Get six interesting facts about what’s happening while you slumber.

The science of sleep

Chances are you go to sleep each night (and maybe take a nap once in a while during the day). But, how much do you really know about sleep? On their Healthy Sleep website, Harvard has collected more than 50 years of sleep research in one place.

Here are six highlights:

  • Your brain stays active while you sleep. Scientists used to think the brain shut down during sleep, but when the EKG machine (which measures brain activities) was invented in 1929, they realized the opposite was true. In fact, the brain is sometimes more active when we're asleep than when we're awake.
  • There are two kinds of sleep: REM and NREM. REM (rapid-eye-movement) sleep is often called “active sleep” because your brain is active and your eyes are moving. Sleep experts think REM sleep is related to dreams, because people often report that they were dreaming if they wake during REM sleep. NREM (non-rapid-eye-movement) sleep is a much calmer state—brain waves become slower and more synchronized, and the eyes remain still. Healthy sleepers go through several cycles of REM and NREM sleep each night. 
  • Muscles in the arms and legs are paralyzed during REM sleep. During REM sleep, muscles in the arms and legs are temporarily paralyzed. Researchers think this might prevent people from “acting out” our dreams.
  • Sleep might give your heart a rest. Researchers don’t know exactly why we sleep, but one possible function of sleep is to give the heart a chance to rest from waking life. There is an overall reduction in heart rate and blood pressure during NREM sleep.
  • Growth hormones are released while you sleep. Did you ever think children look taller when they woke up than when they went to bed? It could be true. One of the greatest changes induced by sleep is an increase in the release of growth hormone. Evidence suggests that cell repair and growth may be an important function of sleep.
  • Dreams are still a mystery. Scientists still don’t know exactly why we dream. Some experts think dreams replay the day’s events and help you form of memories, while others dreams are simply the result of random activity in the brain.