Highlights:

      ✍️what is sleep?

      ✍️Sleep-wake cycle

      ✍️ Theories of sleep

      ✍️ Genesis of Sleep

      ✍️ Factors affecting sleep

      ✍️ Physiological significance of sleep

A. What is Sleep?

     •sleep is referred to the state of unconsciousness from which sensory or other stimuli can arouse the individual.

     •, in other words, sleep is considered to occur as a negative phenomenon, i.e., inhibition of wakefulness center in the posterior hypothalamus by the anterior hypothalamus also contributes to the occurrence of sleep.

    ✍️ Lesion of the posterior hypothalamus produces severe coma.

B. Sleep-Wake Cycle:

    •sleep and wakefulness, like many of the body's regulatory mechanisms, have a circadian rhythm of about 24 hours.

   • A newborn infant has many cycles of sleep and wakefulness in 24 hours, but after 2 years, a single sleep-wake cycle is established.

   • in the normal adult, the sleep-wake cycle consists of 7-8 hours of sleep and 16-17 hours of wakefulness.

C. Control of Sleep-Wake Cycle:

   • sleep-wake cycle, like other circadian rhythms, is endogenous.

   ✍️The biological clock controlling the circadian rhythms is the Suprachiasmatic Nucleus of the anterior hypothalamus.

   • The circadian rhythm is endogenous and can persist without environmental cues; however, under normal circumstances, the rhythms are modulated by external timing cues called Zeitgebers(time givers) that adapt the rhythm to the environment.

   •Sunlight is a powerful timing cue. Light enters this circadian rhythm using the retinohypothalamic tract.

   • Although the Suprachiasmatic nucleus regulates the timing of sleep, it is not responsible for the Genesis of sleep itself.

D. Theories of Sleep:

   1. Pavlov's Theory:

        ✍️ conditioned stimulation of reticular formation causes sleep, i.e., rocking movements, Lullaby songs, Stocking of head and back, Brushing hairs as in children and New-born.

        ✍️ In adults reading, messaging, brushing, and pulling hairs induce sleep. 

   2. Kleitman's Theory:

        ✍️ Reduction in muscle tone and reduced afferents impulses from the periphery by which the cerebral cortex remains inactive induces sleep, i.e., monotony brings sleep.

  3. Biochemical Basis:

        ✍️ Substance like Acetylcholine, Serotonin,5-HT, Nor-Adrenaline, Adenosine, and sleep-inducing peptide also responsible for inducing sleep.

E. Genesis of Sleep:

          • The sleep does not result from the passive withdrawal of arousal due to RAS fatigue as through earlier.

          • Now, it is established that the sleep is produced by an active process, which is different for non-REM sleep and REM sleep.

1. Genesis of Non-REM Sleep:

          • The Non-REM sleep is generated by the interaction of neurons, which are grouped as:

              - Diencephalic sleep zone

              - Medullary synchronizing zone &

              - Basal forebrain sleep zone

✍️ Diencephalic sleep zone:

          • it lies in the hypothalamus and nearby intra-laminar and anterior hypothalamus nuclei.

          • A sleep facilitatory center is considered to be located in the anterior hypothalamus, as it's stimulation causes sleep.

         • posterior hypothalamus acts as a waking center as it's stimulation causes wakefulness.

         • The Diencephalic sleep zone must be stimulated at low frequency (about 8 Hz) to produce sleep.

 ✍️ Medullary synchronizing zone:

              • it lies in the reticular formation of the medulla oblongata at the level of the nucleus of the tractus solitarius.

              • like the Diencephalic sleep zone, this zone also produces sleep when stimulated at low frequency.

✍️ Basal forebrain sleep zone:

           • it includes the pre-optic area and the diagonal area of Broca.

           •, unlike the other two zones (described above), stimulation of this zone at low and high frequency.

✍️ The activity of Non-REM on cells:

          • The Non-REM cells are GABAergic inhibitory neurons that mediate the sleep-inducing action of the above-described sleep zones.

