When you're in a rush to meet work, school, family or household responsibilities, do you cut back on your sleep? Like many people, you might think that sleep is merely a "down time" when the brain shuts off and the body rests. Think again.

What Is Sleep?

Sleep was long considered just a uniform block of time when you are not awake. Thanks to sleep studies done over the past several decades, it is now known that sleep has distinctive stages that cycle throughout the night. Your brain stays active throughout sleep, but different things happen during each stage. For instance, certain stages of sleep are needed for us to feel well rested and energetic the next day, and other stages help us learn or make memories.

In brief, a number of vital tasks carried out during sleep help maintain good health and enable people to function at their best. On the other hand, not getting enough sleep can be dangerous — for example, you are more likely to be in a car crash if you drive when you are drowsy.

How Much Sleep Is Enough?

Sleep needs vary from person to person, and they change throughout the lifecycle. Most adults need seven to eight hours of sleep each night. Newborns, on the other hand, sleep between 16 and 18 hours a day, and children in preschool sleep between 10 and 12 hours a day. School-aged children and teens need at least nine hours of sleep a night.

Some people believe that adults need less sleep as they get older. But there is no evidence to show that older people can get by with less sleep than younger people. As people age, however, they often get less sleep or they tend to spend less time in the deep, restful stages of sleep. Older people also are more easily awakened.

Why Sleep Is Good for You — and Skimping on It Isn't

Does it really matter if you get enough sleep? Absolutely! Not only does the quantity of your sleep matter, but the quality of your sleep is important as well. People whose sleep is interrupted a lot or is cut short might not get enough of certain stages of sleep. In other words, how well rested you are and how well you function the next day depend on your total sleep time and how much of the various stages of sleep you get each night.

Performance: We need sleep to think clearly, react quickly, and create memories. In fact, the pathways in the brain that help us learn and remember are very active when we sleep. Studies show that people who are taught mentally challenging tasks do better after a good night's sleep. Other research suggests that sleep is needed for creative problem solving.

Crash in Bed ... Not on the Road Most people are aware of the hazards of drunk driving. But did you know that driving while drowsy can be just as deadly? Like alcohol, a lack of sleep makes it harder to react quickly enough to a car that stops suddenly, a sharp curve in the road, and other potentially dangerous situations. Watch for these warning signs that you might be too sleepy to drive safely: Trouble keeping your eyes focused Continual yawning Inability to recall driving the last few miles; if you feel sleepy while driving, pull off the road to a safe place and take a nap for 15 to 20 minutes. Tips to Avoid Drowsy Driving Be well rested before hitting the road. Keep in mind that if you skimp on sleep for several nights in a row, it might take more than one night of good sleep to be well rested and alert. Avoid driving between midnight and 7 a.m. This period of time is when we are naturally the most sleepy. Don't drive alone. A companion who's awake and can keep you engaged in conversation may help you stay awake. Schedule frequent breaks on long road trips. Don't drink alcohol! Don't count on caffeine. Although drinking a cola or coffee might help keep you awake for a short time, it won't overcome excessive sleepiness. Remember, if you are short on sleep, stay out of the driver's seat!

Skimping on sleep has a price. Cutting back by even one hour can make it tough to focus the next day and can slow your response time. Studies also find that when you lack sleep, you are more likely to make bad decisions and take more risks. This can result in lower performance on the job or in school and a greater risk for a car crash.

Mood: Sleep also affects mood. Insufficient sleep can make you irritable and is linked to poor behavior and trouble with relationships, especially among children and teens. People who chronically lack sleep also are more likely to become depressed.

Health: Sleep also is important for good health. Studies show that not getting enough sleep or getting poor quality sleep on a regular basis increases the risk of having high blood pressure, heart disease, and other medical conditions.

In addition, during sleep, your body produces valuable hormones. Deep sleep triggers more release of growth hormone, which fuels growth in children, and helps build muscle mass and repair cells and tissues in children and adults. Another type of hormone that increases during sleep works to fight various infections. This might explain why a good night's sleep helps keep you from getting sick — helps you recover when you do get sick.

Hormones released during sleep also affect how the body uses energy. Studies find that the less people sleep, the more likely they are to be overweight or obese, to develop diabetes, and to prefer eating foods that are high in calories and carbohydrates.

Could You Have a Sleep Disorder? If you are spending enough time in bed and still wake up tired or feel very sleepy during the day, you may be one of the estimated 40 million Americans with a sleep disorder. The most common sleep disorders are insomnia, sleep apnea (sleep-disordered breathing), restless legs syndrome and narcolepsy. Although sleep disorders can significantly affect your health, safety and well-being, they can be treated. Talk to your doctor if you have any of these signs of a sleep disorder: You consistently take more than 30 minutes each night to fall asleep. You consistently awaken several times each night and then have trouble falling back to sleep, or you awaken too early in the morning. You often feel sleepy during the day, you take frequent naps or you fall asleep at inappropriate times during the day. Your bed partner says that when you sleep, you snore loudly, snort, gasp, make choking sounds, or stop breathing for short periods. You have creeping, tingling, or crawling feelings in your legs or arms that are relieved by moving or massaging them, especially in the evening and when trying to fall asleep. Your bed partner notices that your legs or arms jerk often during sleep. You have vivid, dreamlike experiences while falling asleep or dozing. You have episodes of sudden muscle weakness when you are angry or fearful, or when you laugh. You feel as though you cannot move when you first wake up. Keep in mind that children can have some of these same signs when they have a sleep disorder, but they often do not show signs of excessive daytime sleepiness. Instead, they may seem overactive and have difficulty focusing or doing their best in school.

It's About Time

How sleepy you are depends largely on how well you've been sleeping and how much sleep you've been getting. Another key factor is your internal "biological clock" — a tiny bundle of cells in your brain that responds to light signals through your eyes and promotes wakefulness. Because of the timing of the biological clock and other bodily processes, you naturally feel drowsy between midnight and 7 a.m. and again in the mid-afternoon between 1 p.m. and 4 p.m.

Night shift workers often find themselves drowsy at work. They also have trouble falling asleep or staying asleep during the day, when their schedules require them to sleep. Being sleepy puts them at risk for injuries on the road and at work. Night shift workers also are more likely to have conditions such as heart disease, digestive disorders and infertility, as well as emotional problems. All of these problems may be related, at least in part, to their chronic lack of sleep.

Adapting to new sleep and wake times also can be hard for travelers crossing time zones, resulting in what's known as jet lag. Jet lag can lead to daytime sleepiness, trouble falling asleep or staying asleep at night, poor concentration and irritability.

The good news is that by using appropriately timed cues, most people can change their biological clock, but only by one to two hours per day at best. Therefore, it can take several days to adjust to a new time zone (or different work schedule). If you'll be moving across time zones, you might want to begin adapting to the new time zone a few days before leaving. Or, if you are traveling for just a few days, you might want to stick with your original sleep schedule and not try to adjust to the new time zone.

Source: National Heart, Lung, and Blood Institute, National Institutes of Health

Updated: April 2006

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