Sleep debt

Sleep debt or sleep deficit is the cumulative effect of not getting enough sleep. A large sleep debt may lead to mental or physical fatigue.

There are two kinds of sleep debt, the results of partial sleep deprivation and total sleep deprivation. Partial sleep deprivation occurs when a person or a lab animal sleeps too little for several days or weeks. Total sleep deprivation means being kept awake for at least 24 hours. There is debate in the scientific community over the specifics of sleep debt, and it is not considered to be a disorder.

There is debate among researchers as to whether the concept of sleep debt describes a measurable phenomenon. The September 2004 issue of the journal Sleep contains dueling editorials from two leading sleep researchers, David F. Dinges and Jim Horne. A 1997 experiment conducted by psychiatrists at the University of Pennsylvania School of Medicine suggested that cumulative nocturnal sleep debt affects daytime sleepiness, particularly on the first, second, sixth, and seventh days of sleep restriction.

In one study, subjects were tested using the psychomotor vigilance task (PVT). Different groups of people were tested with different sleep times for two weeks: 8 hours, 6 hours, 4 hours, and total sleep deprivation. Each day they were tested for the number of lapses on the PVT. The results showed that as time went by, each group's performance worsened, with no sign of any stopping point. Moderate sleep deprivation was found to be detrimental; people who slept 6 hours a night for 10 days had similar results to those who were completely sleep deprived for 1 day.

Sleep debt has been tested in a number of studies through the use of a sleep onset latency test. This test attempts to measure how easily a person can fall asleep. When this test is done several times during a day, it is called a multiple sleep latency test (MSLT). The subject is told to go to sleep and is awakened after determining the amount of time it took to fall asleep. The Epworth Sleepiness Scale (ESS), an eight item questionnaire with scores ranging from 0 to 24, is another tool used to screen for potential sleep debt.

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