Sleep is defined as a reversible state of behavior with reduced responsiveness and interaction with the environment. Sleep has a role to play in the conservation of energy, detoxification processes, restorative functions, in learning and memory consolidation.
The regulation of sleep is mediated by the interactions between homeostatic and circadian processes. Research indicates that inadequate sleep could lead to hypertension, diabetes, depression and early death. Lack of sleep may also be attributed to metabolic, hormonal and cognitive deficits. Insufficient sleep also affects memory, concentration, judgment and decision making.
Sleep and wakefulness are mutually exclusive behavioral states which occur in alternate cycles. Sleep and wakefulness have a circadian periodicity of about 24 hours. Circadian rhythms are endogenous and require an internal clock for its regulation. In mammalian systems, the Suprachiasmatic Nucleus (SCN) of the anterior hypothalamus functions as the internal clock.
However, under normal circumstances, external cues called zeitgebers entrain the circadian rhythm in response to environmental conditions. Sunlight is a powerful timing cue in modulating the active phase of the circadian rhythm in some animals. Thus, most adult humans sleep at night when it is dark.
For the purpose of the study, sleep has been defined by the combination of behavioral observation and electrophysiological recording. There are at least three basic states of consciousness in mammals – Wakefulness, Rapid Eye Movement Sleep (REMS) and Non-REM sleep (NREMS). REMS is characterized by the periodic recurrence of rapid eye movements and a loss of muscle tone.
The dramatic reduction in the amplitude of EEG waves during REMS closely resembles that of waking. Thus, REMS is also called as paradoxical sleep. Most of the dreams are associated with this stage of sleep. On the contrary, NREMS is characterized by a reduction in the physiological activity and an increase in the amplitude of EEG waves.
According to ancient historical records, Greek philosophers had speculated about the nature of sleep and conscious behavior. However, the event that marked the beginning of modern sleep research was the discovery of the human electroencephalogram (EEG), indicative of the electrical activity of the human brain.
By identifying the cyclic pattern of REMS and NREMS by an overnight recording of the EEG and Electro-oculogram (EOG), electrical activity of the eye muscles, it was understood that during the night, 4-5 cycles of REMS occur. NREMS has thus been subdivided into four distinct stages ranging from the stage of lightest sleep to the stage of deepest sleep.
By EEG, EOG and Electromyogram (EMG) recording the electrical activity of muscles, sleep and wakefulness have been divided into the following stages:
• Active wakefulness: In this stage, the subject is alert and physically active. The EEG pattern is desynchronized, the EMG is characterized by increased muscle tone and the EOG shows frequent and repeated eye movements.
• Quiet wakefulness: In this stage, the subject is awake and relaxed but calm. Thus, a predominantly desynchronized EEG pattern, a reduced muscle tone in the EMG and reduced eye movement in the EOG are recorded.
• Slow Wave Sleep: This is the initial stage of sleep characterized by low-frequency EEG waves for nearly 50% of the recorded time. The EMG and EOG recordings indicate a reduced muscle tone and eye movement respectively.
• Deep Sleep (DS): This is the deepest stage of sleep in which the EEG frequency is further reduced, EMG recordings imply a reduction in the muscle tone and very little or no eye movement occurs. Both SWS and DS together constitute NREMS.
• REMS: The EEG resembles that of active wakefulness. REMS is represented by low frequency, high amplitude EEG, EMG, and EOG wave patterns.
The neural regulation of sleep cycle is mediated by the interplay of neurotransmitters such as norepinephrine, serotonin, GABA, and dopamine. Primarily associated with various cognitive functions, these neural associations across different brain areas primarily nuclei in the pons, brain stem, and midbrain regulates the sleep-wake cycle.
More often the neurotransmitters which are associated with an integral role in sleep regulation have also been implicated in behavioral disorders such as Attention Deficit Hyperactivity Disorder, neuropsychiatric conditions such as depression, neurodegenerative disorders such Alzheimer’s and aggression, anxiety and other biological parameters.
Sleep is responsible for maintaining the homeostatic functions in the body and deficits associated with it may have severe consequences regarding neurological disorders and cognitive impairment. To keep the mind and body healthy, a sleep duration of about 8 to 9 hours is recommended to rejuvenate our biological systems and ensure our well-being.