SLEEPINESS, PERFORMANCE, AND NEUROENDOCRINE FUNCTION DURING SLEEP-DEPRIVATION - EFFECTS OF EXPOSURE TO BRIGHT LIGHT OR EXERCISE

The temporal profiles of subjective fatigue (as assessed by the Stanfo rd Sleepiness Scale), of cognitive performance (on a digit symbol subs titution test and a symbol copying task), of body temperature, and of the peripheral concentrations of melatonin, thyroid-stimulating hormon e (TSH), and cortisol were obtained simultaneously at frequent interva ls in 17 normal young subjects submitted to a 43-h period of constant routine conditions involving continuous wakefulness at bed rest in dim indoor light. The subjects had knowledge of time of day. Caloric inta ke was exclusively in the form of an intravenous glucose infusion, and plasma glucose levels were monitored continuously in 8 of the 17 subj ects. Under these conditions, fluctuations in plasma glucose reflect p rimarily changes in glucose use because endogenous glucose production is suppressed by the exogenous infusion. Following the completion of a baseline constant routine study, the volunteers participated in two s ubsequent studies using the same protocol to determine the immediate p sychophysiological effects of exposure to a 3-h pulse of bright light or to a 3-h pulse of physical exercise. Sleepiness and performance var ied in a mirror image, with significant negative correlations. Sleepin ess scores were minimal around noon and then increased at a modest rat e throughout the rest of the normal waking period. Staying awake durin g usual bedtime hours was associated with an acceleration in the rate of increase in sleepiness, which coincided with decreasing body temper ature, rapidly rising cortisol concentrations, and maximal levels of m elatonin and TSH. When body temperature reached its nadir, a further m ajor increase in sleepiness occurred in parallel with a pronounced dec rease in plasma glucose (reflecting increased glucose use). Recovery f rom maximal sleepiness started when blood glucose levels stopped falli ng and when significant decreases in cortisol and melatonin concentrat ions were initiated. Lower levels of subjective sleepiness resumed whe n glucose concentrations and body temperature had returned to levels s imilar to those observed prior to sleep deprivation and when melatonin and TSH concentrations had returned to daytime levels. The synchrony of behavioral, neuroendocrine, and metabolic changes suggests that cir culating hormonal levels could exert modulatory influences on sleepine ss and that metabolic alterations may underlie the sudden increase in fatigue consistently occurring at the end of a night of sleep deprivat ion. Effects of bright light or exercise exposure on subjective sleepi ness appeared to be critically dependent on the timing of exposure.