Sleep is defined as a condition of body and mind such as that which typically recurs for several hours every night, in which the nervous system is relatively inactive, the eyes closed, the postural muscles relaxed, and consciousness practically suspended. It is a state of unconsciousness in which the brain is relatively more responsive to internal than external stimuli. The predictable cycle of sleep and the reversal of relative external unresponsiveness are features that assist in distinguishing sleep from other states of unconsciousness.
Circadian sleep rhythm is one of the several intrinsic body rhythms modulated by the hypothalamus. It sets the body clock to approximately 24.2 hours, with both light exposure and schedule clues entraining to the 24.2-hour cycle.
The hypothalamus allows light cues to be of direct influence. Light is called a zeitgeber, a German word meaning time-giver because it sets the suprachiasmatic clock. Examples of other external zeitgebers are exercise, social activities, and mealtimes. A practical purpose has been proposed for the circadian rhythm, using the analogy of the brain functioning somewhat like a battery charging during sleep and discharging during the wake period.
With decreased sleep, higher-order cognitive tasks are affected early and disproportionately. Tests requiring both speed and accuracy demonstrate considerably slowed speed before accuracy begins to fail. Total sleep duration of seven hours per night over one week has resulted in decreased speed in tasks of both simple reaction time and more demanding computer-generated mathematical problem-solving. Total sleep duration of five hours per night over 1 week shows both reduction in speed and the beginning of the failure of accuracy.
Total sleep duration of seven hours per night over a one-week period leads to impairment of cognitive work requiring a simultaneous focus on several tasks. In driving simulations, for example, accidents increase progressively as total sleep duration is decreased to seven, five, and three hours per night over one week. In the same simulations, three hours total sleep duration was associated with loss of ability to simultaneously appreciate peripheral and centrally presented visual stimuli, which could be termed as a form of visual simultanagnosia and peripheral visual neglect.
In tasks requiring judgment, increasingly risky behaviors emerge as the total sleep duration is limited to five hours per night. The high cost of action seemingly is ignored as the sleep-deprived individual focuses on limited benefit.
BSM Medical Centre