms of the adenosine receptor in different people.⁸ Personally, I take two Zenergize in the morning and drink matcha throughout the day and I sleep fine. I try to kick the caffeine in the head by evening. If we do use caffeine, we each have to find what works well for us individually.

When you pull an all-nighter and the sun comes up.

Have you ever pulled an all-nighter and noticed that as morning rolls around, you seem to suddenly feel more energized and alert again? Why exactly is this, if adenosine levels are still high?

It’s because there’s a second factor which determines whether we’re bright-eyed and bushy-tailed or clinging to the covers for dear life. That factor is the circadian rhythm — the internal body clock. It anchors our sleep-wake schedule to a sinusoidal curve within each 24-hour cycle. In other words, we’re designed to sleep for one solid period each day rather than at random intervals.

Let there be light

As I’m sure you probably know, light, especially sunlight, is the biggest determinant of the circadian cycle. Body temperature is another important factor for setting our internal clocks.² The times of day we eat, socialise and exercise also play a role, but they aren’t as important as light and temperature for calibrating our clocks.⁹

We’re all bombarded with haunting headlines of grim health risks related to checking phones by night or doing shift work, but we rarely hear any constructive advice for how to actually deal with the realities of modern life pragmatically. Some companies sell blue light blocking glasses. Do these glasses actually work? To be quite blunt, no, most of them do not prevent sleep disruption. Certain glasses can absolutely help, as I’ll go on to discuss. However, blue blockers alone won’t prevent circadian shifts. It doesn’t matter what colour the light is. Any bright light will still trigger the setting of our internal clocks.¹⁰ That being said, blue light does have a greater effect than other types of light. It’s true that blue and green light from screens may often be the biggest culprit for misaligned rhythms.

How exactly does light set our circadian rhythm? And how can we utilize light to sleep better and feel more refreshed?

When we wake up, our day starts with a pulse of a hormone known as cortisol. Cortisol is that so-called “stress hormone” with a bad rep. It’s what we often associate with chronic stress. Indeed, if we’re enduring excess stress, we might get pumped full of some extra cortisol pulses throughout the day. However, in the morning, the release of cortisol is actually very healthy. It gets us up and going. Well, at least it should be getting us up and at ‘um.

Up and at them!

Additionally, this cortisol spike is crucial for another reason. It sets a countdown timer in the nervous system. About 12–14 hours after the cortisol pulse, another hormone known as melatonin is released. Melatonin signals that it’s time to wind down and go to sleep. Melatonin is released by the pineal gland. The pineal gland is sometimes called the “third eye,” but I won’t get into that discussion here. If you’re subscribed to the Zenstra newsletter, you’re probably fed up of hearing my somewhat spiritual woo-woo.🙂

Suffice to note that melatonin is only produced by the pineal gland, unless exogenous melatonin is taken as a supplement. At Zenstra, we don’t include any melatonin as an ingredient in our sleep supplement Lemello. The reason is because although melatonin has been shown to help with falling asleep faster, it’s a hormone which may affect other hormones.¹¹ For instance, melatonin levels in children are correlated with the suppression of puberty.¹² We’re not against melatonin supplementation. It works wonders for a lot of people. Many of our customers do combine melatonin with Lemello. We chose to exclude it from Lemello because we want to give you the choice of whether you wish to take it separately or not. For example, if I was worried about hormonal issues, I might be concerned that supraphysiological levels of melatonin may potentially exacerbate the issue. Further research is needed to fully assess the impact of melatonin on human sex hormones and reproduction.¹³ Whatever the case may be, we would always advise consulting with your doctor before making any changes with regards hormone replacement, medications, or indeed even supplements or lifestyle changes.

The below techniques will help to elevate natural melatonin levels for free. I’ll recommend some products that I find helpful, but all of these insights can be applied without spending a single cent.

What happens if we wake up in a dark room?

When we wake up and open our eyes, if we don’t get adequate light, and, as research suggests, the right type of light, the cortisol pulse won’t be timed correctly. It will phase delay our circadian rhythms, meaning we’ll want to stay up later and wake up even later the following day.

This is because we have neurons in our eyes called intrinsically photosensitive melanopsin-expressing retinal ganglion cells (ipRGCs) which perceive light signals for non-visual purposes, such as modulating sleep/alertness, mood and circadian entrainment.¹⁴ Even in blind people, these cells detect light to entrain circadian rhythms. They send a signal to the suprachiasmatic nucleus (SCN), which is like the body’s master clock. The signal triggers the release of cortisol to get us up and moving.

