There is pretty good evidence for how human sleep relates to cycles of light and dark.
Here are some podcasts and books about light and dark, night and day, sleep and wakefulness:
- Digital Human podcast – Dark (series 4, episode 5)
- BackStory Radio – On the Clock
- Monocle 24: The Urbanist podcast – City sleepers (episode 183)
- NPR – Is a Brighter Earth a Better One?
- Last Week Tonight – Daylight Saving Time: How is this still a thing?
- Wikipedia – Segmented sleep
- Sleep: A Very Short Introduction by Steven W. Lockley and Russell G. Foster
- Harvard Business Review IdeaCast – Attacking the Sleep Conspiracy (audio and transcription; episode 359)
As you can probably tell from above, Roger Ekirch was a major force in bringing segmented sleep to popular attention, in part through his book At Day’s Close: Night in Times Past. I actually find the book a pretty slow read – it’s a scholarly examination of how people perceived the night, it’s not just about sleep. The TL;DR is that people basically thought night was pretty weird, with miasmic air drifting about.
Interesting historical sidebar 1: The European Celts apparently measured time not by days, but by nights. They counted time by darkness, not by light.
Interesting historical sidebar 2: Lest you think people always fled the night, ancient Rome was so crowded that delivery vehicles were banned during the day, and had to deliver at night. That being said, night in Rome was dangerous, with roving thuggish gangs.
Disclaimer: I’m not an expert, everything below is based on my understanding of things I’ve learned from the above resources and other sources.
The lost night – how artificial light broke human sleep
We’re basically made for very bright light during the day, and very dark darkness during the night, with dawn and dusk transitions. Our eyes and our brains have special sensors and systems that measure and act based on the light levels and the light change.
Sleep is a very different physiological state from being awake; the transition is traumatic for the body. What’s supposed to happen is that about an hour before you actually wake up, the body is supposed to start preparing, based both on your internal clock and the light intensity and light change (rising with the dawn). This depends on you getting up more or less at the same time each day.
During the day, you’re supposed to be outside in the very bright sunshine. (If you ever emerge from your office blinking at the noonday light, it’s because our offices are dramatically less bright than being outside.)
Dusk is a signal to your body to start relaxing into the evening, and total darkness is the sign for your first sleep.
And slepte hire firste sleepe / and thanne awook/
We’ve learned that sleep based on natural light and dark appears to be broken into two cycles, first sleep from full darkness until a bit after midnight (say 8:30pm to 2am) and second sleep beginning an hour or two later until dawn (e.g. 3am to 6am). The phase between first and second sleep is physiologically different from either full sleep or full wakefulness – it’s a kind of relaxed state, ideal for contemplation. And there is also an alertness cycle during the day, leading to a typical dip after noon, when it would be natural to have a nap (e.g. around 3pm).
You should think of these rhythms as also applying to your mental activity – you’re supposed to rise into alertness, be alert during most of the day, and then wind down in the evening. You shouldn’t either go from rising from bed immediately to a complex mental task, nor do something mentally engaging or stressful and then drop yourself immediately into bed. You’re supposed to ease your way into the day, and then ease yourself back into the night.
So we have these basic rhythms. Our body’s natural clock actually runs longer than 24 hours, but it’s supposed to get reset every day by light. Having predictable wake and sleep times helps too, as does a regular meal schedule (there is a secondary clock in the body, based on when you eat). The body also has a much longer lunar clock, based on the phases of the moon.
Babies and teenagers have different body clock cycles – babies sleep basically all the time, teenagers’ clocks get skewed so that they fall asleep later and wake later.
