Physiology of sleep

Sleep is defined as aroused by sensory or other stimuli. There are multiple stages of sleep, form very light sleep to very deep sleep.

Types of sleep:
Slow-wave sleep:

• Most sleep during each night is of the slow-wave variety this is deep, restful sleep that the person experience during the first hour of sleep.
• Slow-wave sleep is frequently called "dreamless sleep".
• Sometimes dreams and night mare do occur in slow-wave sleep. But those dreams are not remembered because consolidation of the dreams in memory doesn't occur.

REM(Rapid Eye Movement) sleep:

• Rapid Eye movement sleep occupies 25% of the sleep time in young adults. Each episode normally reoccurs about every 90 minutes.
• This type of sleep is not so restful, It is usually associated with vivid dreaming with bodily muscle movement.
• Muscle tone throughout the body is exceedingly depressed, Indicating strong inhibition of the spinal muscle control areas.
• Heart rate and respiratory rate usually become irregular, Which is characteristic of the dream sleep.
• The brain is highly active in REM sleep, and overall brain metabolism may be increased as much as 20 percent.

Mechanism that cause sleep- A possible specific role for Serotonin :

• The most conspicuous stimulation area for causing almost natural sleep is the raphe nuclei in the lower half of the pons and in the medulla. Many nerve endings of fibers from these raphe neurons secrete serotonin.
• Stimulation of some areas in the nucleus of the tractus solitarius can also cause sleep.
• Stimulation of several regions in the diencephalon including, 1. rostral part of the hypothalamus, 2. an occasional area in the diffuse nuclei of the thalamus can promote sleep.

Cause for intense wakefulness:

• Discrete lesions in sleep-promoting centers(raphe nuclei) lead to a high state of wakefulness.
• The excitatory nuclei of the mesencephalon and upper pons seem to become released from inhibition, thus causing the intense wakefulness.
• Indeed, sometimes lesions in the anterior hypothalamus can cause such intense wakefulness.

Basic theories of sleep:

Sleep is believed to be caused by an active inhibitory process. A center located below the midpontile level of the brain stem appears to be required to cause sleep by inhibiting others parts of the brain.

Sleep-wakefulness cycle:

• When the sleep centers are not activated.The mesencephalic and upper pontile reticular activating nuclei are released from inhibition, which allows reticular activating nuclei to become spontaneously active. This in turn excites both the cerebral cortex and peripheral nervous system.
• Then, after the brain remains activated for many hours, event the neurons themselves in the activating system become fatigued which leads to rapid transition from wakefulness back to sleep.

Functions of sleep:

Sleep has postulated to serve many functions,

• Neural maturation
• Facilitation of learning or memory
• Cognition
• Conservation of energy

In the bottom line,

Principle value of sleep is to restore natural balances among the neuronal centers, the specific physiologic functions of sleep. However, remain a mystery and they are the subject of much research.