All animals, including humans, sleep. When animals are prevented from sleeping they eventually die. Despite the thousands of experiments that have been done to study sleep, none has been able to declare with assurance the reason why all animals sleep. It is possible that in different species sleep serves a different function. For instance, most animals spend their time looking for food when they are awake and then find a safe place to sleep and hide from predators. Animals near the top of their food chain, such as lions, seem to sleep when and where they want.

We do not know why we sleep, just as we don’t know why we are awake.
Reasons typically given for why we sleep include conservation of energy, the restoration of important bodily functions, and the repair of damaged tissues.

For example, some hormones are secreted mainly during sleep. Such theories are attractive, but all of these functions could be met by simply resting and having whatever hormones are secreted during sleep secreted during rest, not necessarily having the brain go into an unconscious state. We do know this for sure: if people do not sleep the right amount, they feel rotten the next day, their brains do not work properly, and they might be quite mentally impaired and unable to perform complex tasks. You certainly do not want a sleepdeprived pilot to be flying you across the Pacific. Even the medical profession now admits that sleep-deprived doctors might be a hazard to their patients.

We can conclude that sleep serves many important functions, and that the different types of sleep fulfill different needs. For instance, stages three and four, “slow-wave sleep,” are believed to be the type required to feel refreshed upon awakening. Many scientists believe that REM sleep is the time when memories are saved in our nervous system. The right amount of sleep really does contribute to making you feel vigorous and in a great mood. Shakespeare was probably correct when he wrote in Macbeth that sleep was “Chief nourisher in life’s feast.”

How Much Sleep Do We Need?
How much we sleep, when we sleep, and how much deep sleep and dreaming sleep we need varies with age. Thus, seven to nine hours of sleep, which is adequate for most adults, would leave the average nine-year-old extremely sleepy during the daytime. Figure 1.3 shows sleep requirements for different ages. As you can see, there are ranges for each age group. The amount of sleep a woman needs is an individual characteristic just like her height. Each person needs his or her own unique amount of sleep. The amount of sleep needed is that which makes you wide awake and alert. Research has also shown a link between sleep time and health: women who sleep much more or much less

 

than the average are at increased risk of disease. For example, a study of 71,000 nurses published in 2003 showed that those sleeping five hours or less had a 45 percent greater risk of developing heart disease than those sleeping eight hours. Those sleeping nine to eleven hours increased their risk by 38 percent.

In general, the amount of sleep decreases with age. Babies spend an enormous amount of time sleeping (although it may not feel that way to their sleep-deprived parents); for the first few months of life, most infants sleep at any time during the twenty-four-hour day. Finally and mercifully for the parents, they start to have long periods of sleep mostly at night. Infants and toddlers nap. By the time children go to school, most will no longer nap.

The amount of REM sleep also decreases with age.  Newborns spend roughly half their sleep time in REM sleep. In adults the amount of REM experienced during sleep goes down to between 20 to 25 percent The amount of slow-wave sleep is also much higher in children; the time most people spend in slow-wave sleep decreases with aging. Some elderly people may have no slow-wave sleep. This is the sleep state during which most of the human growth hormone is secreted.

Exasperatingly for their parents, some children start to have a characteristic sleep pattern in their teenage years. Rather than go to bed at the most common times, between 10:30 and midnight in North America, teenagers do not feel sleepy at bedtime, but instead begin to sleep later and awaken later than they did during childhood years. On school nights, it may take them several hours to fall asleep, leaving a teen unable to get the approximate eight to ten hours of sleep he or she needs to wake on time and be alert throughout the day. A mother may find herself dragging an unwilling and unresponsive teenager out of bed. Thus, for the first few hours of the schoolday, a teenager may seem to be in a daze or may actually be asleep and consequently might perform very poorly. On weekends, these teenagers generally sleep until noon or even much later. For them to try to be alert at 9:00 a.m. would be like the average person trying to be alert at 2:00 or 3:00 in the morning. It simply does not work. In the afternoon and evening, these adolescents frequently get a second wind.

Older persons, especially after retirement, may start to nap again. Whether reduced nighttime sleep in the elderly is a consequence of the daytime naps is not clear. Many elderly people sleep poorly not just because of their age, but also as a consequence of a medical condition, medications, pain, sensitivity to their environment, or disruptions and changes in their sleep pattern.

A survey of sleep in the older population, released in 2003, showed that older people without medical problems usually have normal sleep.

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Meir H. Kryger, M.D.
Past President, American Academy of Sleep Medicine