It’s crucial not to miss the morning sunlight opportunity. A delayed cortisol pulse can contribute to numerous ill health effects.¹⁵ A cortisol pulse between 8 p.m. and 9 p.m. has been correlated with anxiety, depression and insomnia.¹⁶ Staying up later probably implies more light exposure at night, which in turn impacts mood and learning through other pathways.¹⁷ A miscalibrated circadian rhythm can also have a detrimental impact on cardiovascular health, metabolism and immune health. It may contribute to obesity, dementia, reproductive disturbances, among many other negative effects that we’re probably all tired of hearing.¹⁵

What about if we wake up by checking our phones or switching on artificial lights?

Those ganglion cells will still be activated by artificial light, but not in the optimal way. Even if your significant other treats you like a teenager, flipping the light switch to coercively eject you from bed, you could still wake up feeling groggy and sluggish at that time the following day.

That’s because those retinal ganglion cells are not just able to detect the overall light intensity. They’re also capable of responding to certain colours of light.¹⁴ In 2020, University of Washington researchers discovered that the type of light these cells respond best to is long wavelength yellow light, contrasted with short wavelength blue light.¹⁴ As it happens, that kind of colour contrast is exactly the type of painting displayed in the sky every sunrise and sunset! While melanopsin is sensitive to blue and green light, it was discovered that cone photoreceptors also send information to those ganglion cells. Those cones respond strongly to a blue-yellow contrast and are a thousand times more sensitive to light than melanopsin.

The solar angle at sunrise and sunset creates the perfect light for entraining the circadian rhythm! Getting adequate sunlight as soon as possible after waking is ideal, but it doesn’t need to be precisely at sunrise or sunset. According to Dr. Andrew Huberman, a tenured Stanford professor of neurobiology, getting light within a few hours of sunrise is fine, but the sooner after waking the better.

Can we just open the curtains then?

Assuming we want to phase advance our circadian rhythms, meaning we want to feel sleepy earlier and feel alert earlier, then going outside or opening the window is still going to be better. This is because window glass filters away some of the UV light which assists in clock setting. Getting sunlight through a window is still great, provided there’s enough light. It’ll take longer though. How much longer? That depends on a number of factors, such as the time of year, cloud cover, the type of window, the location of the window etc. If you really wanted to get technical, Dr. Huberman suggests that you could use a light meter app on your phone to measure the light difference between inside and outside. He claims a colleague of his, Dr. Jamie Zeitzer, told him that viewing light through a window will take 50–100 times longer than viewing direct sunlight, but it’s not clear to me how that figure was obtained. In a 2008 article in Practical Neurology, Dr. Zeitzer stated that bright room light is about 50% as effective as outdoor light. Dr. Huberman has since said that the intensity-response relationship doesn’t scale linearly. I’ve asked Dr. Huberman for clarification and I’ll share an update when he gets back to me. Whatever the case may be, it takes longer to get adequate sunlight through a window.

Usually, between 2–10 minutes of outdoor sunlight should suffice for most people.¹⁸ In the wintertime, in areas such as Scandinavia, it’ll take longer. In such cases, using artificial light might be a good idea. My choice would be the TAO light which mimics sunrise and sunset.

As for duration, during the first few hours of the day, all photon activation is cumulative, meaning you don’t have to get the light all at once. However, after the first few hours of the day have elapsed, it doesn’t matter how much light you get.¹⁹ The middle of the day is known as the “circadian dead zone.” Getting light in the middle of the day won’t alter the clock tuning in one way or another.

There’s no need to stare directly into the sun. I know that some people engage in “sun gazing,” often as part of a spiritual practice, which I respect, but that can actually be damaging to the retina.

How to shoot the lights out

The day’s end is equally important. I’ll give you the bad news first. The bad news is that despite how much light we need in the morning, we paradoxically need very little light at night to throw our clocks out of whack. The longer you’ve been awake, the more sensitive those ganglion cells become to light. As the CDC correctly advises, if you have trouble falling asleep, keep the light levels dim for the 2 hours before you want to go to sleep. In fact, two hours is probably the bare minimum. Exposure to short-wavelength light for two hours, starting 3 hours before habitual bedtime, has been found to decrease deep sleep.²⁰ A 2018 paper published in Cell, showed that chronic light exposure in the middle of the night actually suppresses the release of dopamine.¹⁷ Dopamine is linked with feelings of reward, motivation and satisfaction. Bright light by night actually signals a tiny, pea-sized structure in the brain called the lateral habenula.¹⁷ The habenula triggers feelings of disappointment and hopelessness.²¹ It’s been linked with certain types of depression and learning impairments, as well as misregulation of blood sugar by the pancreas.²² No wonder it’s hard to spring out of bed when we’re feeling disappointed, despondent, demotivated or even depressed.

