Sleep debt

# Overview
Sleep debt is the cumulative effect of not getting enough sleep. A large sleep debt, for example, would suggest that a person is mentally or physically fatigued due to insufficient sleep. There is debate in the scientific community over the specifics of sleep debt.

# Scientific skepticism of sleep debt
There is debate among researchers as to whether there is such a thing as sleep debt.  The Sept 2004 issue of the journal Sleep contained dueling editorials from two of the world's leading sleep researchers: David F. Dinges and Jim Horne.  The popular understanding within the journal that sleep debt can be accumulated indefinitely has however been disproven and is no longer considered plausible.  A 1997 experiment conducted by psychiatrists at the University of Pennyslvania School of Medicine   suggested that cumulative nocturnal sleep debt affects daytime sleepiness, particularly on the first and second, and the sixth and seventh days of sleep restriction.

# Evaluating sleep debt
Sleep debt has been tested/experimented in a number of studies, most notably by Klerman and Dijk through the use of a sleep latency test. This test attempts to measure how easily someone can fall asleep. When this test is done several times during the day, it is called a multiple sleep latency test (MSLT).
However, one does not have to go to a sleep clinic to try this experiment; a home process has been considered: it involves relaxing quietly and alone for a short amount of time.  If the feeling of sleep comes fairly easily, one is considered to have sleep debt. Some also suggest that the quality of sleep can have an effect on the level of one's sleep debt.
The Epworth Sleepiness Scale (ESS) has also been used to measure potential sleep debt along with a variety of other evaluations.  Specifically, the ESS; created by Australian researchers, is a simple eight item questionnaire with scores ranging 0-24. 
A January 2007 study 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.

# Society-wide sleep debt
The National Geographic Magazine 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.  The research behind this article has led some organizations, including the National Sleep Foundation, to create educational campaigns to draw attention to sleep debt.
However, Jim Horne of Loughborough University, a Sleep Researcher, questions such claims.  In a 2004 editorial in the journal Sleep, he notes that available data suggests that the average number of hours of sleep in a 24-hour period has not changed significantly in recent decades.  Comparing data collected from the Bureau of Labor Statistics' American Time Use Survey from 1965-1985 and 1998-2001, shows that the median amount of sleep, napping, and resting done by the average 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.  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.