How much sleep a person needs depends in large part on age. Babies who are 4 to 12 months old need about 12 to 16 hours. School-aged children need 10 to 11 hours. Teens need 8 to 10 hours. Most adults seem to need 8 to 9 hours of sleep a night. People 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 90 minutes after you fall asleep. 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. Sleep inertia is a short-term (temporary) disorientation after suddenly waking up. It can cause poorer short-term memory and slower speed of thinking. 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. Untreated sleep apnea also raises the risk for accidents, including car accidents, because of sleepiness. It can also lead to headaches in the morning.

Many snorers are overweight. In some people, losing weight can reduce or eliminate snoring. Snoring can be caused by a variety of illnesses. These range from the common cold to sinus problems to overgrowth of the tonsils. 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.

Exposure to smoke has been linked to a variety of sleeping problems, such as difficulty falling asleep and fragmented sleep. 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 within a few hours of 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.

Other tips 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.