Bright light at night also suppresses melatonin and shifts the rhythm even later the following night.

If we can’t eliminate light at night entirely, the good news is that there are some simple steps we can take to mitigate these detrimental effects.

One way is to utilise the sunset. When the sun is at a low solar angle, within an hour of sunset, viewing sunlight can mitigate the negative effects of artificial light later in the night. The specific type of light at sunset tells the body that it’s time to unwind. The opponency between yellow and blue light at sunset desensitises the eyes to any other light signals received later in the day.¹⁴ Dr. Huberman refers to it as a “Netflix vaccination.”

The University of Washington has licensed technology based on this discovery to TUO, a tech company that sells white LED light bulbs which emit undetectable sunrise and sunset wavelengths. The company claims that a one-time exposure to the TUO light for just 50 minutes shifted circadian rhythm by about 50 minutes earlier in study participants. In other words, test subjects naturally wanted to wake up 50 minutes earlier the following day. If you struggle to get enough natural light at sunrise or sunset or if you live somewhere with low light levels, particularly in winter, then the TAO light is probably the best solution, short of installing dazzling floodlights.

Another practical approach is to turn off overhead lighting and use dimly lit lamps instead. Most M1 ipRG cells detect overhead light, which we can infer by their concentration in the bottom half of the retina (a lens views an inverted image).²³ Therefore, lights placed low to the floor or below eye level won’t disrupt our melatonin release as much as overhead lighting.

Red light at night, intrinsically photosensitive melanopsin-expressing retinal ganglion cell’s delight.

Overhead fluorescent lights would be the worst offenders. Soft yellow overhead lights would be slightly better. Dimly lit lights set down low would be much better again. The optimal setup would be to use red wavelength lights. I use TrueLight Luna Red bulbs. They can be adjusted to soft yellow in the evenings and then to red at night time. I find their products great, especially for my daughter who’s scared of sleeping in the dark. They also sell portable nightlights with motion sensors, as well as red LED flashlights. Those work great for stumbling to the bathroom at night. A good budget option on Amazon is the Sunlight 100 ln red bulbs. Unlike many others on Amazon, they’re UL listed for safety standards and the manufacturer’s website claims that they only emit red light between 615–640 NM. They’re not colour adjustable or intensity adjustable, but 100 lumens should be dim enough. As for budget portable night lights on Amazon, most of them I wouldn’t recommend, due to safety and/or effectiveness concerns. The only decent ones we could find were the Emagin A portable night lights. They use actual red wavelength LEDs, rather than a red plastic cover over white LEDs. The motion sensor works very well and they look great too. I stick them on the walls of my halls and various other areas around the house, on the lowest brightness setting. My favourite choice is TrueDark though. BlockBlueLight is another good option.

TrueDark also sell stickers for either blocking or filtering the indicator lights on household appliances or alarm screens. If you’re partial to midnight fridge raids like Indiana Jones, then putting these stickers on the interior fridge light will ensure you don’t stay wide awake afterwards.

Indiana Jones, raiders of the last snack.

Using red spectrum lighting might not be necessary, but I do find I sleep much better as a result. The bedside lamps you already have should do fine though, especially in comparison to fluorescent overhead lighting.

For daytime lighting, TrueLight also have bulbs to enhance alertness. Those type of bulbs were originally developed to simulate daylight for astronauts on the International Space station. However, based on the latest research, the TUO bulbs I previously mentioned are probably a better choice.

Saving earth costs sleep — or does it?

While energy efficient white LED bulbs are great for the environment, they’re typically enriched in blue light. A 2020 study published in the esteemed journal Nature found that almost half of homes were lit brightly enough to suppress melatonin by 50% on average (see Figure 1 below).²⁴ They speculated that the impact could be much greater on children or people with sleep or mood disorders. Homes with energy-efficient lights were found to have almost double the impact of those with incandescent lighting.

