While occasional loss of sleep—pulling an all-nighter to wrap up a big project, for example—can generally be made up for by getting more hours of sleep in the following days, people who regularly skimp on sleep may not be able to undo the detrimental effects with the occasional lie-in, according to a new study published in the journal Science Translational Medicine. A team of researchers, led by neurologist and sleep scientist Daniel Cohen, set out to analyze how both long-term and short-term sleep loss impacted performance, as well as whether the timing of sleep—if people normally snoozed during the day and worked night shifts, for example—impacted recovery. They found that, people who suffered acute sleep loss (from the occasional all-nighter, for example) were able to return to optimal performance levels simply by catching a few hours extra rest another day, but those who suffered chronic sleep loss exhibited long-term performance deficits, which occasional snippets of extra sleep did little to improve.
In the 38-day experiment, researchers first primed study participants with plenty of rest. The nine subjects included in the chronic sleep trial got 12 hours of sleep on the first day, followed by the opportunity for four-hour naps the following three days, “to minimize any residual sleep loss.” Finally, they had two nights of 10 hours’ rest prior to beginning a phase of what researchers referred to as “forced desynchrony,” or a schedule intended to mimic that of on-call doctors or other people who work through night shifts, with the equivalent of 24 hours of wakefulness offset by 5.6 hours of sleep. After completing 12 of these cycles, participants in the study group had 10 days of recovery time, with 14 hours of wakefulness and 10 hours of rest.
Both this trial group and control groups—who kept more “normal” sleep schedules, or only suffered the occasional acute sleep loss—were instructed not to consume any caffeine, alcohol or nicotine during the trial or the three weeks before. During the course of the experiment, they also participated in performance testing known as a psychomotor vigilance task (PVT), which was administered every two hours, beginning an hour and a half after they woke up.
For study participants with acute sleep loss, one full night’s rest—10 hours of sleep—was enough to restore performance levels, but for participants who suffered from chronic sleep loss, even after “catching up” on sleep, tests showed that their performance suffered as the day wore on. That is, in the first hours after waking, they performed well on PVT tests, but with each additional hour spent awake, their performance waned.
The findings suggest that there may be two distinct regulatory processes governing sleep—one that functions in the short-term, and one that is impacted by more cumulative sleep patterns across weeks and even months. The cause for concern, researchers say, is that people who endure chronic sleep loss may believe that they have fully recovered, but their performance continues to suffer from the long-term effects of too little rest. This experiment, they write, reveals that “individuals can develop a chronic sleep debt in the face of apparent full recovery of acute sleep loss.” In other words, someone who works long hours and gets by on little sleep during the week may try to make up for it on weekends and holidays, and may actually feel better, but the residual effects of sleep loss persist.
Cohen and his colleagues point out that long periods of wakefulness are associated with dramatic decreases in performance—19 hours awake has been shown to produce performance deficits comparable to having a blood alcohol content (BAC) level of .05%, and 24 hours, a BAC of .10%. (The legal limit is .08%.) Yet, while people are broadly aware of the impact of short-term sleep loss, the authors say that performance deficits caused by acute sleep loss were often indistinguishable from those in the chronic sleep loss group. In addition to performance problems, persistent lack of sleep has also been shown to increase susceptibility to illness, weight gain and other health complications.
The authors suggest that these findings are particularly relevant for individuals who regularly work long, overnight shifts—such as health care workers or long-haul truckers—and the estimated 16% of Americans who regularly get by on six hours of sleep or less per night. They expressed hope that the findings could help guide public health efforts and sleep recommendations within professions, based on models incorporating the effects of both chronic and acute sleep loss, as well as the influence of working off-hours.