When I moved back to Berkeley from Beijing last spring, I noticed that my sleep was worse in Berkeley, months after arrival. I woke up less rested than in Beijing. There was no obvious explanation. My life was similar in the two places, especially on dimensions that influence sleep. I had expected my health to be better in Berkeley than Beijing because of Beijing pollution.
Wondering why my sleep was different, I realized my Beijing bedroom was probably darker than my Berkeley bedroom. In Beijing I live in an apartment complex and cover most of my bedroom windows to block outside light and for privacy. In Berkeley, I live in a house. My bedroom window looks out over an enclosed backyard. That my Berkeley bedroom might not be dark enough had never occurred to me. It was fairly dark — no street light, no alley light, no light from neighbors.
Did the (likely) difference in darkness contribute to the difference in sleep? I tested this possibility by making my Berkeley bedroom much darker. Later I made it lighter, then darker again (an ABAB design). I measured sleep quality by rating how rested I felt when I woke up on a 0-100 percentage scale where I estimated how rested I felt compared to completely rested (= 100%). I have used this scale for many years. Here are the results:
To my surprise, when I made my bedroom darker my sleep improved. It got worse when I returned my bedroom to its original darkness. It improved again when I again made it darker. Until I graphed the data, I hadn’t realized that my baseline ratings probably shifted shortly before I made my bedroom darker. (I kept a paper record of my sleep, which made it hard to graph the data. Failure to notice this baseline shift was the last straw….I have gone paperless.) In spite of the baseline problem, the data are convincing that even at low intensities, light intensity mattered.
Depth of sleep (controlled by the amplitude of a circadian rhythm) is surely controlled by the amplitude of the light/dark rhythm. Below a certain threshold of light intensity, however, reducing light at night won’t make a difference. These observations implied that the threshold was lower than I’d thought. Support for the idea that the threshold is low — lower than other people realize, too — comes from a study published last summer after my experiment. Researchers reanalyzed old data to see if there was a correlation between lunar phase and sleep quality. Their subjects had slept in a windowless laboratory room. Nevertheless, sleep was worse during a full moon. One researcher was baffled. “What I can’t get my head around is, what would that cue be?” he said. In other words, how could the phase of the moon influence sleep? I’m not puzzled. The subjects spent only a few nights in the sleep lab. I believe there was carryover from when subjects slept at home, in rooms open to moonlight. Light from a full moon reduced the amplitude of sleep. This affected sleep later in the lab for the same reason jet lag lasts several days.
Is your bedroom dark enough? The light at night in Person X’s bedroom will differ in many ways from the light at night in someone else’s bedroom so a one-size-fits-all rule (your bedroom should be darker than . . . ) makes little sense. What does make sense is personal science: measure your sleep and test different levels of darkness.
I just wear an eye mask.
Yes. This one is much more obvious to me than the blue blockers. When I blacked out my windows, and reduced the overall amount of light I was exposed to, I saw a great improvement in sleep.
The blue blockers however, didn’t seem to do anything. Some of the Amazon reviews suggest some blue blockers are better than others, so maybe I need a different pair. Without the blue blockers I would get sleepy and go to sleep. With the blue blockers on, it seemed I was not getting sleepy. My sleep quality seemed the same, but I think I might stay up longer if I wear the blue blockers and don’t pay attention to the time.
This makes sense. I am so sensitive to any amount of light when I sleep that I have to put sticky notes over lights on the TV, DVR, clocks, etc when I am in hotels. I find them incredibly bright and annoying. I have a neice and nephew who keep a TV on while they sleep and I worry about how this negatively affects their health.
Seth’s sleep improvement guide:
1. Vitamin D early in the morning
2. Blue light blockers at night
3. Honey before bed time
4. Dark bedroom
I’m surprised he ever wakes up…ha
What I’d like to see/know, is how much improvement each one provides independently of the others; does anyone of them overcome a deficiency of the others; i.e., will darkening your bedroom provide the same quality of sleep as doing all four?
This seems, to me, that it would be a very long experiment, involving many confounders – time of year being the largest (does our sleep improve, naturally, as daylight lessens, or the opposite, or not at all?).
All good stuff for those who have trouble sleeping – inexpensive and easy to do. But, I think the darkened bedroom is probably the winner over all the others. Of course, our early ancestors, before they moved into caves, slept under very bright night skies, especially when the moon was full.
wouldn’t a sleep mask be easier than darkening a room? (that’s what i’ve using for years)
Seth: I have been told a sleep mask is worse. Someday I will try one.
> Support for the idea that the threshold is low — lower than other people realize, too — comes from a study published last summer after my experiment. Researchers reanalyzed old data to see if there was a correlation between lunar phase and sleep quality. Their subjects had slept in a windowless laboratory room. Nevertheless, sleep was worse during a full moon. One researcher was baffled.
I have completely failed to replicate their claim using 1600+ nights of Zeo data from myself & 2 other people: https://www.gwern.net/Lunar%20sleep
There’s nothing there. It was a years post hoc analysis on their part, and probably just data dredging.