Figure 1: Cain et al. (2020) — Evening home lighting adversely impacts the circadian system and sleep. Individual homes are shown as gray curves, averaged across nights and smoothed with a 3-h moving average, ending with a gray dot at the individual’s average bedtime. The blue lines show the median (thick solid line), interquartile range (thin solid lines), and the 10th and 90th percentiles (dashed lines) for homes before bedtime.

On the other hand, according to the US Department of energy, rapid adoption of LED bulbs would save hundreds of millions of tons of greenhouse gases from entering the atmosphere.

That’s why I opt for the aforementioned red LED bulbs. They use less energy than incandescent bulbs and they don’t aggravate my insomnia, anxiety or bipolar depression. Red bulbs save the earth and make my snooze button redundant.

It’s important to be aware that many cheap red lights like these are simply white LEDs with a red plastic cover. These don’t actually emit red light wavelengths. They emit blue light! While the red plastic tint will help to mask most of the blue light, it’s better to get LEDs that emit long wavelength red light such as these from BlockBlueLight. At 80 lumens (lm), those ones are quite bright, so they’re best used in larger rooms, as I’ll explain below. The Emagine A ones on Amazon also use red LEDs. I use some of those along with my Truelight ones. My favourite is the TrueLight Nightlight/Flashlight, which has an adjustable intensity and can be used alternatively as a flashlight.

Regarding light intensity, candlelight doesn’t activate those ganglion cells, so candles or fireplaces or campfires would be fine (apart from the fire hazard). While an open fire might appear bright, it actually gives off very little light in terms of illuminance. Illuminance (lux) is a measure of the light intensity within a given area, such as a room. Luminous flux (lumens) is the total amount of light from a light source. A candle is 1 lumen.1 lux is equal to the brightness of that candle shining on a one metre square surface that is one metre away from it. The same candlelight spread out over a 10 square metre room would be 0.1 lux. In contrast to a romantic, candlelit bedroom, direct sunlight might range between 30,000–100,000 lux, ambient daylight might range between 10,000–20,000 lux and overcast daylight is about 1,000 lux. Moonlight is typically a mere 0.05 to 0.1 lux.²⁵ How much dirty lux is one too many then? Well, a 2016 mouse study published in Nature found that white light disrupted sleep architecture even at an intensity as low as 10 lux. They found that red light at or below 10 lux caused no sleep disturbances, suggesting that this amount of red light could make for a healthy level of romantic night lighting.²⁶ I tell my wife she looks great in red, barely visible.💃🤗

Make the bedroom a red light district.

Now, for my favourite “sleep hack” which requires no effort whatsoever. I find this to be the easiest way to phase advance my clock. The hack is to simply leave the curtains open at night! This ensures that some sunrise light is hitting my eyelids before I even wake up. Light can penetrate the eyelids (unless you sleep under the covers). Leaving the curtains open allows me to gently wake up feeling totally alert by the time I spring out of bed. It also leads me to naturally want to wake up even earlier the next day. It’s incredible. I’ve been told by a trusted source (my wife) that my morning transformation has been “miraculous.” Getting early light exposure can phase advance your clock by 1–3 hours per day.²⁷ That means if you were to fly from New York to Dubai (+8h time difference), you could adjust to the local schedule in as quickly as 3 days!

However, there is one big problem with leaving the curtains open. By now we all know that light exposure at night suppresses melatonin and phase delays our clock. If you live in a brightly lit city like I do, the light pollution at night will have a negative impact on sleep and lead to an undesirable phase delay. Therefore, leaving the curtains open would likely cancel out the positive effects of a natural sunshine alarm clock. The free solution would simply be to set an alarm and actually get out of bed and open the curtains at sunrise. I’m not a big fan of that though. Sluggishly crawling out of bed slightly defeats the purpose of the hack, but more power to those who can muster up the grit. I pondered deeply on this curtain conundrum, which led to a lightbulb moment! I thought that we could create a device to automatically open curtains at sunrise. Good thing I googled “curtain opening device” before consulting anyone about it. It turned out that the product had already been invented! I was a bit disappointed that I wouldn’t be the next Nikola Tesla of the curtain industry, but at the same time, I was glad to be able to buy the device immediately from SwitchBot.

