Sleep Cycles or Sleep Stages
While most people do not spend a lot of time thinking about sleep, everyone agrees sleep is as essential to health and life as air, food and water.
Overview
Extensive studies have shown that most adults require right around eight hours of sleep per any 24-hour period. Most people are awake and active during the day and sleep in the evening, while experiencing a short time of sleepiness in the middle of the day. Not only is this normal, but many cultures recognize this midday lack of vigor and sanction it with a nap or siesta.
An internal biological clock is responsible for determining the sleeping and waking cycle, also called the circadian rhythms (the word "circadian" literally means "about a day"). But this circadian internal clock is not limited to simply the sleep cycle - it also controls the timing of many other bodily and metabolic functions.
An important factor in setting a body's circadian rhythms is the light entering the eyes. This light journeys from the retinas as electrical signals through the optic nerves toward the brain's center to reach the hypothalamus, which contains a small dusting of nerves called the suprachiasmatic mudeus, or SCN. As daylight begins to fade each day, the SCN alerts the brain's pineal gland to produce melatonin, the hormone which helps to promote sleep's onset. At sunrise, the pineal gland is signaled by the SCN to decrease the production of melatonin, thus allowing wakefulness.
Other Factors
Other circadian rhythms controlled by the hypothalamus include the fall and rise of body temperature and the release of other hormones, again throughout a 24-hour timeframe. External and other internal factors can infringe on these body rhythms, with the most noticeable disruption affecting one's sleep.
Hormonal shifts (such as those found with menopause) can severely affect the sleep/wake cycle; extensive travel (where time zones are crossed) often turns sleep cycles upside down; and light exposure at the wrong time, whether from an artificial source or the moon. All of these factors can rest the body's internal clock.
The Stages of Sleep
Beginning in the 1950s, researchers have used polysomnography (the simultaneous recording of electrophysiological and other data) during sleep to try to interpret the activities taking place in the sleeping body and brain.
These scientists discovered five very distinct stages take place during sleep. Each one plays its own essential role in allowing a person to feel alert and well rested when they wake. Four of the stages are put into the non-rapid eye movement (REM) sleep, with the fifth being classified as REM sleep. The stages can be broken down as follows:
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Light Non-REM Sleep - Stage one is also known as the hypnagogic state, a light stage that serves as the transition between wakefulness and sleep. In stage one, external stimuli fades into the background and one's thoughts become broken as consciousness drifts in an out. The muscles begin to relax and blood pressure, heart rate and body temperature slowly decrease. Brain waves in this stage are irregular and rapid. Most are theta waves, at a frequency of 4-7 cycles per second. Alpha waves (intermittent at this point) have a frequency of 8-13 cycles per second and are indicative of "relaxed wakefulness." Serotonin levels begin to increase in the brain. In people with healthy patterns of sleep, stage one sleep lasts for just a few minutes.
Relaxation deepens when stage two of non-REM sleep takes over. In stage two sleep, brain waves become larger and show erratic bursts of electrical activity such as sleep spindles and K-complexes, or waveforms named for their unique appearances on the polysomnograph. Some physical repair processes begin during this stage.
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Deep Non-REM Sleep - In stages three and four, it is much more difficult to awaken a sleeper. In non-REM sleep, brain waves are slow and large (called delta waves), with a low frequency of cycles per second. Somewhere between 20 and 50 percent of the brain activity recorded in stage three is delta waves. In stage four, more than half of the activity is in delta waves, with the remainder being theta waves.
Deep non-REM sleep is very important for maintaining one's health. This restorative time gives the nervous system and brain the chance to bring these body systems back into balance and allows the repair work to take place.
The percentage of time spent in stage four sleep defines the quality of one's sleep, which is also called delta sleep. While researchers are still debating the optimal amount of delta sleep necessary to contribute to optimal health, several important functions take place in this phase. Immune function is strengthened, growth hormone is secreted, and red blood cells are renewed. Those individuals who are regularly deprived of delta sleep often suffer from general aches, pains and fatigue.
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REM Sleep - After approximately an hour of non-REM stages, the sleep pattern shifts into an active stage which is characterized by rapid eye movements, or REM. In this stage, the brain's waves are very similar to those found in wakeful periods, but the person is actually dreaming. The nerves that control movement of the body are suppressed by the brain's inhibition of neurons in the spinal cord, which prevents the body from acting out one's dreams.
It is believed that memories are organized and stored during REM sleep. People seem to enter a REM period about every ninety minutes while sleeping. A REM period usually lasts between twenty and forty minutes.
While most dreams are forgotten upon awakening, being deprived of REM sleep and dreams can lead to confusion, anxiety, and impulse control problems.
References
- Lack, L. et al.(2003). Insomnia : how to sleep easy. Double Bay, N.S.W. : Media 21 Publishing.
- Servan-Schreiber, D. (2006). The Duke encyclopedia of new medicine : conventional and alternative medicine for all ages. London : Rodale.
- Vukovic, L. (2005). Overcoming sleep disorders naturally. Laguna Beach, CA : Basic Health.
- Wilfred, P. (2010). Sleepmanual : how to achieve the perfect night's sleep. London : New Holland.
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