What Makes You Sleep?

     Although you may put off going to sleep in order to squeeze more activities into your day, eventually your need for sleep becomes overwhelming. This need appears to be due, in part, to two sub­ stances your body produces. One substance, called adenosine, builds up in your blood while you’re awake. Then, while you sleep, your body breaks down the adenosine. Levels of this substance in your body may help trigger sleep when needed.
     A buildup of adenosine and many other complex factors might explain why, after several nights of less than optimal amounts of sleep, you build up a sleep debt. This may cause you to sleep longer than normal or at unplanned times during the day. Because of your body’s internal processes, you can’t adapt to getting less sleep than your body needs. Eventually, a lack of sleep catches up with you.
     The other substance that helps make you sleep is a hormone called melatonin. This hormone makes you naturally feel sleepy at night. It is part of your internal “biological clock,” which controls when you feel sleepy and your sleep patterns. Your biological clock is a small bundle of cells in your brain that works throughout the day and night. Internal and external environmental cues, such as light signals received through your eyes, control these cells. Your biologi­cal clock triggers your body to produce melatonin, which helps prepare your brain and body for sleep. As melatonin is released, you’ll feel increasingly drowsy. Because of your biological clock, you naturally feel the most tired between midnight and 7 a.m. You also may feel mildly sleepy in the afternoon between 1 p.m. and 4 p.m. when another increase in melatonin occurs in your body.
     Your biological clock makes you the most alert during daylight hours and the least alert during 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 one-quarter 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 during the early morning hours, because the biological clock is not sending out an alerting signal. 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 troubles, and infertility, as well as emotional problems. All of these problems may be related, at least in part, to the workers’ chronic sleepiness, possi­bly because their biological clocks are not in tune with their work schedules. See “Working the Night Shift”  for some helpful tips if you work a night shift.
     Other factors also can influence your need for sleep, including your immune system’s production of hormones called cytokines. Cyto­kines are made to help the immune system fight certain infections 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. Together, a number of physiological factors 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 generally feeling unwell and irritable. See “Dealing With Jet Lag”