Switchbot Curtain

Their smart curtain device can be set to either open at sunrise or at a specific time. Alternatively, it can be operated by remote, voice or from your phone. Unlike other smart curtains, which can cost thousands to install, this device fits any existing curtain rails and it only takes 30 seconds to install. SwitchBot says the rechargeable battery holds a charge for about 8 months. They also offer a solar panel attachment as an optional extra, so you never need to charge it. Before buying it, I was concerned that the sound of the motor might be unpleasant, but the sound never wakes me. It even has an option to open slowly in Silent Mode. I set it to open and close automatically based on sunlight, but you can choose to set a complex weekly schedule of opening and closing times for each room in the house, as well as variations in how wide the curtains open. You can even control your curtains remotely from your phone when you’re not home. Too bad Kevin McCallister didn’t have these to ward off Christmas intruders.

*Flamethrower sold separately.

It’s a brilliant device. I’d thoroughly recommend it to anyone who struggles with feeling sluggish in the mornings.

Is blue light really the culprit?

There’s a debate about the detrimental impact of blue light. As previously mentioned, apart from the intensity of light, it’s recently been found that the colour of light also plays a role in setting circadian rhythms.¹⁴ As we now know, it’s likely that a contrast between bright yellow and blue light will have the biggest impact. However, a 2019 study by researchers at the University of Manchester, published in Current Biology, suggested that blue light at night might not be as disruptive as previously thought.²⁸ They hypothesized that yellow and orange light more closely resembles daylight, whereas low-intensity blue light actually mimics natural twilight. The study was done on nocturnal mice though, so diurnal humans may respond differently. Further human research is needed.

What does seem clear at this point in time is that the effect of light on melanopsin — so-called melanopic irradiance— is currently the most reliable predictor of both melatonin suppression and circadian shifts. Melanopic metrics such as equivalent melanopic lux (EML) are measured on a weighted scale based on activation of those retinal ganglion cells, as opposed to a standard lighting industry metric such as lux. Standard metrics such as lumens and lux are measured on a weighted scale based on the eye’s perception of brightness. Since blue and green light has the biggest impact on melanopsin, blue light will have a high melanopic irradiance.

Indeed, subsequent reports from the University of Manchester researchers in 2020 seemed to roll back on the 2019 speculation, perhaps somewhat conceding that less blue light at night is preferable.²⁹ A November 2020 report from those researchers layed out guidelines for light exposure, reaffirming the consensus that the melanopic effect in bright environments is mainly dependent on ganglion cells.³⁰ The publication has not yet gone through the peer review process but it does seem pretty clear that a melanopic measure of light is a reliable approximation for most practically relevant situations. Therefore, ensuring that lights are dimmed 3 hours before sleep is probably far more important than adjusting the colour of light. That being said, the authors acknowledge that adjusting the colour of light from screens can decrease overall melanopic irradiance by 50%. The question remains whether this modest decrease + more yellow light is better or worse than no decrease + more blue light. The authors conclude that adjusting the colour of light will be most beneficial when combined with other strategies such as dimming screens and using ambient lighting. As for the blue vs yellow debate, personally, I would think less blue light is preferable. I’m going to stick with my blue blockers. Intuitively, I would speculate that humans might be more evolved to seeing fire at night, which emits very little blue light. Wearing blue blockers also reduces melanopic activation, which currently seems to be the most reliable predictor of how light disrupts circadian rhythms.

Therefore, if like me you enjoy watching TV or using devices late at night, then dimming screens and wearing blue light blockers can help to reduce overall melanopic irradiance.

Many companies sell blue blocking glasses intended for daytime use but personally I never wear blue blocking glasses during the daytime. We want to get plenty of blue light early in the day. That being said, light from a screen does have a very different spectral composition to sunlight. It remains to be seen how daytime screen time impacts health. Therefore, daytime glasses could potentially be beneficial too.

For effective night-time use, it’s extremely important to note that not all “blue blockers” are created equal. Clear lenses usually block a mere maximum of 40% blue light. Some block far less or none at all.

Clear lenses are largely ineffective at preventing circadian disruption. The above image depicts YouTube celebrity “Doctor Mike” promoting MVMT glasses. This particular brand does not even make any specific claims about how much blue light they block. MVMT is a manufacturer and retailer of affordable watches and other fashion accessories.

Yellow lenses typically block a maximum of 75% blue light, while some orange, amber or red lenses can block up to 100% of blue light. Some manufacturers such as TrueDark sell red lenses which they claim can block all blue, green and violet light. While blue light blocking glasses alone won’t prevent circadian shifts, they can certainly help to reduce overall illumination, particularly if the overhead lights are switched off and the only other light you’re getting is from a screen. Glasses which block both blue and green light are necessary to prevent melanopsin activation. I’ll recommend my favourite ones below.