In modern life we basically ignore all of these natural cycles. This is manageable, as humans are not just biological machines – we have neuroplasticity; we can adapt. But it has significant impacts nonetheless. Our misalignment is on many levels:
- we use artificial light to extend the evening, going to bed much later than we would naturally
- we have overlit cities, overlit buildings and overlit bedrooms, which means the sleep itself is disrupted
- including bringing blue light into our bedrooms, whether a television, a laptop, a smartphone or tablet, or bright blue clock LEDs
- the smartphone in the bedroom is basically an anti-sleep bomb
- pre-computer, and still very much for many people today, the TV is on and being watched intermittently for the entire evening (and all day for some)
- this includes the TV in the bedroom, which is very bad, and watching TV to fall asleep, which is terrible for your brain on multiple levels
- we expect to sleep in a single consolidated 8-hour block
- we often force ourselves away to follow the clock, rather than our bodies – including forcing teenagers to get up much earlier than they would normally, and not resting when we feel tired or sleepy
- we change the clock with Daylight Savings Time
- we have months that don’t align with the lunar cycle
- (we have a year that doesn’t align with the lunar months, but this is because the earth’s orbit around the sun doesn’t exactly divide into the lunar cycle)
Basically anything you do that isn’t aligned with the dawn – bright light of day – dusk – very dark night cycle will screw up your sleep. And screwing up your sleep screws up your wakefulness.
In the Protestant work ethic countries, we got this idea that sleep and relaxation are wasted idle time, that we should maximize our wakeful work and activity time, above all other things. So many of us are in perpetual sleep deficit.
There is good news for many people who sleep poorly, on two fronts:
- one is that you may actually be sleeping normally, if you’re waking up for a few hours in the night
- two is that if you’re sleeping poorly you may be able to improve things by taking some basic steps
Things you can do:
- first and foremost, the bedroom must be dark. like really dark. like at the bottom of a coal mine without any lights dark
- if you have way too much artificial light outside to be able to leave the window uncovered for natural dawn, get an artificial dawn alarm – this should both gently raise the light level in the morning, and optionally have a quiet sound that very gradually increases (could be radio, music, white noise, a tone etc.)
- there is basically no natural sudden waking. if you wake suddenly to an alarm, your body basically thinks you’re being attacked by a tiger or something.
- always get up at the same time every day
- take time during the entire work day to look out a window, to get the full brightness of the sun
- go outside at lunchtime, in the full sun
- dusk is basically a lost cause in the modern world
- in the evening, wind down your screen time at least a couple hours before bed
- yes, this means no computer monitor, no laptop, no smartphone, no tablet and no television for a couple hours before bed
- no glowing screens in the bedroom. An e-ink device is ok (e.g. a Kindle) because it’s reflected light, not direct projected light.
- always go to bed at the same time every day
- in general, no blue light in the bedroom (blue is the strongest trigger for the sensors in your eyes). bright blue LEDs are terrible for your sleep.
- set your devices to total silent mode (or just turn them off) – if your brain knows that the phone might beep or buzz with a text during the night, part of you will always be waiting for that to happen
- plus which, as I mentioned above, there’s no natural sudden waking. your phone buzzing and pulling you out of deep sleep makes your brain think it’s an emergency.
- no bright light during the entire span of the night
- cover your windows well (e.g. blackout curtain) if it’s bright outside
- use dim nightlights (e.g. electroluminescents are good)
- no switching on the lights or any screens – maybe a very dim light to read by between first and second sleep, but no screens (due to the blue light)
- eat at the same times every day
- eat breakfast (part of signalling to your body that it’s time to ease itself into the day)
That’s basically the best that I know that you can do.
I used to read (paper books) to fall asleep, but for me it creates two problems, one is that the book falls down when I fall asleep, which wakes me up (often falling on my face), another is that the light stays on and eventually wakes me up. I found a solution in listening to audiobooks (and to podcasts) at night instead. That way I can set the sleep timer and then I just fall asleep in the dark.
As a society, we could also:
- design our cities to be dark-sky, rather than overlighting them
- design our buildings to use natural light, and to be dark by default
- for example in Europe, hallways usually have windows to the outside, and a minuterie (a light timer) that gives you only a few minutes of artificial light to get around
- start school later for teenagers
- let people nap during the day
- spend more time outside, including walking meetings and outdoor lunch breaks
- spend less time facing the computer screen, and more time working on paper (or e-ink screens) using natural light
- use all our screens less
- get rid of Daylight Savings Time, which is stupid on many levels
- go to bed earlier
The above is pretty much all about light and dark and your body clock. There are of course many other factors, including caffeine, stress, and noise.
If you want more info, you can google first sleep second sleep and a bazillion articles will come up.