Throughout the night, our brains cycle repeatedly among five stages of sleep—going all the way through the five cycles about once every ninety minutes. Researchers propose that the cycle of sleep stages allows the brain to perform all its regenerative functions during the course of the night (and rest itself) without leaving the body in a vulnerable state for more than a few minutes at a time.
In my last blog on the lighter sleep stages, we explored the ways the brain balances delicate operations that need uninterrupted sleep with the ability to awaken if needed. Now, let’s have a look at stages 3 and 4—the deeper sleep stages from which it’s more difficult to be awakened.
In the third and fourth sleep stages (also called slow wave sleep) the brain exhibits delta brainwaves, the slowest and broadest waves broadcast by the brain on EEG. In the third sleep stage, less than half of the brain’s activity is comprised of delta waves; in the fourth sleep stage, more than half of the brain’s activity is delta. It’s an arbitrary distinction accepted by sleep researchers as a way to determine just how deeply a subject is sleeping.
In stages 3 and 4 the brain is still sending signals to the muscles and the body is most deeply asleep, so these are the stages where sleep talking and sleep walking are most likely to occur. Once the brain shifts from slow wave sleep briefly to phase 2 and then into REM sleep, the body enters a state of atonia, or lack of muscle tone, that keeps the muscles from acting out the vivid dreams we experience in this fifth sleep stage.
Sleep Cycles and Slow Wave Sleep
The brain tends to either spend more time in slow wave sleep after sleep deprivation, or to exhibit more intensive delta wave activity during periods of slow wave sleep. It isn’t known exactly how delta waves affect the body and mind (according to a 2010 sleep article in The Neuroscientist).
The human brain seems to have adapted sleep patterns over countless generations to protect the body while getting the most from what we erroneously think of as “down time” during sleep. Next time, we’ll have a look at the mysterious REM sleep stage—the fifth stage, the one that produces the vivid dreams that we usually remember.