There are a lot of poor quality and potentially outright fraudulent blue blocking glasses on the market, some of which may not even block blue light at all. For example, I initially bought these Tom Ford ones on Amazon. I’m pretty sure they were actually fake so I had to buy another pair directly from the Tom Ford website. They look nice but any clear lenses don’t really offer much protection. I still wear them in the evenings sometimes, but I’ve since switched to proper ones intended for late-night wear. TrueDark retails some nice glasses which they say block all blue and green light. They also stock glasses which they say block up to 100% of blue, green and violet light, but I don’t think my wife would sleep with me if I wore a pair of those Cyclops goggles. They’re probably the best choice in terms of efficacy though.

I wish TrueDark would make red lens glasses with more aesthetic frames. There are many other brands on Amazon, but just make sure to buy a reputable brand from a seller with a high feedback score. If you already wear glasses, there are models of blue blockers which can be worn as fitovers. If you’re in the UK, a good company which sells some nice looking, effective glasses is blockbluelight.co.uk. They also have websites in Australia and New Zealand for fast and cost efficient shipping to those regions. Luminere is a stylish yet reasonably priced brand endorsed by sleep doctor Michael Breus, who has appeared on TV shows such as NBC News. Although the orange lens style is increasingly fashionable and offers great protection against blue light, Luminere don’t claim to block any green light. Based on the spectral sensitivity of melanopsin, blocking blue light is only half the battle. According to CIE TN 003:2015 standards, the wavelength sensitivity of melanopsin ranges between about 400nm to 600nm, with a maximum sensitivity at 490nm. There’s no definitive definition of what “blue light” is exactly, because it blends into violent and green at either end, but UL, a certification authority commonly used by electronics companies, defines blue as 440–490nm. Therefore, half the spectral sensitivity of melanopsin is to green light.

Figure 2: Photopic normalized spectral power distribution of a warm (3000K) white LED (Hoffmeister LED-104–471) in comparison to the melanopsin sensitivity function (based on CIE TN 003:2015).

To get a melanopic luminosity metric, we can simply multiply the melanopic sensitivity function by the spectral power distribution (SPD) of a light source, on a per-wavelength basis.

Figure 3: The melanopic flux of a warm (3000K) white LED (Hoffmeister LED-104–471), calculated by multiplying its photopic normalized spectral power distribution by the melanopsin sensitivity function (based on CIE TN 003:2015). The blue area beneath the curve represents the contribution of blue light to the activation of melanopsin. The green area beneath the curve represents the contribution of green light.

I’m not going to do the calculations for every type of light source, but we can eyeball the melanopsin function in comparison to the SPDs of 54 modern smartphones. We can see that blocking blue light alone won’t cut it, as far as melanopsin is concerned. You can do the same by looking at the SPD of any light source.

Figure 4: Relative melanopic sensitivity curve (based on CIE TN 003:2015) in comparison to the spectral power distributions of 54 different smartphones, taken from Yao et al. (2020). The blue area roughly represents the blue wavelengths that activate melanopsin and the green area represents the green wavelengths that activate melanopsin. We can see that green light from modern LCD, OLED and AMOLED phone displays significantly contributes to overall melanopsin activation.

If we assume that the the Luminere Sleep Doctor glasses perfectly block all blue beyond 490nm, then they are only about 45% effective in blocking melanopic luminosity from 3000K (warm white) LED lights. Therefore, the TV celebrity “Sleep Doctor” and other sellers cannot claim that blocking blue light alone will prevent melatonin suppression or circadian disruption. That’s why I opted for a pair from Ra Optics. I love the logo, aesthetic frames and they block both blue and green light. The lenses are made from CR39, a polymer used in military aircraft for its light weight and durability. Their handcrafted frames are made in California from Italian plant-based cellulose acetate and German steel. I also like the video of their CEO Matt Maruca using a spectrometer to demonstrate the effectiveness of the glasses. Ra Optics have offered us 15% off for the Zenstra community. You can enter the coupon code “Zenstra” at checkout to claim your discount.

The spectral power distribution of sunlight filtered through a Ra Optics night lens.

