What Do You Know About Sleep?

How much sleep a person needs depends in large part on age. Babies need about 16 hours. School-aged children need 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 4 stages of sleep during one sleep cycle: N1, N2, N3, and REM. REM stands for rapid eye movement. One complete cycle usually takes 90 to 110 minutes. N1 is the transition from wakefulness to deeper sleep. N2 is intermediate sleep and makes up 40% to 50% of adult sleep time. N3 is deep sleep and makes up about 20% of sleep time. The amount of N3 decreases as you age. Adults spend about 20% of their time in REM sleep, when dreams occur. (Babies 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 have shorter REM periods and longer periods of deep sleep. As the night progresses, REM sleep periods become longer and the deep sleep periods get shorter. Your sleep doesn't have to follow the stages in order. For instance, you can fall asleep right in N2 and progress from N2 to REM.

This is normal. It's sometimes known as sleep inertia. 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 leaves the person without oxygen for up to a minute many times a night. It can also 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.

If you have sleep apnea, you have many periods of interrupted breathing while asleep. When you try to take a breath (inhale), your windpipe collapses, blocking the flow of air. After 10 seconds to a minute, your brain senses that the supply of oxygen has been cut off, and wakes you up enough so that airway muscles contract, opening the windpipe. You then snort or gasp to breathe and go back to sleep until the cycle is repeated. You may have 5 or more of these no-breathing periods every hour. You may not be aware that you are having them, but you don't get enough sleep to feel rested. You are at higher risk of having sleep apnea if you are overweight or obese. You are also at higher risk if you have lost muscle tone because of aging.

Many snorers are overweight. Losing weight can reduce or eliminate snoring. Snoring can be caused by a variety of illnesses. These range 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 less 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. Exercising before bedtime can be stimulating and interfere with the body's ability to fall asleep. Being around 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 against most school schedules. This results in a sleep deficit for teens.