Researchers in the US found that chronic sleep loss is not easy to recoup and severely impairs performance later in the day, particularly late at night when performance is naturally low. The study, which examines how sleep lost in the short term (last night) and the long term (over several weeks) combine with the body's natural circadian rhythm in our 24-hour internal clock, appears to dispel the idea that one can quickly recover from chronic sleep loss by just sleeping really well for a couple of days.

The researchers said the findings suggest people with occupations that potentially affect health and safety in a critical way, like surgeons, lorry drivers and heavy machine operators, should make sure they don't accumulate sleep loss or they may find their performance is so reduced it becomes dangerous.

The study appears in the 13 January issue of Science Translational Medicine and is the work of lead author Dr, Daniel Cohen, a researcher in the Division of Sleep Medicine at Brigham and Women's Hospital (BWH) in Boston, Massachussets, USA and colleagues. It was sponsored by funds from the Air Force Office of Scientific Research, the National Institutes of Health, and the National Space Biomedical Research Institute.

Cohen told the media that:

"Many people have a false sense of reassurance that they can quickly recover from a chronic sleep debt with just one or two days of good sleep."

He said their research showed that while one long night of sleep can bring performance back to normal levels for about six hours after waking, and the late afternoon and early evening alerting signal of the circadian rhythm can mask the effects of chronic sleep loss for the rest of the day, when people with chronic sleep loss try and stay awake for an extended period, such as "pull an all-nighter" they find that:

"The lingering effect of chronic sleep loss causes performance to deteriorate dramatically."

The researchers discovered that when individuals with a history of chronic sleep loss try to work extra hours into the night, their reaction times become about 10 times slower than what they were earlier in the day, increasing the risk of accidents and errors.

Although we don't know exactly why we sleep, we do know that it is necessary for optimal performance, including cognitive functioning like memory and thinking and learning, and physiological functioning such as in the cardiovascular, metabolic and immune systems, said the researchers.

The body's 24-hour circadian rhythm and cycles of being awake and being asleep are intertwined, so Cohen and colleagues designed a way to tease them apart for the study.

They recruited 9 healthy volunteers and asked them to follow a pattern of sleep and awake that was out of step with the natural 24-hour cycle. So for three weeks, the volunteers lived a 43-hour day comprising 32.85 hours of scheduled wakefulness and 10 hours of scheduled sleep, an equivalent of 5.6 hours sleep for every 24 hours or a reduced sleep-to-wake ratio of 1:3.3.

This way the researchers could look at the sleep-wake cycle unhooked from the body's natural circadian cycle, because, as they explained:

"Sleep and wake episodes were distributed across all circadian phases, enabling measurement of the effects of acute and chronic sleep loss at different times of the circadian day and night."

They found that:
  • After a 10-hour sleep, the volunteers performed well at first, but performance fell as the 33-hour day progressed.

  • As the chronic sleep debt increased, volunteers' performance on reaction time tests got poorer faster for each hour awake, despite being within normal limits when they first awoke.

  • The reaction times were always slower, especially with acute and chronic sleep loss, when the body's natural 24-hour circadian rhythm was at the lowest-performing point in the late night/early hours of the morning.

  • Reaction times were relatively normal, even with substantial acute and chronic sleep loss, when the circadian rhythm was at the highest-performing point in the late afternoon/early evening.
The researchers concluded that:

"Thus, extended wake during the circadian night reveals the cumulative detrimental effects of chronic sleep loss on performance, with potential adverse health and safety consequences."

Senior author Dr Elizabeth Klerman, an Associate Professor in the Division of Sleep Medicine at BWH, said:

"Individuals who get too little sleep during the work or school week but try to catch up on weekends may not realize that they are accumulating a chronic sleep debt."

"This may lead to a dangerous situation in which individuals do not realize the extent of their sleep deprivation and their vulnerability to sudden sleepiness when they try to drive or work late into the night," she warned.

Co-author D. Charles Czeisler, who heads the Division of Sleep Medicine at BWH, said that the findings:

"Contribute to the growing body of research showing that resident physicians who are required to work in hospitals for 30-hour shifts twice per week often make fatigue-related errors."

"Burning the candle at both ends at the expense of sleep renders tasks such as driving a truck, operating heavy machinery or performing surgery dangerous, especially during the hours ordinarily reserved for sleep," he added.

"Uncovering Residual Effects of Chronic Sleep Loss on Human Performance."
D. A. Cohen, W. Wang, J. K. Wyatt, R. E. Kronauer, D.-J.Dijk, C. A. Czeisler, E. B. Klerman
Science Translational Medicine, Vol. 2, Issue 14, p. 14ra3, 13 January 2010
DOI: 10.1126/scitranslmed.3000458

Sources: Harvard/BWH.

Written by: Catharine Paddock, PhD