Stages of Sleep:

REM, Dreaming, and Stages of Sleep

by Orlin Sorensen

If Shakespeare wrote Hamlet today, his famous line "To sleep, perchance to dream" might sound a little different: "To sleep, certainly to dream--but I only remember the dreams I have during REM sleep stages."

Not nearly as poetic, but then again, much more intriguing. In Shakespeare's time, most people thought of sleep as an unchanging, dormant period of little interest. Hardly anything was known about sleep or dreaming. In fact, it wasn't until the discovery of REM sleep at the relatively late date of 1953 that scientists knew much about sleep at all!

Studying infants in the 1950s, American physiologists Eugene Aserinsky and Nathaniel Kleitman reported that periods of eye movement and twitching occur during sleep. They named these periods "rapid eye movement" (REM) sleep. At first they assumed that REM sleep occurred only in babies, but later investigations proved it occurs not only in all humans, but in all mammals-even in birds.

Sleep Stages

REM Sleep

We typically have about three to five periods of REM sleep per night. The first rapid eye movement period usually begins about 70 to 90 minutes after we fall asleep. These periods occur at intervals of one to two hours and vary in length from five minutes to more than an hour. Periods of REM sleep seem to be shorter at the beginning of the night and longer toward morning.

About 20 percent of sleep is REM sleep. If you sleep seven or eight hours a night, only around an hour and half of that time is spent in REM sleep. However, the relative amount of REM sleep varies considerably with age: babies spend more than 80 percent of total sleep time in REM mode, while people over 70 spend less than 10 percent.

For those of us who think of sleep as a peaceful, largely static activity, we should think again: REM sleep is characterized by rapid, low-voltage brain waves and irregular breathing and heart rate. And the eye movements themselves, called saccades, are indeed rapid: The saccade is the fastest movement of an external part of the human body. The peak angular speed of the eye during a saccade reaches up to 1,000 degrees per second. However, our muscles do largely "turn off" during REM sleep, effectively paralyzing the body, except for involuntary muscle jerks.

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Non-REM (NREM) Sleep

NREM sleep is divided into four stages of increasing depth, leading to REM sleep. In stage one, people often believe they are fully awake, although they are beginning to lose conscious awareness of the external environment. This stage can be thought of as a gateway between waking and sleep.

Stage four is the deepest stage of sleep. In contrast to REM sleep, during stage four the brain waves are slow and high voltage; breathing and heart rate are slow and regular; and blood pressure is low. It is very difficult to wake people who are in this state, and, if they do wake up, they are extremely groggy. This is the stage in which night terrors and sleepwalking occur.


There's no "perchance" about it: Everyone dreams, whether they remember their dreams or not. But most dreams-and almost all the dreams we can recall upon waking-occur during REM sleep.

Perhaps that is the reason our bodies' muscles turn off during REM periods: If they didn't, we would physically respond to the vivid dreams we have during REM sleep. Studies of people with malfunctioning REM systems show that those folks thrash around in their sleep, often punching their bedmates or hurting themselves as they act out their dreams.

Unanswered Questions, Theories, and Fun Facts

We know sleep is vital to our bodies and that REM sleep, in particular, seems to be intrinsically important, as it is so physiologically different from the other phases of sleep. If dreams turn out to have some important function for humans (we still don't know whether animals dream), then perhaps we need REM sleep solely in order to dream.

Another theory is that the amount of REM sleep per night in a species is closely correlated with the developmental stage of newborns. The platypus for example, whose newborns are completely helpless and undeveloped, has eight hours of REM sleep per night; animals that can hunt, eat, keep warm, and defend themselves soon after birth (for example, dolphins or horses) need almost no REM sleep.

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