Neurobehavioral performance in young adults living on a 28-h day for 6 weeks.

IN MAMMALS, THE SUPRACHIASMATIC NUCLEUS IS A CENTRAL NEURAL PACEMAKER THAT GENERATES CIRCADIAN RHYTHMS IN PHYSIOLOGIC AND BEHAV-IORAL measures, including the timing of sleep, hormone production, body temperature, and waking neurobehavioral performance.1–4 Studies of humans isolated from daily changes in the environment have led to the realization that the timing and organization of sleep, alertness, and performance vary with the circadian phase of the underlying, near 24-hour oscillation.5–7 Neurobehavioral performance in many sleep-deprivation studies has also been reported to be closely linked to homeostatic factors influenced by the duration of prior wakefulness.8 Previous studies have suggested that it is the precise balance between sleep-wake homeostatic factors and the circadian rhythm of alertness and sleep propensity that allow for stable alertness and performance throughout the waking day during entrainment.9–11 A recent study in which chronic misalignment between internal biologic time and the imposed sleep-wake schedule was imposed across 4 weeks showed that cognitive performance (i.e., learning) was significantly impaired, whereas subjects in the same study who remained entrained improved their performance.12 Using forced desynchrony (FD) studies, the circadian modulation of performance and alertness has been revealed.9,13–15 A 28-hour FD protocol has shown that, even within the range of 0 to 18 hours of wakefulness, the contribution of the circadian system to variations of alertness and performance is approximately equal to the contribution of the sleep-wake homeostat in young subjects.10 A study using a 20-hour FD protocol (with a 13.7-hour wake episode) also found circadian modulation of alertness and several aspects of cognitive performance.14 Previous 28-hour FD studies have found that, when scheduled to sleep at an adverse circadian phase, healthy young adults cannot take advantage of the 9.33-hour sleep opportunity on every night.16,17 In fact, when the end of the scheduled sleep episode is at an adverse circadian phase (beginning in the biological daytime between about 110° and 290°), the latter quarter of the scheduled sleep episode is severely disrupted, with sleep efficiency averaging below 70%.17 However, how that lost sleep then impacts subsequent sleep and wake episodes and daytime performance was not explored in those studies. A widely used performance test in studies of circadian rhythms or sleep deprivation is the Psychomotor Vigilance Task (PVT).18 This task has been reported to show no long-lasting training effects.8,18 In contrast, many other performance tests do have learning or practice effects, whereby, with repeated practice, performance on the task gradually improves.12,19,20 Therefore, consideration of those learning or practice effects should be made in analyzing data from long-duration studies. In a preliminary report, we found that a calculation performance task, a test of throughput, showed learning effects across a 4-week FD study, in which a saturating exponential function provided the best fit to the data for all subjects.21 Most prior studies in which both circadian and wake-dependent influences on performance have been assessed were of a 1- to 4-week duration. In the study we report here, subjects participated in a 6-week 28-hour FD study designed to assess circadian period estimates. Several performance measures were given to the subjects throughout the protocol. To examine the practice effects on a calculation performance test, we included a similar addition task that had been used in our prior preliminary report.21 We also included the PVT, given the prior reports suggesting that it has no such practice effects. We aimed to examine whether performance on the addition and PVT would show significant changes across the 6-week FD study, as well as to analyze the circadian and wake-dependent homeostatic modulation on both types of performance, taking into account interindividual differences.