Sleep is one of the most essential biological processes of an organism. Humans view sleep as a peaceful activity that marks the end of the day’s activities. The process of sleep is much more than merely being in a restful state. Sleep is a cyclic phenomenon that has been divided into four distinct stages. During each of these stages, there are varied brain wave activities and bodily reactions thus making the process of sleep an active one. Each sleep cycle (of the four stages) lasts for ninety minutes and an individual experiences five sleep cycles on an average each night.
Sleep is an altered state of consciousness, as opposed to the popular belief that sleeping is unconscious. The brain is active throughout the process, as seen with the help of devices such as Electroencephalogram (EEG), Electrocardiogram (ECG or EKG), Electrooculagram (EOG) and Electromyogram (EMG). The changes in body temperature and heart rate are further evidence that establish sleep as an active process.
What is Sleep?
Sleep is a biological imperative critical to the maintenance of mental and physical health. It is a state of lessened consciousness or altered consciousness and decreased physical activity during which the organism slows down and repairs itself. The sleep cycle involves two distinct phases that alternate from light sleep to deep then deeper and deepest sleep throughout the sleep period. There are two main phases of sleep.
- Rapid eye movement (REM) sleep
- Non-rapid eye movement (NREM) or slow-wave sleep (SWS)
The Stages of Sleep and the Sleep Cycle
Sleep is divided into four stages and these stages constitute one sleep cycle. The timing and progression of the sleep cycle and the total amount of nightly sleep required for optimal health varies from infancy to adulthood, depending on developmental stage and temperament. Children, particularly infants, require the most sleep during a 24-hour period. The natural sleep-wake cycle, governed by an internal "biological clock," tends toward a 25-hour day. It is affected by the relative balance of light and darkness in the environment. As darkness approaches, the hormone melatonin is secreted by the pineal gland and signals the brain that it is time to sleep.
An individual is alert and awake before sleeping and the brain produces beta waves when it is active. As the brain begins to relax, slower waves called the alpha waves are produced. The individual slowly begins to feel ‘sleepy’.
Stage 1 Sleep
It is the state of transition between wakefulness and sleep, characterized by relatively rapid, low voltage brain waves. The brain produces high amplitude or theta waves, which are very slow brain waves. This period of sleep lasts only a brief time (around 5-10 minutes). During the consecutive sleep cycles, stage 1 is the REM sleep stage.
Stage 2 Sleep
It is a deeper stage that the first and is characterized by slower, more regular wave patterns along with momentary interruptions of sleep spindles or sharply pointed, spiked waves. This stage lasts approximately twenty minutes and the body temperature begins to decrease, along with the heart rate.
Stages 1 and 2 dominate the second half of the night.
Stage 3 Sleep
In this stage, the brain produces slower waves or delta waves, with greater peaks and valleys in the wave pattern. In stage 3, 20% to 50% of the brain waves are delta waves, the rest being the theta waves. It is also a transitional period between light sleep and deep sleep.
Stage 4 Sleep
This is the deepest stage of sleep and it is almost impossible to awaken a person from this stage. Individuals are least responsive to the environmental stimulation and the body temperature, heart rate and blood pressure decrease further. More than 50% of the brain waves constitute delta waves and this stage is also called Delta Sleep. Bed-wetting and sleep walking usually occur at the end of this stage.
Stage 3 and 4 dominate the first half of the night.
REM Sleep
Some refer to this as the fifth stage of sleep. Research has shown that REM sleep occurs during stage 1 sleep from the second sleep cycle onwards. REM or Rapid Eye Movement sleep occupies 20% of an adult’s sleeping time, characterized by increased heart rate, blood pressure, erections, eye movements and the experience of dreaming.
One explanation for the rapid eye movements is that, the eyes follow of the action of their dream, like watching a movie on the screen.
In an experiment conducted to determine the importance of REM sleep, subjects were constantly awakened during their REM sleep and they began to show signs of what is now called Rebound Effect. It was found that the REM deprived sleepers spent more time in the REM stage the following night. It was concluded that this was the body’s method to restore the lost REM sleep, thus establishing the requirement of REM sleep and consequently, the importance of REM sleep.
Although sleeping seems to be tranquil and inactive, it is far from being an idle process. A sleeping individual constantly experiences the four stages of sleep, REM sleep and brain wave activity during the night’s sleep cycles.
Source:
Feldman, R S (2004). Understanding Psychology. Tata McGraw Hill Publications
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