Uncovering Residual Effects of Chronic Sleep Loss on Human Performance

The cumulative effects of chronic sleep loss may be overcome at certain times of day, but the residual effects of sleep deprivation profoundly degrade performance and may thereby compromise safety. More Than Beauty Sleep: Long Days Hamper Performance We all know the dangers of driving when tired; indeed, being awake for 24 hours straight can impair our abilities as much as a blood alcohol level of 0.10%. But all too often these real effects of too little sleep are dismissed with a laugh—or accepted. A careful dissection of the effects of short-term and long-term sleep restriction by Cohen et al. now shows that the situation is even worse than we thought. When chronic sleep loss is superimposed on the natural low-performance periods of our body’s 24-hour rhythm, reaction times slow to about 10 times normal, even if we got a good night’s sleep the night before—a truly hazardous situation. Because our circadian rhythms and sleep-wake cycles are usually intertwined, it has been difficult to dissect precisely their individual influences on how well we function. To independently determine the effects of circadian rhythm and those of acute and chronic sleep loss, the authors completely controlled the schedules of nine healthy volunteers for 38 days. For 21 of these days, the volunteers slept 10 hours out of every ~43-hour cycle, equivalent to about 5.6 hours of sleep per night. On this schedule, the subjects’ circadian rhythm, acute sleep deprivation (which they experienced during each of their long days), and chronic sleep restriction (which got worse and worse as the experiment went on) were all decoupled. By frequently giving the subjects a set of behavioral tests that measured reaction time, the authors were able to discern which factor had the most effect and when. Some of the results could be predicted from our own experience. For a few hours after waking from a 10-hour sleep, subjects’ performance was always normal, but it deteriorated as the ~33-hour waking day went on. But there was an additional effect of chronic sleep restriction. As the weeks of the experiment went by and the subjects’ sleep debt increased, their performance deteriorated to a greater extent each day, although it was still within normal limits just after they woke up. What was surprising was the very large effect of the circadian rhythm. When the subjects’ independently cycling internal clock was at its lowest point in the late night, it always extended reaction times, but its influence was considerably larger when the individual was experiencing acute or, especially, chronic sleep deprivation. Even more surprising was that when the internal clock was at its highest point in the late afternoon, reaction times were relatively normal despite substantial acute and chronic sleep loss. This leads to a dangerous situation in which individuals may not realize that they have a severe chronic sleep debt and a high vulnerability to sudden sleepiness a few hours later into the night. These findings translate into a warning for employers. Workers who need to remain awake for extended periods of time cannot maintain normal performance—and may not be aware of this vulnerability—if they are suffering from chronic sleep loss, especially if they are working at times during which their circadian rhythms are at a nadir. Sleep loss leads to profound performance decrements. Yet many individuals believe they adapt to chronic sleep loss or that recovery requires only a single extended sleep episode. To evaluate this, we designed a protocol whereby the durations of sleep and wake episodes were increased to 10 and 32.85 hours, respectively, to yield a reduced sleep-to-wake ratio of 1:3.3. These 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. Despite recurrent acute and substantial chronic sleep loss, 10-hour sleep opportunities consistently restored vigilance task performance during the first several hours of wakefulness. However, chronic sleep loss markedly increased the rate of deterioration in performance across wakefulness, particularly during the circadian “night.” 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.