Abstract
STUDY OBJECTIVES: Although impairment of daytime functioning is a symptom of many sleep disorders, there are limited data on their nature for some patient groups. The role of the circadian system on impaired functioning, specifically the wake maintenance zone (WMZ)-a ∼3-h window of reduced sleep propensity that occurs shortly before the onset of melatonin synthesis-has received little attention. The study examined the influence of the WMZ on neurobehavioral performance under normal conditions and following sleep deprivation. METHODS: Thirty-one adults (8 F; 18-29 y) completed an in-patient protocol including a baseline day (8-h sleep:16-h wake) and a ∼50-h constant routine (CR), including regular assessment of plasma melatonin and neurobehavioral performance (i.e., auditory and visual psychomotor vigilance tests [aPVT, vPVT], Digit Symbol Substitution Test [DSST], and subjective sleepiness). RESULTS: Performance in the 3 hours before the onset of melatonin secretion (i.e., the expected WMZ) was significantly improved compared to performance during a 3-hour block earlier in the biological day, despite a longer time awake. The improvement during WMZ was most prominent after extended wakefulness (i.e., day 2 of the CR). CONCLUSIONS: These results suggest that alignment of circa-dian phase with respect to sleep-wake timing may affect cognitive performance, particularly when homeostatic sleep pressure is high, and especially when performance is assessed in the evening, near the predicted WMZ. The potential contribution of the WMZ to sleep-onset insomnia complaints should be assessed further, using objective neurobehavioral testing and simultaneous circadian phase measurement.