What Happens When We Sleep?

We sleep every night. In fact, we spend about one third of our lives asleep. When it gets dark, the brain makes more of a hormone called melatonin. It helps us feel sleepy. Sleep is not the same all night. There are two kinds of sleep: REM sleep and non-REM sleep. We have most of our dreams in REM sleep. The two kinds of sleep come again and again during the night. Experts say adults need seven to nine hours of sleep.

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We spend about one third of our lives asleep, but what actually happens while we sleep? Our bodies have an internal clock that follows a cycle of about 24 hours. Light is the main signal for this clock. When it gets dark, the brain produces more of a hormone called melatonin, which helps us fall asleep. Another chemical, adenosine, builds up in the brain the longer we stay awake and makes us feel sleepy. Caffeine keeps us awake by blocking it. Once we fall asleep, we move between two main types of sleep: non-REM sleep and REM sleep. REM stands for "rapid eye movement", and it is when most dreaming happens. A full cycle takes about 80 to 100 minutes, and we usually go through four to six cycles a night. Experts recommend that adults sleep between seven and nine hours.

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Sleep can look like nothing more than lying still with our eyes closed, but the brain stays surprisingly busy all night. According to the U.S. National Institutes of Health, two main systems decide when we feel sleepy. The first is the circadian rhythm, an internal clock of roughly 24 hours that is set mainly by light. A small area of the brain receives signals from the eyes, and when darkness falls, it tells the pineal gland to release more melatonin, a hormone that prepares the body for sleep. The second system is sleep pressure. The longer we are awake, the more of a chemical called adenosine builds up, making us drowsy. Caffeine works precisely because it blocks adenosine. Once we fall asleep, we pass through repeated cycles of non-REM and REM sleep. Non-REM sleep has three stages, ending in deep sleep, when it is hardest to wake up. REM sleep, named after the rapid eye movements that occur under closed eyelids, is when brain activity rises and most vivid dreaming takes place. During REM sleep, the muscles of the arms and legs are temporarily paralyzed, which may stop us from acting out our dreams. Each cycle lasts about 80 to 100 minutes, and a typical night includes four to six of them. Scientists believe sleep helps the brain store memories and clear away waste. Experts recommend seven to nine hours a night for adults, and those who regularly sleep less than seven hours may have more health problems.

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