ReadingGlassesEtc is another option for designer frames that block most of the melanopic spectrum. You can choose your favourite frames and then select “BPI 550 Red Orange” lenses at checkout. They block up to 550mn of green light. Other brands that block green light include the aforementioned TrueDark range, as well as some models from BlockBlueLight. Ra Optics is definitely my #1 choice though, both in terms of efficacy and aesthetics.

As for the colour debate, the other argument, made by University of Manchester researchers in 2019 is that blue light blockers lead to a modest reduction in melanopic illuminance at the expense of perceptible changes in color. As previously mentioned, they suggested that dim blue coloured lighting might actually be preferable to dim yellow coloured lighting — flying in the face of the general consensus that blue light is worse. If you’re the type of person who likes to walk on the wild side and break from the crowd, you could try experimenting with very dim lighting and maybe even go as far as to use blue tinted lenses to mimic twilight! I wouldn’t recommend it, but I’d love to hear how you get on. Who knows, maybe the conventional wisdom is wrong. Personally, for now, I’m going to stick to viewing the world through rose-tinted lenses.

Ra Optics Popp Night

Roses are red, my iPhone is blue…

Because humans are so sensitive to evening light, even levels below 30 lux can cause a 50% suppression of melatonin.³¹ The measured corneal illuminance from a smartphone’s display is likely to be over 100 lux.²⁰

Dimming the screen to its minimum level may potentially reduce melanopsin activation to less than 1% of the activation at maximum display brightness.³² Newer phones with OLED and AMOLED screens have higher concentrations of blue and green light, with peak wavelengths around 450, 530, and 630 nm. While some OLED manufacturers might boast of having lower blue light, the large amount of green light also needs to be considered, in terms of overall melanopic impact. For instance, Samsung boasted that their Galaxy S10 reduced ‘harmful’ blue light (which they asserted were wavelengths in the 415 to 455nm range) to below 7%. Their marketing photo showed that they had shifted the peak concentration of blue light from about 450nm toward around 460 nm.

Figure 5: Samsung marketing image for their Galaxy S10 smartphone, in comparison to those peak wavelengths roughly plotted against the melanopsin sensitivity function.

Considering melanopsin is maximally sensitive to blue light at between 479–490nm, this means Samsung actually made the spectral power distributions worse for circadian rhythms, based on a melanopic metric! And they had the audacity to boast about it. The study they cited indicates that by “harmful” they meant in terms of macular degeneration rather than circadian disruption. Therefore, those health claims could potentially be misleading to an average consumer who’s more concerned about sleep disruption than macular degeneration. Samsung proudly boasted of being awarded TÜV Rheinland’s official ‘Eye Comfort’ certification, whatever that means. I was unable to find exact criteria on TÜV’s website. A search for Samsung’s claimed certification number on TÜV’s “Certipedia” database yielded “currently no valid certificates.”

I admire Samsung’s effort to make harm reduction a prominent selling point but they should have been more clear that by “harmful” they meant contributing to age-related macular degeneration and not detrimental effects on sleep. Hence why I wrote this article, so you can spot marketing spin in full 4k resolution. I’m not saying that Samsung phones are any better or worse for sleep than other brands. I’m just saying that certain aspects of their marketing for the S10 might have been misleading.

To be fair, Samsung’s night-time blue filter setting is a brilliant feature, as it is on other modern smartphones. Enabling the blue light filter mode on devices is a free and easy way to reduce blue and green light. On iPhones, iPads, Macs, the setting is called “Night Mode,” which automatically adjusts the colour of your screen to warmer tones at night. On Windows 10, it’s called “Night Light.” On most Android devices, the display setting is usually called “Blue Light Filter.” If your Android doesn’t have this setting, there’s an App called “Blue Light Filter” which does the same. You can set your device to automatically adjust at sunset or you can set a custom schedule tailored to your own routine. When I began using Night Mode it had a tremendous benefit on my sleep.

Alternatively, if you don’t like your phone screen being tinted orange or don’t wear night glasses, there are some phone covers which block blue light. Research suggests that such covers are still most effective if used in conjunction with a fully dimmed screen and Night Mode enabled. If you dim the screen and still keep the tone cool, the screen cover will still filter a lot of the blue light. I don’t personally use one of these covers, because I always wear night glasses instead.

When cooler is better…

Body temperature has a circadian rhythm too. Body temperature increases during the daytime and decreases during the night time. It fluctuates by about 1°C (33.8°F).³³ We tend to feel alert when our temperature is increasing and we tend to be feel sleepy when our temperature is decreasing.

