When we sleep, we go through a number of sleep stages. The first stage is a very light sleep from which it is easy to wake up. The second stage moves into a slightly deeper sleep, and stages three and four represent our deepest sleep. Our brain activity throughout these stages is gradually slowing down so that by deep sleep, we experience nothing but delta brain waves - the slowest brain waves.
About 90-110 minutes after we go to sleep and after the fourth sleep stage, we begin REM sleep. Rapid eye movement (REM) was discovered in 1953 by University of Chicago researchers Eugene Aserinsky, a graduate student in physiology, and Nathaniel Kleitman, Ph.D., chair of physiology.
Our brains cycle through four types of brain waves, referred to as delta, theta, alpha and beta. Each type of brain wave represents a different speed of oscillating electrical voltages in the brain. Delta is the slowest (zero to four cycles per second) and is present in deep sleep. Theta (four to seven cycles per second) is present in stage one when we're in light sleep. Alpha waves, operating at eight to 13 cycles per second, occur during REM sleep (as well as when we are awake). And beta waves, which represent the fastest cycles at 13 to 40 per second, are usually only seen in very stressful situations or situations that require very strong mental concentration and focus.
REM sleep is primarily characterized by movements of the eyes and is the fifth stage of sleep. This eye motion is not constant but intermittent. It is still not known exactly what purpose it serves, but it is believed that the eye movements may relate to the internal visual images of the dreams that occur during REM sleep.
During REM sleep, several physiological changes also take place. The heart rate and breathing quickens, the blood pressure rises, we can't regulate our body temperature as well and our brain activity increases to the same level (alpha) as when we are awake, or even higher. The rest of the body, however, is essentially paralyzed until we leave REM sleep.
This paralysis is caused by the release of glycine, an amino acid, from the brain stem onto the motoneurons (neurons that conduct impulses outward from the brain or spinal cord). Because REM sleep is the sleep stage at which most dreaming takes place, this paralysis could be nature's way of making sure we don't act out our dreams. Otherwise, if you're sleeping next to someone who is dreaming about playing kickball, you might get kicked repeatedly while you sleep.
REM sleep occurs in cycles of about 90-120 minutes throughout the night, and it accounts for up to 20-25% of total sleep time in adult humans, although the proportion decreases with age (a newborn baby may spend 80% of total sleep time in the REM stage).
The four stages outside of REM sleep are called non-REM sleep (NREM). Although most dreams do take place during REM sleep, more recent research has shown that dreams can occur during any of the sleep stages. Most NREM dreams, however, don't have the intensity of REM dreams.
Throughout the night, we go through these five stages several times. Each subsequent cycle, however, includes more REM sleep and less deep sleep (stage three and four). By morning, we're having almost all stage one, two and five (REM) sleep. Generally speaking, the deeper the level of sleep, the slower, stronger and more synchronized the brain waves become. Also, the deeper the level of sleep in the cycle, the higher the arousal threshold, so that it is quite difficult to wake someone in stage 3 sleep, but relatively easy in stage 1 or REM sleep. There is also a general tendency towards decreased muscle tone as deeper and deeper sleep stages are achieved, although in this case REM sleep is anomalous in that muscle tone is at its lowest during that stage.
Each sleep stage in any particular sleep cycle fulfills a distinct physiological and neurological function, each of which appears to be necessary for the health of the body and mind, to the extent that, if sleep is interrupted or if certain stages are missing for any reason, their physiological functions are not fully executed, and the person may feel tired or groggy even after an apparently sufficient sleep period.
What happens if you don't get any REM sleep? Originally, researchers thought that no REM sleep meant no dreams. Although lack of REM sleep leads to surprisingly few negative effects on behavior, it has been shown to impair the ability to learn complex tasks.
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