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Losing Sleep: the Causes and Effects of Sleep Deprivation
Types of Sleep LossSleep experts define sleep deprivation as either partial or total lack of sleep, whether voluntary or involuntary. Sleep deprivation can be either an acute (occasional) or a chronic lack of sleep. Partial sleep deprivation is the term used when an individual gets some, but not all, of the sleep necessary for waking alertness during the day. Partial sleep deprivation can be caused by medical conditions, sleep disorders, as well as lifestyle (e.g., shiftwork, jet lag or working overtime).
Total sleep deprivation is defined as a complete lack of sleep lasting for 16 hours or more in a healthy adult. When total sleep deprivation lasts longer than 24 hours, a divergence occurs between the sleep/wake cycle, which begins to build an escalating sleep debt, and the circadian clock, which maintains its normal cycle. The result is counterintuitive — when we remain awake for 40 hours, we will feel less sleepy at the 36-38 hour point than at the 22-24 hour point.(10)
Fragmented SleepAs many people know, sleep is not a continuous state; it follows a series of stages, including rapid eye movement (REM) and other types of sleep. Sleep fragmentation, a form of partial sleep deprivation, occurs when the normal progression and sequencing of sleep stages is disrupted. If sleep fragmentation is limited to a specific sleep stage, (e.g., when sleep apnea or medications disrupt a particular stage of sleep), this is called selective sleep stage deprivation.
The elderly are particularly prone to this kind of fragmentation and subsequent loss of sleep quality. Selective sleep stage deprivation is characterized by waking up frequently at night, difficulties falling asleep and waking up unusually early in the morning.
Sleep fragmentation is also a symptom of sleep disorders such as obstructive sleep apnea, in which patients experience repetitive nocturnal respiratory pauses that produce chronic sleep deprivation and excessive sleepiness. Narcolepsy, in which patients show recurrent episodes of irresistible sleep (or sleep attacks), cataplexy (sudden, brief, loss of muscle control in response to strong emotions such as laughter or anger), hallucinations and sleep paralysis (the inability to move while falling asleep or awakening) also produces excessive daytime sleepiness.(11)
Parkinson's Disease can also cause daytime sleepiness (up to 45% of cases); roughly 1% of those with Parkinson's are at risk for sleep attacks.(12)(13)(14)
Chronic Sleep DebtSleep debt, or sleep restriction,15 is a common form of partial sleep deprivation. Researchers have studied the changes that occur when sleep is steadily reduced in duration from 8 to 4 hours each day, and the effects of these changes on sleep and waking functions.
Measuring SleepThe effects of chronic sleep restriction are evaluated using one of two tests.(16)(17)(18) In one test, subjects are instructed to close their eyes and try to fall asleep while lying down, during which their sleep patterns are evaluated with a specially-designed instrument called a polysomnograph (PSG). In the other type of test, subjects are seated upright and instructed to try and remain awake. For both tests, sleep propensity is measured as the time is takes to fall asleep.(19)(20)Unsurprisingly, chronic shortening of nocturnal sleep increases daytime sleep propensity.(21)
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