The circadian rhythm of body temperature (CRBT) tends to cause your temperature reach its lowest level usually a few hours before your average wake time. The time of this daily temperature minimum is called the bathyphase. Conversely, body temperature tends to reach its highest level in the afternoon. The time of this daily temperature maximum is called the acrophase. In a 2019 study of 93,000 people, the average minimum was found to occur between 06:00–08:00 and the maximum between 18:00–20:00.³⁴ The bathyphase was found to be timed roughly to sunrise, with the acrophase occurring about 12 hours later. The timings vary widely across studies though. Many studies report the average bathyphase as falling around 03:00–04:00. Morris et al. (1990) found that the temperature rhythms of insomniacs were about 2.5 hours later than those of good sleepers (see figure 6).³⁵

Figure 6: Fitted Fourier curves to control vs insomniacs mean 24-h temperature relative to their usual sleep onset times (vertical solid line). The usual mean lights out times (LOT) for each group are indicated as vertical dashed lines. The estimated mean wake maintenance zone (WMZ) for each group is indicated as a shaded area. WMZ is what the authors used to describe the 6 to 9 hour period before mid-trough temperature and about 4 to 7 hours after mid-trough temperature. During the so-called WMZ, subjects tend not to want to sleep. From Morris et al. (1990).

In fact, temperature is actually the main effector by which a diverse range of cells in the body are entrained.³⁶ Not all cells speak the same language, but every cell in the body understands temperature. The SCN ‘master clock’ controls the circadian component in core body temperature. The SCN uses heat as a cue to set the clocks.

Meals have a thermogenic effect, so eating will cause minor fluctuations, as will exercising and showering. Needless to say, going for a sauna or jumping in a freezing lake like Wim Hof will significantly disrupt the cycle. Well, actually maybe Wim Hof isn’t the best example to use, considering he’s able to literally raise his core body temperature while sitting in an ice bath!³⁷ Wim claims anyone can do the same by applying his breathing techniques.

Wim Hof, the ”Iceman,” having a relaxing bath.

Superhumans and breathing techniques aside, we mere mortals can utilize cold exposure to shift the CRBT and begin waking up earlier.

A change in core body temperature is actually not what sets the clock. Swallowing a bucket of ice will probably drop your core temperature, but it won’t help you nod off to sleep.³⁸ In one study, subjects who were fed ice actually became more alert (unsurprisingly). It turns out sleepiness induction is actually caused by an increase of distal vasodilation (when blood vessels widen in our extremities), and hence heat loss.³⁸ This is why a warm bath, a hot drink, lying down, relaxing or having sex all promote sleepiness. They all induce vasodilation. If you’ve ever experienced the feeling of not being able to sleep because you have cold feet, this is the reason. Whether you or your partner have noticeably cold feet or not, wearing socks to bed can increase distal vasodilation and may help you to fall asleep faster.

As for circadian rhythm setting, your bathyphase is your reference point for shifting your clock. In the hours before this temperature minimum (which usually occurs at night when you’re asleep), any light exposure, increase in temperature, exercise or food is going to make your body think its daytime and it will phase delay the clock. In the few hours after the bathyphase, any of those zeitgebers (external cues) will phase advance the clock.

Now that we understand the importance of the bathyphase, we can utilize it. Typically when we wake up in the morning, our temperature is rising. If you were to jump into a sauna first thing in the morning, your body would begin rapidly cooling down after you get out. That’s not what you want. That will delay the CRBT.

On the other hand, if you were to go for a cold shower or if you’re masochistic enough to do an ice bath first thing in the morning, of course it’s going to immediately wake you up (to say the least). Additionally though, it’ll shift your clock earlier the following day.³⁹ When you hop out, there’ll be a sharp rebound thermogenic effect. The rate at which your body heats up will be increasing. This will lead your temperature to peak earlier in the afternoon.

Finally, eating earlier and exercising earlier can help to advance the clock. Eating late at night can shift the clock later.⁴⁰ Exercising in the morning can phase advance your clock while exercising at night will phase delay it.⁴¹ Interestingly, exercising in the evening has actually been found to cause phase advances for some people but phase delays for others.⁴² It may be related to whether someone is a “night owl” or a “morning lark.” I might talk more about exercise as a zeitgeber in another article. The most important factor is light though. Swap the disco strobe for the sun and step into the light!🌞🙌

Figure 6: Image of the Sun taken from ESA/NASA’s Solar Orbiter satellite.

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