Below is an answer to the simple question what is sleep?
What is Sleep?
Physiologically, sleep is regarded as absence of wakefulness, where the responses to environmental stimuli are greatly reduced.
But, in fact, it is an active state, related to definite anatomic structures as well to specific biogenic amines. Yet, its exact mechanism is not known.
The determinants of natural sleep are many but the most important regulator is probably a “central pacemaker” (or the biological clock) in the suprachiasmatic nuclei of the hypothalamus.
The normal circadian rhythm is related to production of melatonin by the pineal gland. We all need sleep. It is believed that restoration of natural balance among the neuronal centers in the brain takes place chiefly during sleep, and the association between sleep and growth in the early years of life is generally accepted.
What is NREM sleep and What is REM sleep?
The types of sleep are basically of 2 types.
Based on electrophysiological studies (EEG, clectromyogram and electro-oculogram), sleep has been classified into two types:
Non Rapid Eye Movement sleep (NREM); and
Rapid Eye Movement sleep (REM).
While falling asleep, one passes sequentially through stages 1, 2, 3 and 4 of NREM sleep. After about 90-120 minutes of NREM sleep, REM sleep occurs, lasting for 5-30 minutes.
The NREM-REM cycle repeats 4-5 times during the night, with progressive lengthening of the REM bouts until one awakens from REM (not NREM) sleep in the morning.
There are important differences between NREM and REM sleep.
NREM sleep is very peaceful with preponderance of the parasympathetic activity and diminution of the metabolic rate, heart rate, cardiac output and peripheral vascular resistance. Dreaming is infrequent and the dreams are rarely recalled on awakening.
On the other hand, the sympathetic activity predominates during REM sleep. The sleep is not so restful; 75% of the dreams occur in this type of sleep; the dreams tend to be more vivid, bizarre and often sexual, with erections occurring in the males.
Dreams are accompanied by appropriate cardiovascular responses to the perceived dream activities such as running or escaping. As a result, the heart rate, BP, cardiac output, peripheral vascular resistance, small airway resistance and metabolic rate rise markedly. Short periods of central apnea may occur during REM.
During NREM sleep, the EEG shows alpha rhythm together with sleep spindles. The sleep becomes deeper during the four stages of NREM sleep and it becomes progressively less easy to awaken a person from this type of sleep.
On the other hand, during REM sleep the EEG resembles that of an awake and alert person (it shows a beta rhythm) and the brain is highly active with increased oxygen consumption. In spite of this, it is difficult to awaken a person from REM sleep.
There is no eyeball movement during NREM sleep, whereas the eyeballs move rapidly and jerkily during REM sleep; hence the name.
Muscle tone diminishes progressively during NREM sleep, but the muscles which hold the chin up and keep the middle respiratory passages open are active. By contrast, all voluntary muscles except extraoccular muscles are profoundly flaccid during REM sleep.
- Growth hormone secretion occurs during stages 3 and 4 of NREM sleep, but none during REM sleep.
REM sleep normally occupies about 25% of the sleeping time; this pattern is altered by various diseases, sleep disorders and drugs. The notion that, sleep is a uniformly quiescent and peaceful state, and therefore devoid of stress, is not correct. Both NREM and REM types of sleep expose a person to different types of stress.
For example, relative hypotension during stage 4 of NREM sleep accounts for some of the ischemic cerebral-vascular strokes during sleep. On the other hand, the extreme hypotonia of the small muscles that hold the middle respiratory passages open can lead to obstructive sleep apnoea, with possible disastrous consequences, during REM sleep. Further, the rise in the cardiovascular parameters due to catecholamine secretion during REM sleep can lead to hypertensive-hypoxic cardiovascular events.
A normal person spends about one-third of his life in sleep. Adequate sleep is a necessity of life. In practice, many individuals complain of lack of sleep, insomnia.
A hypnotic drug is one which produces sleep resembling natural sleep. A sedative, on the other hand, is a drug that reduces excitement, calms the patient, and is commonly used as an anxiolytic. Hypnotics and sedatives both depress the CNS, the difference being quantitative.
What is Sleep Apnea?
Sleep Apnea is the temporary cessation of breathing that can occur during sleeping especially in overweight individuals due to airway obstruction. This usually happens in deep sleep.
What is Sleep Deprivation?
Sleep deprivation occurs when there is inadequate sleep like in insomnia and can cause long term health problems.
What is Sleep Walking?
Sleep walking i s also called as Norcolepsy. It is the involuntary walking during sleep, of which the person effected does not know anything of.
This was in brief about what is sleep.
If you have any similar questions you can Ask a Doctor here.