Deep sleep is a mysterious state that we usually enter several times each night. Its nature is somewhat less well known than the more dramatic "dream sleep." While we are very aware today of dream sleep, and of the ability of some people to be conscious of and even direct their dreams in a state known as lucid dreaming, the nature of deep sleep continues to invite a sense of mystery.
Sleep stages were first discovered in the 1930s when Loomis and his colleagues began doing overnight EEG recordings of sleeping people. Stages of sleep were recognized by the regular changes noted in the EEG. By the late 1960s, it was possible to specify ways of reliably scoring these stages, and some knowledge existed about their presumed role in the restorative process of sleep. Because of the dramatic nature of dreams and their historical role in religion, literature, and psychoanalysis, REM sleep, during which these vivid experiences occur, became well known to the public.
Less emphasis has been placed on the non-dream state of deep sleep.
This stage of sleep is also known as delta sleep, slow wave sleep or, more recently, N3. It is called delta sleep because of the presence of high-amplitude, low-frequency delta waves that are seen to occur in the EEG. In the past, this stage was divided into two stages, stage 3 and stage 4, depending on the percentage of delta waves present. Stage 4 has a greater amount of delta wave activity than stage 3, and was thought of as a deeper state of sleep. Research has not, however, been able to clearly show any significant difference in the benefit of these two stages, and more recently they have been combined into a single stage—N3.
Subjectively deep sleep is a time of nearly complete disengagement from the environment. It is very difficult to awaken a person in deep sleep, and children in this state may be nearly impossible to wake up. It is from this stage that sleepwalking emerges. This happens when there is a sudden arousal from deep sleep that causes the motor centers of the brain—but not the higher centers—to awaken, so that the person is in a sleep state dissociation characterized by complex motor activity with limited judgment and awareness.
Many important physiological processes occur during deep sleep. Most deep sleep occurs during the first two sleep cycles, with the greatest amount of deep sleep typically occurring in the first cycle. As the night progresses, deep sleep decreases and is replaced by the lighter stage 2 sleep and there is an increasing amount of REM sleep toward morning. Deep sleep is extremely effective in decreasing the sleep drive that builds steadily over the course of the day—far more effective than stage 2 sleep in this regard. One reason that short afternoon naps of about 20 minutes may not affect nighttime sleep, while longer ones may result in difficulty falling asleep, is that in a short nap there is not enough time to cycle into deep sleep; most sleep is stage 2. If N3 sleep occurs during a nap, it will rapidly decrease your sleep drive and make it hard to fall asleep later that night.
Human growth hormone is released in pulses during deep sleep, and interruption of this stage abruptly stops its release. The drug gamma hydroxybutyrate (GHB) was once used by body builders because it causes an increase in deep sleep. With this increase in deep sleep there is also a release of growth hormone. In adults, growth hormone promotes cell repair that is necessary after the stress of weight training. In the past GHB was freely available as a supplement. Unfortunately, the ability of this drug to induce sleep was misused by some, who turned it into a "date rape" drug. It is now tightly controlled and available only by prescription for treatment of disorders such as narcolepsy.
There are psychological benefits of deep sleep: By rapidly reducing sleep need, this stage of sleep is an especially refreshing part of the sleep cycle, unless you are wakened out of it, in which case you will feel very sluggish and may experience "sleep drunkenness," during which it is unsafe to drive. Some recent neural network research also indicates that deep sleep may be important in helping clear the brain for new learning the next day.
In some Eastern mystical traditions, the state of consciousness through which we ordinarily experience the world is thought of as being one of gross awareness. There is a more subtle state of awareness that may be developed through meditation and can be thought of as being more subtle and like that of the dreaming state of consciousness. The deepest level of consciousness, in this scheme, is the one in which it is possible to become aware of the emptiness in which all phenomenon are thought to occur. According to the integral philosopher Ken Wilber, it is possible, with training in advanced meditation, for people to be aware of the subtler states of consciousness, including the states of dreaming and even deep sleep.
Whether or not we are aware of the state of deep sleep, it functions to restore us physically and mentally. Unfortunately, our access to deep sleep is also vulnerable to the effects of stress, sleep disruption, aging, and many drugs. By preventing adequate deep sleep, these factors contribute to the run-down feelings that many people dealing with financial pressure, sleep apnea, aging, or taking certain drugs experience daily.
What can we do to get more of this marvelous, mysteriously restorative sleep stage? There is no easy fix—we can't turn back the clock on aging, or eliminate life's ubiquitous stressors. But when we take steps to ensure that we have a regular, pre-midnight bedtime, that we get any apnea problem treated, and that we use relaxation breathing or other meditative techniques to slip into sleep, we may indeed find that the mysterious and restorative powers of deep sleep are within our reach.
It's well worth the attempt.