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Example: A traveler leaves New York City at eight in the morning and arrives in London about seven hours later. For her, it’s three in the afternoon, but because of the time change, in London it’s eight in the evening. Her body, thinking it’s mid-afternoon, will be confused by the lack of sunlight, and she’ll experience jet lag.

The Function of Sleep

Although everyone sleeps, no one really knows why people sleep. Researchers have proposed several theories to explain how sleep evolved to be a necessary behavior:

  • People conserve energy by sleeping periodically.
  • Sleep has a protective function, as it keeps people tucked away at night, safe from predators.
  • Sleep restores body tissues that are depleted during daily activities.

Sleep Research

Sleep research has provided a lot of information about what happens to the brain and body during sleep. Researchers study sleep by monitoring subjects who spend the night in labs, and they use various instruments for different purposes:

  • Electroencephalographs (EEGs): record brain waves
  • Electromyographs (EMGs): record muscle activity
  • Electrooculographs (EOGs): record eye movements
  • Electrocardiographs (EKGs): record the activity of the heart

Other instruments monitor breathing, body temperature, and pulse.

Sleep Stages

During one night’s sleep, people pass through several cycles of sleep, each lasting about ninety to one hundred minutes. There are five distinct stages of sleep in each cycle: 1, 2, 3, 4, and REM.

Stages 1–4

When people are relaxed and ready to fall asleep, their EEG will show mostly alpha waves. When people fall asleep, they enter into stage 1 sleep, which lasts just a few minutes. In stage 1, the EEG shows mostly theta waves. Heart rate, breathing rate, and body temperature drop, and muscles relax. Fantasies or bizarre images may float around in the mind.

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