Why can't you get a good night's sleep? The problem is that you probably don't realize what makes you fall asleep in the first place.

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Despite what it might feel like, sleep isn't something that you can "turn on" using the power of your own will. It's actually something that the environment triggers in your body, at the cellular level.


The amazing thing about sleep is that most of us feel the urge to do it at roughly the same time every night. That's because sleep is triggered partly by a clock in your brain that is tracking night and day cycles on Earth — just the way almost every other life form on the planet does, from bacteria and plants, to fish, insects and mammals.

Circadian Rhythms: How the Earth Rules Your Body

Your brain keeps time in several ways, but perhaps the most basic involves circadian rhythm, which tracks a standard Earth day's 24-hour cycle. That's why most of us fall asleep when it's dark, and wake up when it's light.


What's incredible about this biological clock is that it can adjust to new cycles of light and dark. That's why you eventually recover from jet lag when, for example, you fly from the United States to Europe. The light/dark cycle on the other side of the globe slowly alters your internal clock, and sooner or later you'll find yourself falling asleep during nighttime hours that were once your daytime hours. But in the absence of a light/dark cycle, it will continue to run, ticking off the hours and getting you to sleep roughly on time.

Life forms with circadian clocks have been around on Earth for a very long time. It's likely that circadian rhythms evolved in cyanobacteria — bluegreen algae — over 3 billion years ago. Evolutionary biologists aren't certain why these clocks evolved, but there are a few theories.


One is that these single-celled bacteria needed energy, but to get it, their bodies had to carry out two different chemical processes that interfered with each other. So the bacteria began keeping time by tracking the sun. When it was light outside, the cyano would get energy from photosynthesis. When it was dark, they would get energy by sequestering nitrogen. These chemical processes would cancel each other out if done simultaneously. But done sequentially? A perfect way to gather energy during the day, and during the night.

Another theory is that life forms who shared the same environments needed a way to compete with each other for food, so some evolved to feed during the day and others to feed at night. Both groups would need an internal clock tracking daylight hours to arrive promptly at their mealtimes.

Whatever the evolutionary reason for the first circadian clocks, they are found everywhere in nature. They are perhaps the most profound way that the motion of our planet affects our bodies on a fundamental level. Plants use them to determine when to bloom, and animals use them to determine when to sleep. There are also seasonal circadian rhythms, which send bears into hibernation and trigger the growth of apples on trees. In humans, the circadian clock is part of a brain region called the suprachiasmatic nucleus (SCN). When that region is damaged, all kinds of things can go wrong with our sleep cycles.


But the main reason why human sleep is disturbed has nothing to do with SCN damage. It has to do with the way our circadian clocks are wound, as it were.

Sleep Disorders Are Clock Disorders

When you get a good night's sleep, you wake up feeling refreshed — physically and mentally. It's not unusual for aches and pains of the night before to have faded. The same can be said for psychological aches. The things that annoy you at 11 PM are often easy to dismiss as no big deal when you get out of bed the next day. And there's a good reason.


Earlier I talked about how cyanobacteria could perform different chemical processes at night to gain energy. Humans do a similar kind of thing, though not to gain energy. At night, our bodies are busy cleansing our tissues of toxins, solidifying memories in our brains, and even repairing damage to our DNA. Sleeping isn't about resting, in other words — it's about being active in ways that are very different from when we are awake.

Recent studies have shown that people whose sleep is disrupted, or who get very little sleep, are more likely to develop obesity, depression, inflammatory diseases, and even cancer. This doesn't mean that if you get a bad night's sleep you're going to get sick — or even that insomnia leads to disease. It just means that there is a strong correlation between troubled sleep and health problems. More specifically, it means that many aspects of human health are affected by the proteins and genes involved in keeping that circadian clock in your brain running smoothly.

You may have noticed that what I'm calling "sleep disorders" here aren't the typical ones you might expect, such as sleep walking, nightmares, restless leg syndrome, and many others. Those disorders are certainly related to sleep, but they're not directly governed by how the Earth's rotation tunes your circadian clock. And what we're focusing on here are the most typical sleep disorders, which are caused by changes to your night/day cues.


There are still a lot of questions about how circadian clocks govern our well being, but anybody who has ever worked overnight shifts, or has to deal with lots of jet lag, has probably noticed some of these health problems. When your body isn't getting regular day/night cues from the environment, it affects the functioning of your circadian clock. You sleep badly, or not at all. Your dream cycle may be interrupted, too, which can wreck havoc with mood and memory.

Given that circadian clocks are a very ancient evolutionary development, it's likely that many of our bodily processes rely on them for "stop" and "start" signals. If your clock is off, it may not be giving the right "start" signal to the molecules that would normally clear toxins out of swollen tissues — or repair damaged DNA. And over time, this situation can lead to chronic problems.


Entrainment: Getting the Right Kind of Sleep

If you are having a hard time sleeping, it's likely that the problem isn't genetic or neurological. It's probably your environment. More precisely, it's your relationship to the daily rotation of the Earth, which your body evolved to use as its most fundamental measure of time.


You may not be sleeping right because you're not allowing your circadian clock to be properly entrained, or set, by day and night cycles. This is a particularly acute problem for people who work night shifts — they have to be awake when the body is getting the "go to sleep" signal from their environment. But it can be a problem for anybody who doesn't get out into the sunlight during the day. Without that sunlight cue, your body may not be getting entrainment from the daylight.

Of course, not all of us want to fall asleep the moment it gets dark, or wake up at dawn. Everybody has a slightly different sleep schedule that is right for them. As sleep researcher Till Roenneberg puts it in his book Internal Time, every human has a "chronotype," or a typical time period when they fall asleep and wake up. On average, people prefer 8-hour sleep periods; most of us have our "peak" sleep period, halfway through our sleep cycle, around 3 AM. But these are only averages. In reality, there are early birds who prefer to fall asleep at 8 PM, and there are night owls who don't get to bed until 4 AM. Everybody's chronotype is slightly different.


Forcing yourself to sleep against chronotype is going to make you tired, and possibly even sick in the long term. Ideally, to sleep well, you should be falling asleep when you're sleepy — even if it's not at a "typical" bedtime. The problem is that many people's work schedules don't fit their chronotypes. Still, there are workarounds.

One system that works for many people is to make sure your body is getting enough signals from the environment. Basically, make sure you're entraining your body with exposure to sunlight during the day and darkness at night. If you work a night shift, you might try taking a walk right before sunset, to trigger wakefulness — and be sure to avoid sunlight or even bright lights right before bed if you can. If you work inside all day without any sunlight, try to expose yourself to sunlight (even if it's cloudy) at least once during morning or afternoon.

Obviously this simple solution isn't going to work for everybody — especially if you suffer from a sleep disorder that's unrelated to circadian clocks. But many people have found that proper entrainment into light/dark cycles helps them fall asleep during the night. This, in turn, can lighten your mood and enhance your health.


Annalee Newitz is the author of the book, Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction. Follow her on Twitter.