Q: I’m sure everyone will agree that a good night’s sleep makes one refreshed, alert and ready to tackle the day’s works. But why do we need to sleep? Also, how much sleep does a person need? My father is 74 years old and he sleeps only four hours a night. Is that normal? --Mary R., Pasay City

MANILA, Philippines -- A: There is indeed no doubt that sleep makes us feel better. After a good night’s sleep, we feel rejuvenated—more alert, more energetic, happier and more functional. In contrast, lack of sleep leaves us mentally, emotionally and physically fatigued. But more than just a refreshing activity, sleep is actually a vital function—much like breathing, eating and drinking—that our body needs to undertake. Animals that are completely deprived of sleep do not simply become lethargic, they lose all their immune capability and die in just a matter of weeks. But why do we need to sleep?

There are several theories that experts are currently entertaining regarding this matter, the more important ones are as follows:

First, sleep is needed to carry out our body’s restorative functions more effectively. Scientific findings show that many of the major physical restorative functions in the body, like muscle growth, tissue repair and protein synthesis occur mostly, or in some cases only, during sleep. Likewise, anabolic substances such as the human growth hormone that promotes the buildup of tissues are secreted preferentially during sleep. Attesting to the restorative powers of sleep are the facts that when we are asleep, wound healing occurs faster and our white blood cells (the body’s immune cells) increase in number.

The restorative functions of our brain are also carried out during sleep. While we are awake, our brain cells produce metabolic by-products that accumulate during the day and cause us to feel tired. Sleep gives our body the chance to clear itself of these substances because most brain cells are inactive during this period.

Second, in newborns, infants and young children, sleep may be necessary for proper growth and development. Sleep deprivation in early life causes behavioral problems, decreased brain mass and increased death rate in neurons.

Third, sleep may be a vital part of our memory process. There are two forms of sleep: rapid eye movement (REM) sleep and non-REM (NREM) sleep. REM sleep, the period when most dreams occur, is associated with rapid, roving movements of the eye while NREM sleep, which has four stages, is not.

Evidently, REM sleep helps us consolidate spatial (i.e., information about one’s environment and the body’s orientation in relation to space) and procedural (i.e., skills and procedures) memories, while NREM sleep helps us consolidate declarative (i.e., facts and experiences) memory.

Also, some experts say that all new information that we encounter while awake is initially stored in our short term memory. This information is only transferred to our long-term memory during sleep.

As to your second question, there is no consensus among experts on the length of sleep a person must regularly have to maintain optimal health and wakeful alertness. Scientific studies show that the amount of needed sleep varies from individual to individual.  Most people sleep between seven to eight hours a night, but many function normally with less than seven or more than eight hours of sleep. However, epidemiologic studies indicate that regular sleep of less than four or more than nine hours is associated with a shorter life expectancy.

If your father does not complain of sleepiness during the day, he might be getting enough sleep, which means it is normal for him to just have four hours sleep each night.

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