Assistant Professor HAYASHI Yu, International Institute for Integrative Sleep Medicine

Why do we sleep? Moreover, why do we dream? In truth, we do not really know the answers to these questions. However, an experiment from a quarter of a century ago showed that rats prevented from sleeping died after two to three weeks. The rats exhibited symptoms such as weight loss and decreased body temperature, but the direct cause of death is not known. Despite sleep being a phenomenon that we experience every night, we know almost nothing about it.

People often talk of sleeping so soundly that they did not even dream. However, normal sleep is a repeating cycle of non-REM sleep, during which we do not dream, and REM sleep, during which we are more prone to dreaming. REM is short for "rapid eye movement." This phenomenon was named in 1953 by American researchers who discovered that although a person may be sleeping soundly, there is still rapid movement of just the eyes beneath the eyelids. Later research showed that besides REM sleep, there is another type of sleep called non-REM sleep during which there is no eye movement. By waking up people in each of these states, researchers discovered that we mainly dream during REM sleep. When brain waves are monitored, brain waves during REM sleep appear as narrow and small waves like those seen during wakefulness whereas brain waves during non-REM sleep appear as unique waves that fluctuate widely (called "slow-wave"). In a normal sleep sequence, deep non-REM sleep comes first, then REM sleep, then alternating cycles of the two types. Although REM sleep has only been observed in mammals, it is said to also occur in birds. REM sleep does not occur in reptiles. Furthermore, no animal is known to only have REM sleep. The evolution of sleep quality is a very interesting topic.

Article by Science Communicator at the Office of Public Relations