What Do You Know About Sleep?

How much sleep a person needs depends in large part on age. Infants need about 16 hours; school-aged children, 10 to 11 hours. Teens need about 9 hours. Most adults seem to need 7 to 8 hours of sleep a night. That amount can vary to as few as 5 hours to as many as 10, according to the National Institute of Neurological Disorders and Stroke (NINDS). Women in their first trimester of pregnancy often need extra sleep.

You pass through five stages of sleep during one sleep cycle: 1, 2, 3, 4, and REM, which stands for rapid eye movement. One complete cycle takes 90 to 110 minutes. Stage 1 is the transition from wakefulness to deeper sleep. Stage 2 is intermediate sleep and accounts for 40 to 50 percent of adult sleep time. Stage 3 and 4 are deep sleep and account for approximately 20 percent of sleep time. Adults spend about 20 percent of their time in REM sleep, when dreams occur. (Infants spend about half their sleeping hours in REM sleep.) During REM cycles, the brain is as active as when you are awake. The length of REM periods varies with the time of night. The first REM sleep period usually occurs about 70 to 90 minutes after you fall asleep, according to the NINDS. The first sleep cycles of the night contain relatively short REM periods and long periods of deep sleep. As the night progresses, REM sleep periods become longer and the deep sleep periods decrease in length.

This is a normal occurrence. According to the NINDS, someone awakened after sleeping for more than a few minutes is usually unable to recall the last few minutes before falling asleep. This phenomenon explains not only why middle-of-the-night phone calls are forgotten, but also why people don't remember the alarm ringing after they turn it off, roll over, and go back to sleep.

Sleep apnea also is linked to heart failure, irregular heartbeat, stroke, and mood disorders such as irritability and depression. Sleep apnea, because it leaves the person without oxygen for up to a minute many times a night, can lead to headaches in the morning, according to the NINDS. Other effects include a loss of interest in sex and a decline in mental functioning.

A person with sleep apnea has many periods of interrupted breathing while asleep. When a person with sleep apnea attempts to inhale, the windpipe collapses, blocking the flow of air. After 10 seconds to a minute, the brain senses that the supply of oxygen has been cut off, and wakes the person enough so that airway muscles contract, opening the windpipe. The person then snorts or gasps to breathe and resumes sleeping, until the cycle is repeated. A person who is overweight or obese or who has experienced a loss of muscle tone because of aging is more at risk of developing sleep apnea.

Many snorers are overweight, and losing weight can reduce or eliminate snoring. Snoring can be caused by a variety of illnesses, ranging from the common cold to sleep apnea, according to the National Sleep Foundation (NSF). Although snoring is a sign of sleep apnea, most people who snore do not have it. People who drink alcohol in the evening also may snore.

Heavy smokers often sleep very lightly and have reduced amounts of REM sleep, according to the NINDS. They may also often wake up after 3 or 4 hours of sleep because of nicotine withdrawal. Although many people think that alcohol can help them get to sleep, it keeps them in a light sleep, instead of allowing them to move into REM and deeper sleep. Exposure to bright light, such as a computer monitor, late at night also can affect the quality and quantity of sleep.

Teens' internal body clock is set later than that for children and adults, according to the NSF. They usually aren't able to fall asleep until 11 p.m. and wake up later in the morning. This pattern of sleep usually runs counter to most school schedules, resulting in a sleep deficit for teens.

Other tips from the NSF for getting a good night's sleep include: Go to bed and get up at the same time every day. Make your bedroom sleep friendly by keeping it cool, quiet, and dark. Limit what you eat or drink before bedtime. Exercise regularly, but not too near bedtime. If you want to take a nap during the day, limit it to no more than 20 to 30 minutes.