The study looked at three things: the number of consecutive hours awake, the number of days or weeks of chronic sleep reduction, and what time it was in the person's day. "How those three factors combine determine how well we perform at any moment," says Cohen.
"It's very hard to cheat the sleep system. You will pay a price sooner or later," says David Dinges, a professor of sleep studies at the University of Pennsylvania school of medicine. This research suggests "it takes longer to recover from sleep debts than has been believed in the past."
This study shows for the first time in humans that sleep regulation is actually composed of at least two separate processes acting on different time scales. The short-term process causes performance to decline with each hour awake, and this process can be rapidly overcome with one extended sleep episode.
The long-term process builds over weeks of too-little sleep. It causes a faster decline in performance for each hour a person is awake, particularly during the biological late night, the equivalent of 3 a.m. to 7 a.m. It is unknown how many nights of good sleep it takes to recover from this longer-term component.
This work in humans parallels work in animals showing more than one mechanism that promotes sleepiness in the context of reduced sleep hours. The sleep-inducing chemical adenosine appears to increase with hours spent awake. Recently a second mechanism, which is affected by long-term sleep deprivation, has been found. In this, the number of receptors in the brain for adenosine increase as long-term sleep deficit becomes bigger.
In effect, the brain becomes sensitized to the effects of adenosine, and the same number of hours awake has a bigger impact on performance.
"Sleep appears to be a crucial process, and evolutionary mechanisms have evolved so that more than one mechanism kicks in to promote sleepiness," says Cohen.
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