Sleep debt is the result of insufficient sleep. It’s also called sleep deficit, and it can have serious effects on a person’s physical and mental health. Sleep deficit is cumulative, so the longer a person goes without sleep, the deeper the fatigue they suffer and the more extreme the changes in their thoughts and behaviors.

Two Types of Sleep Deficit

There are two kinds of sleep deficit that are recognized: total and partial.

Total sleep deprivation. This occurs when someone stays awake for at least 24 hours. The wakefulness may be involuntary through insomnia or voluntary by purposely staying awake.

Partial sleep deprivation. This occurs when someone sleeps for only limited stretches over the course of days or weeks. The amount of sleep the person gets is not sufficient for them to feel rested or refreshed.

The Course of Sleep Deficit

The scientific journal “Sleep” publishes research by leading sleep scientists. The September 2004 issue contains editorials by Jim Horne and David F. Dinges that highlight different results from studies on the course of sleep debt.

A 1977 study at the University of Pennsylvania School of Medicine found that sleep deficit translates into daytime sleepiness on very specific days. Cumulative nocturnal debt affected the study subjects on days one, two, six and seven of sleep deprivation.

In a different study, the Psychomotor Vigilance Task was used to evaluate the effects of sleep deficit. This study found that the effects were steadily cumulative over the course of the study rather than peaking on certain days, and they did not appear to hit a ceiling.

In this second study, the PVT test was given daily over a two-week period to the study subjects. The subjects were allowed to sleep for four, six or eight hours per night, or they endured total sleep deprivation. For all groups, their performance on the PVT declined over the two week period. Even subjects that were allowed to sleep six hours per night showed marked deficits. Their performance over a 10-day period resembled the performance of subjects that had been sleep deprived completely for one day.

Measuring Sleep Deficit

Scientists who study sleep debt have several tests and measures available to use in their research.

– The Sleep Onset Latency Test. This test measures how long it takes a person to fall asleep. When it’s used repeatedly over the course of a day, it’s called the Multiple Sleep Latency Test. In this case, the person is allowed to sleep for some set interval. After being awakened, they’re allowed to go back to sleep, and the researchers again measure how long this takes.

– The Epworth Sleepiness Scale. The ESS is a survey that asks the subjects to report how likely it is that they would fall asleep in various circumstances. It includes situations like watching TV and sitting in a stopped car. The subject can choose from four options that range from “Would never nod off” to “High chance of nodding off.”

– Amylase testing. Researchers at Washington University in St. Louis reported in 2007 that the level of amylase in saliva may be a valid measure of sleep deficit. Amylase is an enzyme that’s always present in saliva, and the Washington University study found that its activity correlates with how long the subjects have been deprived of sleep.

– Orexin manipulation. Orexin is a protein that’s involved in regulating wakefulness. Orexin keeps a person awake, but orexin antagonists may be used to produce sleep. Some interesting connections have been found between orexin and amyloid beta, which is implicated in Alzheimer’s disease. Further research may discover a link between chronic sleep deficit and Alzheimer’s.

Cultural Causes of Sleep Deficit

In 2007, USA Today reported that American adults were sleeping about an hour less per night than they had 40 years previously. Some researchers believe this may be a result of modern technology like 24-hour television channels and the internet. The ease of email and texting may also pressure many people to feel that they must always be “on” even though messages interrupt their sleep.