Although you may put off going to sleep in order to squeeze more activities into your day, eventually your need for sleep becomes overwhelming and you are forced to get some sleep. This daily drive for sleep appears to be due, in part, to a compound known as adenosine. This natural chemical builds up in your blood as time awake increases. While you sleep, your body breaks down the adenosine. Thus, this molecule may be what your body uses to keep track of lost sleep and to trigger sleep when needed. An accumulation of adenosine and other factors might explain why, after several nights of less than optimal amounts of sleep, you build up a sleep debt that you must make up by sleeping longer than normal.
Because of such built-in molecular feedback, you can’t adapt to getting less sleep than your body needs. Eventually, a lack of sleep catches up with you. The time of day when you feel sleepy and go to sleep is also governed by your internal “biological clock” and environmental cues—the most important being light and darkness. Your biological clock is actually a tiny bundle of cells in your brain that responds to light signals received through your eyes. When darkness falls, the biological clock triggers the production of the hormone melatonin. This hormone makes you feel drowsy as it continues to increase during the night. Because of your biological clock, you naturally feel the most sleepy between midnight and 7 a.m.
You may also feel a second and milder daily “low” in the midafternoon between 1 p.m. and 4 p.m. At that time, another rise occurs in melatonin production and might make you feel sleepy. Your biological clock makes you the most alert during daylight hours and the most drowsy in the early morning hours. Consequently, most people do their best work during the day. Our 24/7 society, however, demands that some people work at night. Nearly onequarter of all workers work shifts that are not during the daytime, and more than two-thirds of these workers have problem sleepiness and/or difficulty sleeping. Because their work schedules are at odds with powerful sleep-regulating cues like sunlight, night shift workers often find themselves drowsy at work, and they have difficulty falling or staying asleep during the daylight hours when their work schedules require them to sleep.
The fatigue experienced by night shift workers can be dangerous. Major industrial accidents—such as the Three Mile Island and Chernobyl nuclear power plant accidents and the Exxon Valdez oil spill—have been caused, in part, by mistakes made by overly tired workers on the night shift or an extended shift. Night shift workers also are at greater risk of being in car crashes when they drive home from work. One study found that one-fifth of night shift workers had a car crash or a near miss in the preceding year because of sleepiness on the drive home from work. Night shift workers are also more likely to have physical problems, such as heart disease, digestive disturbances, and infertility, as well as emotional problems. All of these problems may be related, at least in part, to the workers’ chronic sleepiness.
Other factors can also influence your need for sleep, including your immune system’s production of cellular hormones called cytokines. These compounds are made in large quantities in response to certain infectious diseases or chronic inflammation and may prompt you to sleep more than usual. The extra sleep may help you conserve the resources needed to fight the infection. Recent studies confirm that being well rested improves the body’s responses to infection. People are creatures of habit, and one of the hardest habits to break is the natural wake and sleep cycle. A number of physiological factors conspire to help you sleep and wake up at the same times each day. Consequently, you may have a hard time adjusting when you travel across time zones. The light cues outside and the clocks in your new location may tell you it is 8 a.m. and you should be active, but your body is telling you it is more like 4 a.m. and you should sleep. The end result is jet lag—sleepiness during the day, difficulty falling or staying asleep at night, poor concentration, confusion, nausea, and general malaise and irritability.