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Thursday, June 13, 2024

Understanding The Concept Of Sleep

When the sleep centers are not activated, the release from inhibition of the mesencephalic and upper pontile reticular nuclei allows this region to become spontaneously active. This in turn excites both the cerebral cortex and the peripheral nervous system, both of which send numerous positive feedback signals back to the same reticular nuclei to activate them still further. Once wakefulness begins, it has a natural tendency to sustain itself because of all this positive feedback activity.

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By: Dr. Suman Bharali

Sleep is defined as unconsciousness from which the person can be aroused by sensory or other stimuli.

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Two types of sleep:

  • Slow wave sleep [Brain waves are very slow.]
  • Rapid Eye Movement (REM) sleep [Eye undergoes rapid movement despite the fact that the person is still asleep.]

Slow wave sleep:

  • Most of the sleep during each night is of the slow wave variety.
  • This is deep, restful type of sleep that the person experiences during the first hour of sleep after having been kept awake for many hours.
  • Dreams do occur during this type of sleep but usually are not remembered. That is during slow wave sleep consolidation of the dreams in memory doesn’t occurs.

REM sleep {Paradoxical sleep, Desynchronized sleep}:

  • In a normal night of sleep, bouts of REM sleep lasting 5 to 30 minutes usually appear on the average every 90 minutes.
  • First such period occurs 80 to 100 minutes after the person falls asleep.
  • When the person is extremely sleepy the duration of each bout of REM sleep is short and may even be absent.
  • On the other hand, as the person becomes more and more rested through the night, the duration of the REM bouts greatly increases.
  • Brain is highly active in REM sleep, and the overall brain metabolism may be increased as much as 20 %. This type of sleep is also called paradoxical sleep because it is a paradox that a person can still be asleep despite marked activity in the brain.
  • It is usually associated with active dreaming.
  • The person is even more difficult to arouse by sensory stimuli than during deep slow wave sleep, and yet people usually awaken in the morning during an episode of REM sleep.

Basic theories of sleep:

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  1. Passive theory of sleep: Earlier theory of sleep was that the excitatory areas of the upper brain stem, which were called the reticular activating system, simply fatigued during the period of a waking day and therefore, become inactive as a result.
  2. Now the view is that the sleep is probably caused by anactive inhibitory process.It was found that some center or canters located below the midpontine level of the brain stem that required to cause sleep by inhibiting other parts of the brain.

Neuronal centers, neurohormonal substances, and mechanisms that can cause sleep:

Stimulation of several specific areas of the brain can produce sleep with characteristics near those of natural sleep. Some of these areas are the following:

  1. Raphe nuclei in the lower half of the pons and in the medulla.

Many nerve endings of fibers from these raphe neurons secrete serotonin. When a drug that blocks the formation of serotonin is administered to an animal, the animal cannot sleep for next several days. Therefore, it has been assumed that serotonin is a transmitter substance associated with production of sleep.

  1. Nucleus of the tractus solitarius.
  2. Stimulation of several areas in the diencephalon can also promote sleep, including (a) the rostral part of the hypothalamus, mainly in the suprachiasmal area, and (b) an occasional area in the diffuse nuclei of the thalamus.
  3. One likely substance has been identified as muramyl peptide, a low-molecular-weight substance that accumulates in the cerebrospinal fluid (CSF) and urine in animals kept awake for several days.
  4. Another substance that has similar effects in causing sleep is a non-peptide isolated from the blood of sleeping animals.
  5. A third sleep factor not yet identified molecularly has been isolated from the neuronal tissues of the brain stem of animals kept awake for days.

Possible cause of REM sleep:

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Drugs that mimic the action of acetylcholine increases the occurrence of REM sleep. Therefore, it has been postulated that the large acetylcholine secreting neurons in the upper brain stem reticular formation might, through their extensive efferent fibers activate many portions of the brain. This theoretically could cause excess activity that occurs in certain brain regions in REM sleep, even though the signals are not channelled appropriately in the brain to cause normal conscious awareness that is characteristics of wakefulness.

Cycle between sleep and wakefulness:

When the sleep centers are not activated, the release from inhibition of the mesencephalic and upper pontile reticular nuclei allows this region to become spontaneously active. This in turn excites both the cerebral cortex and the peripheral nervous system, both of which send numerous positive feedback signals back to the same reticular nuclei to activate them still further. Once wakefulness begins, it has a natural tendency to sustain itself because of all this positive feedback activity.

After the brain remains activated for many hours, even the neurons within the activating system presumably will become fatigued. Consequently, the positive feedback cycle between the mesencephalic reticular nuclei and the cortex will fade and the inhibitory effects of the sleep centers will take over, leading to the sleep state.

Then one could postulate that during prolonged sleep, the excitatory neurons of the reticular activating system gradually become more and more excitable because of the prolonged rest, whereas the inhibitory neurons of the sleep centers become less excitable because of their overactivity thus leading to a new cycle of wakefulness.

