When it comes to sleep, it’s your brain, and the chemicals that affect it, that plays the biggest role in when you feel awake and when you feel sleepy.
Deep inside your brain, there is an “internal clock”, a part of the system that also controls your heart rate, body temperature, and appetite, and other bodily functions.
Our natural circadian rhythm, or a behavioral change that follows a 24-hour cycle, tends to make the desire for sleep strongest between midnight and dawn, and to a lesser extent in midafternoon. In one study, researchers instructed a group of people to try to stay awake for 24 hours. Not surprisingly, many slipped into naps despite their best efforts not to. When the investigators plotted the times when these unplanned naps occurred, they found peaks between 2 a.m. and 4 a.m. and between 2 p.m. and 3 p.m.
Cues that keep your circadian rhythm on track
Light
Light striking your eyes is most influential on your circadian rhythm. When researchers invited volunteers into the laboratory and exposed them to light at intervals that were at odds with the outside world, the participants unconsciously reset their biological clocks to match the new light input. Exposure to light at the right times helps keep the circadian clock on the correct time schedule. However, exposure at the wrong time, or lack of exposure to natural light, can shift sleep and wakefulness to undesired times. For example, 90% of blind people suffer from circadian rhythm disturbances and sleep problems due to altered light exposure to the eye.
Time
Reading clocks, following work and train schedules, and demanding that the body remain alert for certain tasks and social events, results in a constant knowledge of what time it is. This knowledge has an effect on your circadian rhythm, and you’ll naturally tend to get sleepy at certain times.
Melatonin
Melatonin is a hormone that is predictably produced at certain times of day by a small gland in the brain. Receptors for melatonin reside in the part of your brain that controls how sleepy or awake you feel.
Levels of melatonin typically begin climbing after dark and ebb after dawn. The hormone induces drowsiness, and scientists believe this cycle of hormone production helps keep the sleep/wake cycle on track.
What is biphasic sleep?
Most Americans sleep in a single block of time at night, but this is not a universal practice. In societies where taking a siesta is the norm, people can succumb to a drop in alertness with a 1-2-hour afternoon nap and a correspondingly shorter sleep at night. The siesta sleep schedule is an example of biphasic sleep, meaning sleep that occurs in two distinct phases.
Another pattern of biphasic sleep involves two nighttime sleeps, with a period of wakefulness in between, which may be part of the reason that many people experience an often-unwelcome period of wakefulness in the middle of the night. Today, the practice of “two sleeps” is practically unheard of, but there is evidence that prior the Industrial Revolution, it was quite common.
According to many historical documents, letters, diaries, newspaper articles, and medical textbooks, England and many European countries participated in a practice of “two sleeps”. A typical pattern was to sleep from 9 p.m. to 11 p.m., then wake for two hours, then go back to sleep. The “awake hours” were used for sex, conversation, and other purposes. Various religious orders even required (and still require) rising at midnight for prayer. This practice appears to have ceased with the Industrial Revolution, when artificial light became more accessible, and people started staying up later. As a result, sleep became compressed into a single block of time.
Although biphasic sleep doesn’t appear to have practical applications today, many sleep doctors commonly advise that if you don’t fall asleep again within 20 minutes, you should get up and read or do some other quiet activity until you grow sleepy. When you think of this as biphasic sleep, you realize it’s not necessarily a cause for alarm, and you may relax and find it easier to let yourself naturally get sleepy.
What makes your internal clock “tick”?
All living things follow circadian rhythms, including animals, insects, and even plants. A study of mimosa plants found that the leaves open in the morning and close in the evening. The scientists assumed the plants were responding to the sunlight, but when the plants were placed in darkness around the clock, they continued to open and close, following their circadian rhythms rather than the presence or absence of light.
Since then, scientists have learned that circadian rhythms in plants control multiple functions, such as what time of day or night flowers produce scents to attract specific pollinators. If they are pollinated by moths, for example, they’ll put out their fragrance at night or in the evening, when moths are most active. Similarly, they’ll time their production of unpleasant-tasting chemicals to deter their most common predators from munching on them.
Ongoing research has revealed three genes that are largely responsible for “winding and setting” of our internal clocks.
- The “timer” gene causes a buildup of a protein that stimulates the brain to produce melatonin.
- The “timeless” gene controls a protein that binds with the protein that stimulates melatonin, halting production of the sleep-inducing hormone.
- The “double-time” gene produces a protein that delays the accumulation of the protein that stimulates melatonin, promoting wakefulness during the day.
These discoveries have helped pave the way for the new and growing field of circadian biology, which has helped us understand that our internal clocks regulate not just sleep patterns, but also hunger, hormone release, blood pressure, body temperature, and other functions.
Scientists are still exploring the implications for human health. But it’s clear that ongoing misalignment between your circadian rhythm and your schedule (for example, if you do shift work) can increase your risk for various chronic diseases. And research is showing that the time of day you take certain medications—such as blood pressure drugs or cancer chemotherapy—can also make a difference.