Excerpt from a delanceypost article ------
The five stages of sleep:
"Until the middle of the twentieth century, scientists thought that sleep was an unchanging condition during which time the brain was quiet. The discovery of rapid eye movements in the 1950s upended that. Researchers then realized that sleep is made up of five distinct stages that the body cycles through over roughly ninety-minute periods. The first is so light that if you wake up from it, you might not realize that you have been sleeping. The second is marked by the appearance of sleep-specific brain waves that last only a few seconds at a time. If you reach this point in the cycle, you will know you have been sleeping when you wake up. This stage marks the last stop before your brain takes a long ride away from consciousness.
"Stages three and four are considered deep sleep. In three, the brain sends out long, rhythmic bursts called delta waves. Stage four is known as slow-wave sleep for the speed of its accompanying brain waves. The deepest form of sleep, this is the farthest that your brain travels from conscious thought. If you are woken up while in stage four, you will be disoriented, unable to answer basic questions, and want nothing more than to go back to sleep, a condition that researchers call sleep drunkenness. The final stage is REM sleep, so named because of the rapid movements of your eyes dancing against your eyelids. In this type of sleep, the brain is as active as it is when it is awake. This is when most dreams occur.
"Your body prepares for REM sleep by sending out hormones to effectively paralyze itself so that your arms and legs don't act out the storyline you are creating in your head. This attempt at self-protection doesn't always work perfectly, and when that happens, what follows is far from pleasant. Sometimes, it is the brain that doesn't get the message. This can lead to waking up in the middle of the night with the frightening sensation that you can't move your limbs. In the Middle Ages, this was thought to be a sign that a demon called an incubus was perched on the chest. Instead, this condition is simply a flaw in the sleep cycle, a wrong-footed step in the choreography of the brain's functions that allows a person to become conscious when the body thinks the brain is still dreaming.
"At other times, the body doesn't fully paralyze itself like it is supposed to. This is the root of a series of problems called parasomnias, of which sleepwalking ... is by far the most mild. Patients with REM sleep disorder, for instance, sometimes jump out of a window or tackle their nightstand while they are acting out a dream. Some patients I spoke with who have this disorder have resorted to literally tying themselves to the bedpost each night out of the fear that they will accidentally commit suicide."
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