           •  These cells are thought to produce sleep by inhibiting the histaminergic cells in the posterior hypothalamus and nucleus Reticularis Pontis Oralis (RPO)  in the midbrain that mediate arousal.

✍️ Mechanism of production of sleep spindles and slow waves of Non-REM Sleep:

       •The Non-REM sleep is characterized by the EEG( Electroencephalogram) spindles and slow waves produced by synchronized postsynaptic potentials in the cortical neurons.

       • These synchronized postsynaptic potentials are generated by the rhythmic firing of thalamic relay neurons that project to the cortex.

       • The rhythmic firing of thalamic relay neurons results from the action of GABAergic inhibitory neurons in the nucleus reticularis that forms the shell around the thalamus.

2. Genesis of REM sleep:

            • Rapid eye movement sleep is generated by neurons' interaction in the caudal midbrain and pons with the neurons in the medulla oblongata and forebrain.

           • REM sleep is characterized by:

                 - Blockage of EEG spindles and slow waves,

                  - Occurrence of PGO waves

                  - Muscle Antonia

                  - Phasic motor action

✍️ Role of Cholinergic neurons of the midbrain and the adjacent dorsal pons:

           • These cells form an important component of the mid-brain arousal system and are maximally active during waking and REM sleep.

           • There activity contributes to the blocking of the slow waves of EEG.

✍️ Role of Nucleus Reticularis Pontis Oralis (RPO):

           • Three classes of neurons in the RPO of particular interest are:

1. Cholinergic PGO-on cells:

           • The discharge of these neurons produces the PGO spikes that are the characteristic feature of REM sleep.

2. REM waking-on cells of RPO:

          • These cells fire at a high rate during active waking as well as REM sleep.

          • some of These cells project to the motor neurons in the spinal cord, and other projects to motor neurons that drive the extracurricular muscles.

         • Bursts firing of REM-waking-on cells during REM sleep produces rapid eye movements and muscle twitches.

3. REM-on cells of RPO:

          • it shows a high level of activity during REM sleep but has very little or no activity during waking and Non-REM sleep.

           • Although, few of these cells play a key role in REM sleep.

F. Factors affecting sleep:

     • sleep time remains fairly stable from day to day, even under widely varying conditions, and is modestly affected by variations in activity and sensory stimuli. however, the factors which minimize sensory stimulation and favor the onset of natural sleep are:

       1. Darkened room,

       2. Comfortable surrounding temperature,

       3. Silence

       4. Physical and mental relaxation,

       5. Consumption of a basic urge, such as hunger or sex and

       6. Low-frequency stimulation, such as patting or knocking in a cradle or sitting in a moving vehicle.

G.Physiological Significance of Sleep: 

   1.Sleep serve as a period of body's rest and metabolic restoration:

         • pulsatile release of growth hormone and gonadotropin from the anterior pituitary gland.

         • Decrease in blood pressure, heart rate, and respiration

   2.Sleep is necessary for certain forms of learning and memory.

         • sleep is considered to be responsible for the conversion of short-term memory into long-term memory. And also responsible for developing certain methods of learning.

       • In experimental animals, learning sessions do not improve performance until a period of Slow Wave Sleep (SWS) or SWS plus REM  sleep has occurred.

        • However, it is not known why sleep is necessary, and there is no clinical correlation to this experimental observation.

   3.REM sleep is necessary for mental well being.

       •the correlation between dreaming and REM sleep indicates that the brain is highly active at this time. This may allow for the expression through dreams.

       • in most human beings, the dreams that come in sleep are also responsible for the motivation of Full fill that dream or doing work afterward person's life becomes changed.

   4.REMsleep plays an important role in the homeostatic mechanism.

         •, it is evident from the observation that when the experimental animals are completely deprived of REM sleep for long periods, they lose weight despite eased calorie intake and finally die.

        • certain world records of a long time waking without taking sleep also there but without taking sleep person's behavior become continually changing and eventually die.