Physiology of Sleep

Theories of sleep

Why do we sleep?

       Sleep plays a vital role in our daily lives. We spend more time sleeping than doing any other single activity. We abandon vigilance in order to sleep, we render ourselves vulnerable when we sleep. But, what is sleep for? And why do we spend so much time sleeping? How do the benefits of sleep outweigh the risks? Lets take a look at different theories.

Restorative theories

        Sleep helps our body save and restore energy by slowing down our metabolism. By lowering our metabolism, we conserve energy. While we sleep, the brain's learning processes are turned off. Still another theory predicts that sleep helps replenish our stores of neurotransmitters. This is because most neurons decrease their activity during sleep.

       Slow wave sleep has restorative effects. It seems like a period of rest for the brain. Without rest, our brain cannot function properly. Brain areas that are very active during wakefulness show an increase in slow wave sleep, as if they were resting, restoring themselves.

Learning theories

       During sleep, we can reorganize and store information. Neurons that are involved in memory and attentive learning do rest in sleep, specially during REM.[i]Maybe this is why we feel mentally sharper when we have had a good night of sleep, compared to how we feel after staying up all night long.

       REM sleep plays a role in memory retention and consolidation. Many studies support this theory, here is one: There are two groups of students, who had learned the same material the evening before. One group was deprived of REM sleep during one night, the other group slept normally without interruption. The group with REM sleep deprivation showed that their retention of material covered was poor compared to the other group that slept without interruption. Their retention of complex material (stories) was greatly reduced by REM sleep deprivation. Still, other theories say that sleeping, particularly REM sleep is designed to remove useless information from memory. This theory suggest that it is of equal importance to remove unwanted information and to store important data. Memory has to work two ways: one for storage of important information and one for removal of trivial information.

Developmental theories

       Other theories say that sleep plays a role in the development of the brain. REM sleep is a major component of sleep for babies in utero and infants. REM sleep is thought to activate visual, motor and sensory areas in the brain. In the baby's brain, this increases the ability of neurons to make the correct connections and function properly. For example, observation of lamb fetuses through plexiglass windows implanted on the uterine walls, has shown that during REM sleep, the fetuses, without having air to breathe do move their chest as if they were breathing. This suggest that REM plays a role in preparing, teaching us how to behave, even before we come out to this world.

       Another theory proposes that sleep is an adaptive behavior to keep us away from trouble at night. In a natural environment we are daytime beings; in the darkness of night we cannot perform the same activities we engage during the day. So by sleeping we simply refrain from being active at night when our vision is poor and this keeps us away from trouble.


       Perhaps sleep has all these functions and more that we do not know about yet. The bottom line is that sleep is so essential that long periods of sleep depravation eventually results in stress-related deaths (this has happened with lab animals, the same could happen with us). Missing even one single night of sleep may affect our mental performance in decision making on the next day.

       Fatal familial insomnia is an inherited disorder characterized by the slow withdrawal of slow wave sleep until it disappears. There is only small bouts of REM sleep without paralysis when the patient sleeps. The symptoms include confusion, loss of control over the endocrine system and autonomic nervous system activities, insomnia, decreased attention and diminished memory. The disorder is fatal when advanced and it shows how important sleep really is.

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  • Stages of Sleep
  • Physiology of Sleep
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