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Changes in anatomical and biochemical processes in the body are influenced by sleep. We address the physiology of stress, as well as the roles of sleep and the implications of sleep deprivation, in this section.
Sleep is essential for good health. Guyton and Hall described sleep as "a state of unconsciousness from which the individual may be awakened by sensory or other stimuli" (p.51). Sleep is a dynamic phenomenon; sleep is characterized by very particular physiological and behavioral changes. Behavioral aspect of sleep is manifested by humans and animals sleeping at a particular time for a specific duration. However, considerable variation in total hours of sleep required varies among different species, and also among individuals of a given species. Variation also exists between the preferred timings for sleep in humans. Studies monitoring the sleep cycle of several individuals classified them into two groups: 'M-type' or morning type, and 'E-type' or evening type of people. Sleep also correlates with drop in body temperature (p.52-53).
Melatonin hormone, secreted in response to low light is an important regulator of sleep in humans. Melatonin is conditions. As the retina detects low light intensity in the surrounding, it signals the pineal gland in the brain via optic nerves to secrete the hormone melatonin (p.54). Most of our current understanding of the neurophysiology of sleep has been obtained from EEG studies, which revealed two types or phases of sleep: rapid eye movement (REM) sleep and non-REM (NREM) sleep. Brain activity during REM sleep is similar to that during wakefulness, but the muscles of the body are in a state of relaxation. NREM is a state of deep sleep, and occurs in the early hours of sleep, while the REM states are common during the later hours. Awakening a person from deep sleep leads to mental confusion and poor physical coordination in the awakened individual; waking up from REM sleep, however, allows the individual to regain alertness quickly, and function normally (p.56-57).
The biological clock, located in the suprachiasmatic nuclei (SCN) of the hypothalamus of the brain controls the timing and duration of sleep. The existence of biological clock in humans and other animals was proven by several studies conducted by decoupling the subjects' sleep cycle from environmental conditions. In these "free-running experiments" it was shown that the biological clock is rather flexible, and responds strongly to external cues of light and darkness (p.58-60). And once it is established, sudden changes are resisted, as for example the effect on sleep pattern following jet-lag in individuals travelling across time zones.
Sleep plays a crucial role in maintaining "homeostatic equilibrium" in organisms. Research has shown that 3-4 hours of sleep deprivation in a week reduced carbohydrate processing, stress management, and affected the hormonal balance in the body (p.67). Sleep also affects leptin secretion, which in turn affects an individual's appetite and body weight. A major function of sleep is to conserve energy by maintaining a state of inactivity, at times when the organism will most likely be unable to function best. Sleep has also been shown to enhance tissue repair, and REM sleep has been linked with improved memory and brain functionalities. Sleep deprivation adversely affects the physiology and general state of health by negating the beneficial effects of regular sleep (p.66-69). The restorative role of sleep is especially apparent when an individual requires more sleep while suffering from an infection. While deprivation of REM sleep has moderately adverse effects, studies on the deprivation of NREM sleep have been linked to several deleterious effects including lowered recuperating capacity in the studied subjects (p.68). Sleep is therefore indispensable for keeping the organism healthy.
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