This overall theory can explain the rapid transition from sleep to wakefulness and from wakefulness to sleep. It can also explain arousal, insomnia that occurs when a person’s mind is preoccupied with a thought, the wakefulness that is produced by bodily activity, and many other conditions that affect the person’s state of sleep or wakefulness.

Sleep at different age

  • New born: Sleeps 17 to 18 hours a day. Spending half of the time in REM sleep.
  • By 5 years of age: 10 to 12 hours a day. Spending 20% of the time in REM sleep.
  • Average young adult: 8 hours of sleep per night to function optimally during working hours.
  • Some people, however, sleep just 6 to 7 hours a night, while others need more than 9 hours to feel rested.
  • Elderly spend less time in deep NREM sleep and their sleep is more easily interrupted.

EEG changes in the different stages of wakefulness and sleep

(Gamma waves), 25 to 100 Hertz, highest state of focus possible, brain running at full capacity

Alert wakefulness (Beta waves), frequency of 12 and 30 Hertz

Quiet wakefulness, (Alpha waves), frequency of 8–12 Hertz

Slow wave sleep (Delta wave), 0.5 and 2 hertz

Here, we observe that more the activity of the brain more is the frequency of the brain wave and as the brain goes to rest the frequency of the brain wave decreases. Thus, all those things which increases the frequency of the brain wave will affect the sleep. For example, bright light, loud sound, electromagnetic radiations, from any source, now a days mostly from electronic devices like TV, Computers, Mobiles etc.

Psychosomatic effects of sleep

Sleep causes two major types of physiological effects:

  1. Effects on the nervous system itself.
  2. Effects on other structures of the body.

Effect of sleep on central nervous system

  • Principle value of sleep is to restore the natural balance among the neuronal centers.
  • Lack of sleep effect the functions of the central nervous system.
  • Prolonged wakefulness is often associated with progressive malfunction of the mind and sometimes even causes abnormal behavioural activities of the nervous system.
  • Sluggishness of thought occurs towards the end of a prolonged wakeful period.
  • A person can become irritable or even psychotic after forced wakefulness for prolonged periods.
  • Overuse of some brain areas during wakefulness could easily throw these out of balance with the remainder of the nervous system.

Effect of sleep on autonomic nervous system

  • During wakefulness, there is enhanced sympathetic activity as well as enhanced number of skeletal nerve impulses to the skeletal musculature to increase muscular tone.
  • During slow wave sleep, sympathetic activity decreases while the parasympathetic activity increases. Therefore, during restful sleep –
    • Arterial blood pressure falls.
    • Pulse rate decreases.
    • Skin vessel dilates.
    • Activity of the gastrointestinal tract sometimes increases.
    • The overall basal metabolic rate of the body fall by 10 to 30 %.
    • Respiratory rate becomes more regular.
    • Minute ventilation decreases out of proportion to decrease the metabolic rate at sleep onset, resulting in a higher Pco2.
  • During REM sleep,
    • Cardiac dysrhythmias may occur selectively.
    • Respiratory rate becomes very irregular.
    • The muscle tone throughout the body is exceedingly depressed indicating strong inhibition of spinal projections from the excitatory areas of the brain stem.
    • Despite extreme inhibition of the peripheral muscles, a few irregular muscle movements occur; these include rapid movements of eyes.

Effect of sleep on endocrine functions

  • Sleep in general is associated with augmented secretion of prolactin.
  • During puberty, sleep is associated with increased luteinizing hormone secretion, whereas sleep in mature women inhibits LH secretion in the early follicular phase of the menstrual cycle.
  • Sleep onset (and probably slow wave sleep) is associated with inhibition of thyroid stimulating hormone and of the adrenocorticotropic hormone – cortisol axis.
  • The pineal hormone melatonin is secreted predominantly at night.
  • Exogenous melatonin increases sleepiness and may potentiate sleep when administered to good sleepers attempting to sleep during daylight hours at a time when endogenous melatonin levels are low.

Effect of sleep on thermoregulatory functions

  • NREM sleep is associated with an attenuation of thermoregulatory responses to either heat or cold stress and animal studies of thermosensitive neurons in the hypothalamus document an NREM sleep dependent reduction of the thermoregulatory set point.
  • REM sleep is associated with complete absence of thermoregulatory responsiveness, effectively resulting in functional poikilothermy. However, the potential adverse impact of this failure of thermoregulation is blunted by inhibition of REM sleep by extreme ambient temperatures.

Sleep in animals

  • All mammals and birds sleep. But it is not confirmed if all invertebrates or insects sleep.
  • Large mammals tend to sleep less than small mammals.
  • Giraffe and elephants sleep only 2 to 4 hours.
  • Bats, opossums and armadillos sleep 18 hours a day.
  • While sleeping most mammals close their eyes and adopt sleep postures. For example, humans sleep by lying straight on their back, a giraffe sleep by kneeling down its feet and turning his long neck around its body etc.
  • Some animals such as dolphins can sleep while they are moving.

(The author is an M.D. Ayurvedic Medicine & can be reached at 9954154192)

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