The Mind and Body During Sleep
During the night, we pass through five stages of sleep that, collectively, are termed REM (rapid eye movement) sleep and non-REM sleep. REM sleep is commonly called dream sleep while non-REM sleep consists of stages 1, 2, 3, and 4 sleep.
When we close our eyes to go to sleep at night, we first enter a relaxed state of wakefulness that is characterized by a brain wave (EEG) pattern called alpha. Good sleepers usually spend less than ten minutes in relaxed wakefulness before falling asleep, and some spend as little as a few seconds in this state. Falling asleep almost immediately at bedtime means one of two things: you are very sleep deprived; or, you are a very good sleeper.
After spending a few minutes in relaxed wakefulness, you drift into stage 1 sleep. This is not a “true” sleep stage because it is really a drowsy, transitional state between waking and sleep from which we are easily awakened. You’ve experienced stage 1 sleep if you’ve ever found yourself “drifting off” while lying on the couch or during a boring lecture or meeting. Stage 1 sleep is characterized by a brain wave pattern called theta and the simultaneous disappearance of the alpha rhythm. Our bodies relax deeply during stage 1: respiration slows, the muscles relax, and heart rate drops.
Most people who are awakened from this stage will report that they were “drifting off” but were not really asleep. This state is often described as drowsy, drifting, floating, and wandering. Mental activity during stage 1 sleep involves an abundance of mental imagery that sleep researchers refer to as “hypnagogic imagery”. Here there is a letting go of our usual conscious thought processes and awareness of the external world. Relaxation techniques facilitate stage 1 sleep.
Most good sleepers only spend a few minutes in stage 1 sleep before descending to stage 2 sleep. This is considered the first true sleep stage. It is defined by specific changes in brain wave patterns called sleep spindles and K-complexes that probably represent intermittent attempts by the brain to preserve awareness before we lose conscious awareness of our surroundings. Here we become even more detached from conscious thought and the outside world as our brain and body become more deeply relaxed. Although stage 2 is deeper than stage 1, it is still a light stage of sleep for we are easily awakened from it. Insomniacs underestimate how much sleep they obtain, in part, because they are more likely to perceive stage 2 sleep as wakefulness.
We spend about thirty to forty-five minutes in stage 2 before entering the deeper stages of sleep called stage 3 and stage 4. We produce delta brain wave patterns during these two sleep stages, which are collectively called deep sleep. During deep sleep we reach the lowest levels of physiological activity during the 24-hour day. Deep sleep is also a period of dramatically reduced blood flow and energy use for the brain, which is probably crucial for restoring energy that is used during the daily demands of self-conscious awareness (e.g., thinking). It is very hard to wake up from deep sleep because the brain has turned off its awareness of the external world. We spend about forty-five minutes in deep sleep, revert to stage 2 for a few minutes, and then enter rapid eye movement (REM), or dream sleep.
Dream sleep is an intriguing stage of sleep. It is characterized by rapid eye movements (hence the name rapid eye movement sleep), intense visual imagery in the form of dreams, significant physiological activity, and heightened emotional activity in the brain (particularly emotions such as fear and anxiety). Because REM sleep is such an active period for the brain and body, we are more likely to awaken from this sleep stage than deep sleep. Many areas of the brain are highly active during dream sleep, particularly those involved in emotions and stress, as evidenced by increased blood flow and energy expenditure (increased use of oxygen and glucose). However, the area of the brain that is involved in complex cognitive functions and self-awareness, the frontal lobes, become inactive during dream sleep. Dream sleep is often called paradoxical sleep because, except for muscle paralysis so that we can’t act our dreams, our heart rate, blood pressure, and breathing rate are as high or higher than during waking, and our brain wave patterns resemble wakefulness.
The eye movements that occur during dreaming do, in fact, appear to coincide with the actions in our dreams. Most individuals who are awakened from dream sleep will report a dream, and even though many of us don’t remember our dreams in the morning, everyone dreams. The content of a dream is often detailed, has a continuous quality, and is vivid, bizarre, and sometimes frightening. Despite the fact that dream content is often impossible or improbable, we experience dreams as if they are real because there is a complete suspension of self-reflection (we are not aware that we are dreaming) and logic (impossible experiences of time, places, people, and actions) during dreaming.
During the course of a night’s sleep, we progress from Stage 1 to Stage 4 and then through REM sleep in about ninety minutes. Therefore, a six-hour sleeper will move through four of these one and one-half hour cycles during the night while an eight-hour sleeper will experience about five of these cycles. We spend about 5 percent of the night in Stage 1, 50 percent in Stage 2, 20 percent in deep sleep, and 25% in REM. Early in the night, deep sleep periods are longer (sometimes lasting up to one hour) whereas REM periods last only a few minutes. Later in the night, deep sleep periods grow shorter and the duration of REM periods increase so that, by the final REM period of the early morning, REM sleep may persist for an hour. As a result, we obtain most of our deep sleep during the first half of the night and most of our dream sleep during the last half of the night. And because sleep grows lighter in the second half of the night, awakenings are more prone to occur in the second half of the night.