Let There Be (Morning) Light
How light can improve your sleep and mood.
Over the past few months, I’ve changed the amount of light I am exposed to throughout the day and now I sleep better and earlier. I went from convinced that I was a night owl to regularly falling asleep around 10:30pm and waking up around 7:30am. I haven’t noticed any mood benefits, but there is strong evidence that light therapy can improve your mood (and very strong evidence if you have seasonal depression). My interest in lighting was sparked by the Huberman Lab podcast, but I wanted to dig in myself. Why is lighting important? Does it make that much of a difference? Is this tweet really true?
The science here is relatively new. But improving our light exposure has a huge upside and needs little effort. In other words, it’s low hanging fruit. It comes down to two things: increasing daytime light, and reducing artificial evening light.
The Brain Clock
To see why light matters, we have to learn how to keep time.
Cycles are fundamental to our health, and we have LOTS of cycles. We have eating cycles, mood cycles, menstrual cycles, social cycles, and of course, sleep cycles. Cycles that repeat once a day are known as circadian (circa/about, dian/day) rhythms. The most famous circadian rhythm is the sleep-wake cycle.
Here’s a fun diagram from wikipedia showing a typical sleep-wake cycle:
There’s a lot going on here, but all that matters is our sleep-wake cycle repeats once every 24 hours. If I asked you to close your eyes and tell me when exactly 1 hour has passed, I’d bet some serious money that you wouldn’t get within 1 minute. Maybe you’d get within 10 minutes — that’s a 16% margin of error. So if it’s hard to gauge when one hour has passed, or even one minute, how does your body time when 24 hours has passed?
It turns out we all have a central clock (technical term is the supra-chiasmatic nucleus or SCN) nestled within us, sitting directly above where our eyes connect to the rest of our brain (spoiler alert: this location isn’t a coincidence).
But how does the clock actually work? We can guess by thinking about how other clocks work. We have grandfather clocks, mechanical watches, and quartz watches. Does our brain clock work like any of those?
Grandfather clocks work by swinging a pendulum back and forth. This could be a fun function for the uvula, but sadly we don’t work this way.
Mechanical watches work by winding a spring and then letting that push a gear that consistently ticks. Throughout the day, sleep pressure (homeostatic sleep drive) builds up, increasing the need for sleep. This is somewhat similar to the spring winding of a mechanical clock, but doesn’t explain what starts the sleep pressure of how we keep time.
Quartz watches are pretty cool. They work by electrifying a tuning fork, which starts to vibrate at a specific frequency (32,768 times a second). Then a meter in the watch counts the number of vibrations, and once it hits 32,768, emits a pulse that one second has past. We have something called “clock genes” that output precise rhythmic signals to the rest of our body, but how do the clock genes know when to fire? What is the tuning fork equivalent for our clock?
So what’s the answer? The primary way our clock tells the time is... light! First, our eyes see how bright the sun is. Then, that signal gets passed along to the clock, which then uses that information as the primary way to keep pace. The sensors in our eyes that detect sunlight are called melanopsin cells, and they directly communicate with our clock genes. Like any smart manager, we delegate the annoying job of keeping a consistent pace to someone else — the sun — which works well because the sun is very consistent.
Our clock has just two very reasonable assumptions: (1) we’ll be outside most of the time, and (2) the only significant light source is the sun. But in the last couple hundred years or so, it has become a very bad way for humans to tell time.
We invented lights.
Fixing the Clock
I hope you are convinced that lighting is important. It might seem that the only true solution to fixing our clock is to be outside all day and fall asleep with the sun.
There’s an easier way. We can get a big bang for our buck without becoming Amish. Here’s what you can do, in order of most to least evidence.
Step 1: Morning Sunlight
The first thing we want to fix is the beginning of our cycle. Morning light, often referred to in studies as “bright light therapy,” is as simple as this: go outside in the morning. I think the best way to do this is a daily 15-20 minute walk soon after waking.
Why outside? Typical illuminance in a living room is around 50 lux. In full daylight, you get a whopping 10,000 - 25,000 lux. Although it’s possible to make inside as bright as the outdoors, it’s much easier to step outside for 15 minutes. Why 15 minutes? I don’t know. Samer Hattar (Chief of the Section on Light and Circadian Rhythms at National Institute of Mental Health) recommends 15min. I don’t think there’s anything magical about this number, but the more the better.
Think of morning light as the starting gun for the rest of the circadian rhythm.
Step 2: Dim Evenings
The next most important thing it to make our evenings as dim and warm as possible. We want maximal blue light during the day, but by 9-10pm, that same blue light will inhibit melatonin production, increase cortisol, and throw off our cycle. In the evening, we want light that is much closer to candlelight than fluorescent.
