Introduction to Sleep Science: Duration Needs, Physiological Rhythms, and the Significance of Mental Recovery

If you live to the average lifespan, you will sleep 220,000 hours in your lifetime, or about 20-25 years, roughly one-third of your life, spent sleeping. The average person needs 7-8 hours of sleep per day. However, many Americans sleep less than 6 hours or more than 9 hours a day and live well. Einstein slept 12 hours a day, while Edison only slept 5 hours. The amount of sleep needed each day is determined by each person's genetics, health condition, lifestyle, and other relatively unchanging factors. A person's sleep duration also decreases with age. After age 60, people become significantly weaker, sleeping less than 7.5-8 hours a day. Many older people also often compensate for insufficient nighttime sleep by napping during the day. In some cases, even after several days of not sleeping, a single long day of sleep may be sufficient to recover, and you don't need to make up for the missed hours. Sometimes you might sleep extra long on weekends to make up for a week's sleep deprivation. However, this compensation can sometimes be harmful because sleep is a rhythmic biological process. Our bodies are sensitive not only to changes in the amount of sleep but also to changes in its rhythm. Some researchers hypothesize that sleep allows the body to rest and rejuvenate, restoring the body's immune function. During sleep, the body secretes growth hormone, which scientists believe promotes the renewal of aging red blood cells, bone synthesis, and growth and development in adolescents. Although your brain doesn't truly rest during sleep, its greatest benefit is mental recovery. What are the consequences of sleep deprivation besides drowsiness? Research on sleep shows that occasional sleep deprivation can affect your mood but doesn't harm your health. However, chronic sleep deprivation can lead to drowsiness, irritability, fatigue, weakness, anxiety, and poor concentration, while also increasing the risk of accidents and sports injuries. Simply lying still can sometimes have the same resting effect as sleep. Only you can determine if you've slept enough. If you wake up feeling refreshed, whether you slept for 6 or 10 hours, you've slept enough. If you fall asleep during the day when you shouldn't, or if you feel sleepy while driving and cause or are about to cause an accident, it indicates you may need more sleep. Dr. William DeMante of Stanford University believes that most Americans are "chronic sleep deprived." To some extent, sleep is a learned behavior. Human sleep typically occurs in cycles of about 90 minutes. Each cycle includes rapid eye movement (REM) sleep and non-REM sleep. Sleep begins with non-REM sleep, during which brain waves become slow and regular, body temperature decreases, and sensory abilities decline. About 80 minutes after non-REM sleep, sleep enters the RME stage, during which the eyes make rapid movements behind closed eyelids, heart rate increases, brain waves and breathing become rapid and irregular, fingers and toes twitch, but the trunk remains limp. This stage is characterized by vivid dreams and typically lasts 10 minutes. Then another cycle begins, with RME sleep increasing thereafter. By the end of the last sleep cycle, RME sleep reaches almost 40 minutes, while non-REM sleep becomes increasingly shorter; hence, the first few hours of sleep are often the most restful. Psychologists once believed that REM sleep was essential for maintaining mental health, but now they believe it has other uses, possibly related to learning and memory. Recent experiments on humans and animals have shown that a lack of REM sleep can lead to decreased memory.