Sleep debt

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

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

Scientific debate
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 contained 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.

Evaluation
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.

A January 2007 study from Washington University in St. Louis suggests that saliva tests of the enzyme amylase could be used to indicate sleep debt, as the enzyme increases its activity in correlation with the length of time a subject has been deprived of sleep.

The control of wakefulness has recently been found to be strongly influenced by the newly discovered protein orexin. A 2009 study from Washington University in St. Louis has illuminated important connections between sleep debt, orexin, and amyloid beta, with the suggestion that the development of Alzheimer's disease could hypothetically be a result of chronic sleep debt or overly long periods of wakefulness.

Across society
National Geographic Magazine has reported that the demands of work, social activities, and the availability of 24-hour home entertainment and Internet access have caused people to sleep less now than in premodern times. USA Today reported in 2007 that most adults in the USA get about an hour less than the average sleep time 40 years ago.

Other researchers have questioned these claims. A 2004 editorial in the journal Sleep stated that according to the available data, the average number of hours of sleep in a 24-hour period has not changed significantly in recent decades among adults. Furthermore, the editorial suggests that there is a range of normal sleep time required by healthy adults, and many indicators used to suggest chronic sleepiness among the population as a whole do not stand up to scientific scrutiny.

A comparison of data collected from the Bureau of Labor Statistics' American Time Use Survey from 1965–1985 and 1998–2001 has been used to show that the median amount of sleep, napping, and resting done by the average adult American has changed by less than 0.7%, from a median of 482 minutes per day from 1965 through 1985, to 479 minutes per day from 1998 through 2001.