We want to avoid screens because they’re usually high in a similar wavelength of blue light that mimics sunlight, and nighttime screen use results in worse sleep, circadian timing, and next morning alertness. Does iPhone night shift mode help? A recent study at BYU says no, it’s comparable to using an iPhone without night shift. But, if you’re going to use a screen, I don’t think it can hurt to use the lowest brightness / highest warmth available.
Most of the cells in our eye that signal the central clock are located in the bottom half of our retina. Because our lens inverts light (see below), these light sensors are most sensitive to light coming from above. This makes sense considering sunlight is usually overhead (unless you’re 50 Cent).
This means that overhead lights are likely much more likely to interfere with your sleep-wake cycle than low placed lights. So make your lights dim in the evenings, and avoid overhead lights.
As far as I can tell, blue light blockers are not worth it. Not only is blue light actually beneficial for you during the day, but blue light blocking glasses have poor consistency and aren’t good at blocking blue light. Plus, blue blockers don’t seem to improve sleep quality, reduce eye fatigue, or conserve eye health. So, 👎.
Step 3: Watch the Sunset
What if all those sunset instagrammers were on to something? A study by Rivera & Huberman 2020 proposes that sunsets may have “unique chromatic features” that alerts the central clock that the day is ending. Melanopsin cells in our eyes may be specifically tuned to the wavelengths of a setting sun. This isn’t definitive, and as far as I can tell there aren’t studies on sunset watching and sleep. But watching the sunset can’t hurt, and it may help align your sleep-wake cycle.
The study includes the driest possible description of a sunset:
At sunset the sky transitions from blue to yellow, to orange, before turning dark blue and, eventually, black. Built into these transitions is yellow–blue contrast with blues getting progressively darker and yellows brighter.
You will sleep better and earlier, and you may feel happier, if you:
Go outside in the morning for 15min
Dim and lower evening lights
Watch the sunset
Bonus: get as much daylight as you can. If you can’t be outside and you want to be hardcore, you could try this.
Light and Sleep
Sleep is the first casualty of a disrupted circadian rhythm. Here’s a scary excerpt from Bedrosian & Nelson, 2017:
Individuals who slept with a night light of ~40 lux had shallower sleep and increased arousals, as well as markedly decreased brain oscillations during sleep... it is possible that even relatively low levels of nighttime light could have detrimental effects on cognitive function through sleep disruption.
FYI, 40 lux isn’t that bright. For reference, a standard 40W bulb emits about 400 lux. Here’s another scary one:
A striking number of mood disorders are either characterized by sleep and circadian rhythm disruption or precipitated by an irregular light cycle. Sleep disruption is a diagnostic criterion for major depression, bipolar disorder, post-traumatic stress disorder, generalized anxiety and other mood disorders.
According to Bloom et al. 2019, evening light exposure via an e-reader led to reduced evening fatigue (you’re less tired late at night), reduced melatonin secretion (it’s harder to fall asleep), and reduced alertness the next morning.
But light exposure at the right time can have positive impacts. What happens if you get a lot of daytime light? Bloom et al. 2019 explain:
Natural daylight at high intensities as experienced outside buildings has previously been shown to (1) advance the timing of sleep to earlier hours, (2) affect the duration of sleep, and (3) improve sleep quality.
Light and Mood
Bright light at the right time can also improve our mood, from Bloom et al. 2019:
Light can affect mood in several ways: by directly modulating the availability of neurotransmitters such as serotonin, which is involved in mood regulation, and by entraining and stabilising circadian rhythms…light therapy has found an increasingly widespread use for treating mood and other psychiatric disorders.
Plus, windows make us happier:
In one study, women who had a window were exposed to higher light levels during the day and reported better sleep and lower depressive symptoms than women working in similar jobs, but without office windows.
Some more direct evidence for morning light: Choi et al. 2019 took melatonin and cortisol measurements before and after exposure to bright blue light in the morning (to mimic the sun). The light reduced melatonin levels by 53%, and reduced cortisol levels by 43% (although other studies showed no reduction in cortisol). Blue light also improved measures of subject mood by 18% (as compared to viewing a regular white light in the morning).
There are so many more studies on this that I won’t cite. To summarize:
not getting the right levels of light exposure can disrupt your circadian rhythm
circadian rhythm disorders are associated poor sleep and most mood disorders
bright light during the day (light therapy) can be an effective treatment for mood disorders and can improve your sleep
Other interesting papers: