The absolute number of hours of sleep necessary for someone to function properly is not known. Some people can function with full effectiveness with only three to five hours of sleep per night, while others need at least eight hours or more of sleep per night.
A chronically sleep-deprived state can cause tiredness, excessive daytime sleepiness, clumsiness and weight gain. It impairs the normal functioning of the brain. It is impossible for humans to go completely without sleep for long periods of time - brief microsleeps cannot be avoided. Total sleep deprivation has been shown to cause death in lab animals.
Microsleeps occur when someone is significantly sleep-deprived. The brain can automatically shut down, falling into a sleep state that can last from a second to half a minute.
You can fall asleep no matter what you are doing. Microsleeps are similar to blackouts and you will not be aware that they are occurring when you are experiencing them.
Individuals who are sleep-deprived may not recognise the effects of being so. Small amounts of sleep loss over many nights may result in subtle cognitive loss, which appears to go unrecognised by the individual.
More severe sleep deprivation for a week can lead to profound cognitive dysfunction similar to those seen in some stroke patients, which may also appear to go unrecognised by the individual.
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