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How the brain initiates and regulates sleep

Updated on April 19, 2012
Sleep cycle
Sleep cycle

The reasons why we eat and why we breathe have been scientifically understood. The processes involved and the need for it has been understood. On a comparative note, the reason why we sleep has not been thoroughly understood and still remains a mystery. Use of advanced technology in recent times has shed some light so as to make researchers give some reasonable proposals on the need and exact benefits of sleep.


The belief that brain activity ceased during sleep was proved wrong by Nathaniel Kleitman the pioneer of sleep research and his student Eugene Aserinsky of the University of Chicago way back in 1953. They had discovered REM sleep which is marked by periods of rapid eye movement (REM) which indicated brain activity during sleep. REM sleep has been observed in a majority of the examined terrestrial mammals. REM sleep alternates with non-REM sleep also known as quiet sleep in a regular cycle.


Physical and mental health is in many ways affected by sleep, a better understanding of sleep will in many ways help you sleep better!


Neurotransmitters are chemicals which control our being asleep or awake by acting upon different groups of nerve cells, also called neurons, in the brain. Serotonin and norepinephrine are produced by neurons in the brainstem (which connects the brain with the spinal cord). These chemicals keep some parts of the brain active when we are awake. There are other neurons which starts signaling when we are asleep. These signals are known to kind of cut out the signals that keep us awake. Further it is known through research that there is a buildup of the chemical adenosine while we are awake and a considerable amount of buildup triggers drowsiness. The chemical breaks up as we sleep.


While asleep, sleep phases through 5 stages: stages 1, 2, 3, 4 and REM (Rapid Eye Movement) sleep. The stages are cyclic starting from stage 1 to REM sleep and then repeating itself. Of the total sleep time, 50% of it is spent in stage2, 20% in REM sleep and the remaining 30% in the other stages.

Stage 1: Light sleep is what best describes this stage as the sleeper is easily awakened. The sleeper drifts in and out of sleep. Muscle activity slows down and there is a slow movement of eyes. When awakened from this sleep the sleeper experiences hypnic myoclonia, which are sudden muscle contractions, which is often preceded by the sensation of falling. This being similar to the jump one makes when startled.


Stage 2: Eye movement ceases and the fluctuations of electrical activity, termed brain waves, become slower. Sleep spindles, which are sudden bursts of rapid waves, are also observed.


Stage 3: Delta waves which are extremely slow brain waves make an appearance which is interspersed with smaller, faster waves.


Stage 4: Deep sleep is what best describes this stage. By now delta waves are produced exclusively. It is difficult to arouse someone from sleep during stages 3 and 4. The absence of eye movement and muscle activity marks this stage. Disorientation and grogginess is experienced by sleepers forcibly awakened while in this stage. It takes time for the sleeper to adjust himself.


REM sleep: During this stage the eyes jerk rapidly in various directions, breathing becomes more rapid, irregular and shallow, heart rate increases, blood pressure rises, males can have penile erections, and females can have clitoral enlargement though with absence of sexual thoughts. Sleepers awakened from REM sleep often recall dreams. 70 to 90 minutes into sleep is the time needed to progress into the first REM sleep period. 90 to 110 minutes is the average time a sleep cycle lasts. The initial sleep cycles contain fewer REM sleep periods and longer periods of deep sleep. With progression of the night, REM sleep periods increase while deep sleep period decrease. By the time of normal awakening sleepers spend their sleep in stages 1, 2, and REM.

Disrupted REM sleep of one night is compensated for the next time we fall asleep. The normal sleep cycle may be skipped and we immediately slip into REM sleep and go through extended periods of REM until the REM sleep debt is paid off.


Recent advances made in technology have enabled researchers to guide fine microwires (32 microns wide) into various brain regions, these wires producing no pain on implant and have been implanted into humans as well as the other lab animals, without affecting their normal activities. These studies have enabled characterization of sleep at the neuron level. It has been observed that brain neurons activity was at the peak when the subjects were in the wake state, which was expected. But the brain neuronal activity while asleep was interestingly variable. Despite the non-responsiveness to the environment and similar posture while in both REM and non-REM sleep, the brain behavior was completely different in both the states.


In the duration of non-REM sleep the breathing and heart rate tended to be normal. There is no dreaming during this state. It has been observed that during this sleep cells in different brain regions do very different things. Almost all the neurons in the brain stem, immediately above the spinal cord reduce or stop firing. Neurons in the cerebral cortex and adjacent forebrain regions reduce their activity by a small amount. It is in the overall pattern of activity that a dramatic change is seen. During the wake state the neurons go about its own activity, but during the non-REM sleep, adjacent cortical neurons fire synchronously, with a relatively low frequency rhythm. Though higher-voltage brain waves are generated than during waking, less energy is consumed.


Sleep-on neurons, a very small group of brain cells totaling to about 100,000 in humans and found at the base of the forebrain is maximally active during non-REM sleep. These cells seemingly are responsible for inducing sleep, though the exact signals that activate these neurons is not completely understood. Body heat while the individual is awake has shown a clear activation of some of these cells. Drowsiness following a hot bath is an explanation for this.

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