Sleep is just one of many types of consciousness we experience, and sleep itself comprises several states of consciousness. Even when we’re sleeping, our brains and bodies continue to work.
Sleep is affected by biological rhythms or periodic physiological changes. Biological rhythms are regular, periodic changes in a body’s functioning. There are three types of biological rhythms:
Biological rhythms usually synchronize with environmental events such as changes in daylight. However, experiments have shown that many biological rhythms continue to have the same cycle even without cues from the environment. Such biological rhythms are endogenous, which means that they originate from inside the body rather than depend on outside cues.
Endogenous rhythms exist because the body has biological clocks that keep time. Biological clocks can be adjusted by environmental cues, such as changes in temperature.
In humans, the suprachiasmatic nucleus (SCN) is the main biological clock that regulates circadian rhythms of sleep. The SCN lies in the brain’s hypothalamus. When light stimulates receptors in the retina of the eye, the receptors send signals to the SCN. The SCN then sends signals to the nearby pineal gland, which secretes melatonin, a hormone that regulates the sleep cycle.
Jet lag is the fatigue and disorientation air travelers feel after a long flight. Although traveling itself drains energy, the time change also contributes to fatigue. People experience jet lag when the events in their environment are out of sync with their biological clocks.
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.
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:
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:
Other instruments monitor breathing, body temperature, and pulse.
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.
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.
After a few minutes of stage 1 sleep, people move into stage 2 sleep. Stage 2 lasts about twenty minutes and is characterized by short bursts of brain waves called sleep spindles. People then pass into slow-wave sleep, which occurs during stages 3 and 4. In stages 3 and 4, which together last about thirty minutes, the EEG displays mostly delta waves. People in stage 3 and 4 sleep show slow breathing and pulse rates, have limp muscles, and are difficult to rouse.
At the end of stage 4, people go back through the stages in reverse, from stage 4 to 3 to 2 to 1. When they reach stage 1, instead of waking up, people go into REM, or rapid eye movement, sleep. A single cycle might look like this:
REM sleep is a stage of deep sleep in which, paradoxically, brain wave activity resembles that of an alert person. REM sleep is also called paradoxical sleep.
During REM sleep, pulse rate and breathing become irregular, eyes move rapidly under closed lids, and muscles remain very relaxed. Genital arousal also happens during REM. In women, the clitoris becomes swollen with blood, and vaginal lubrication increases. In men, the penis becomes erect. EEGs show mostly beta waves during REM sleep. Although dreaming happens in other sleep stages as well, dreams are most vivid and frequent during REM sleep.
People typically go through about four sleep cycles during one night of sleep. The REM stage of sleep gets longer and longer as the night passes, while stage 3 and 4 sleep gets shorter and shorter. During the night’s first sleep cycle, the REM stage lasts about ten minutes. During the night’s last sleep cycle, people may spend about forty to sixty minutes in REM sleep. Non-REM sleep becomes more shallow as the night goes on, and eventually the sleeper awakens.
Different people need different amounts of sleep. Some people can function with fewer than six hours of sleep a night, while others can’t manage without at least nine hours. Research shows that getting insufficient sleep can have negative effects on health, productivity, and performance.
Researchers have also studied the effects of insufficient REM sleep. Experiment subjects who are intentionally deprived of REM sleep tend to enter the REM stage of sleep more and more frequently during the night. After an REM-deprivation experiment has ended, subjects usually experience a REM rebound effect, spending more time in the REM stage on subsequent nights to make up for lost REM time.
Sleep patterns change as people get older. Newborn babies spend about two-thirds of their time in sleep. As people age, they tend to sleep less. The amount of time spent in REM sleep also changes over time. In very young babies, about half of all sleep is REM sleep. As babies get older, the proportion of REM sleep decreases.
Everyone has occasional difficulty sleeping, but some people have insomnia, a chronic problem with falling or staying asleep. Another kind of sleep disorder is narcolepsy, which is a tendency to fall asleep periodically during the day. Narcolepsy can be dangerous, as people who experience it may fall asleep while driving or operating machinery.
Sleep apnea is another condition that can have negative effects on health and safety. People who have sleep apnea stop breathing many times during a night’s sleep, and each time they stop breathing, they wake up briefly and gasp for air. This prevents them from getting enough deep sleep, which leads to irritability and sleepiness during the day. Chronic sleep apnea can also result in high blood pressure.