Sleep Disorders and Research Center
Henry Ford Hospital
2921 West Grand Boulevard
Detroit, Ml 48202
tudies in a variety of populations have demonstrated that decreases in total sleep time lead to increased levels of daytime sleepiness. The question remains, however, as to whether daytime sleepiness in a population of normal subjects is reflective of an accumulated sleep deficit. To test this hypothesis, a series of studies have been carried in normal healthy volunteers, asymptomatic as to sleep wake complaints, routinely sleeping 6.5 to 8.0 hours with no daytime napping, but with varying objective levels of daytime sleepiness as measured by the MSLT. "Sleepy" (i.e. MSLT < 6 min) and "alert" (i.e. MSLT > 12 min) were compared as to basal sleep-wake function and in response to various experimental manipulations. The sleep of "sleepy" subjects resembles that of sleep deprived individuals in that it is longer, with an elevated sleep efficiency and fewer awakenings during the night. Evaluation of auditory awakening threshold shows that "sleepy" subjects have thresholds comparable to those of acutely sleep deprived subjects and higher than that those of "alert" individuals. On daytime testing, "sleepy" subjects exhibit poorer performance than their ̉alert" counterparts on tasks known to be sensitive to sleep loss. In experiments with ethanol administration, "sleepy" subjects experience greater performance impairment than "alert" ones. A circadian challenge, (i.e. a 4 hour phase advance) does not reduce sleep efficiency in "sleepy" subjects as it does in "alert" subjects. Finally, sleep extensions for periods of 1-14 days has a greater benefit on level of alertness and daytime performance in "sleepy" subjects compared to "alert" subjects. In fact, 2 weeks of 10 hours in bed normalizes the sleep wake function of "sleepy" subjects. These data suggest that short daytime sleep latency in healthy subjects is reflective of a homeostatic deficit in sleep. These data have implications for individual differences in biological sleep need.
