OCCURRENCE OF MICROSLEEPS DURING DAYTIME SLEEP ONSET IN NORMAL SUBJECTS

The main aim of this study was to explore whether the multiple sleep l atency test (MSLT) could be made more sensitive to low daytime sleepin ess in normal, healthy subjects by adopting a shorter period of sleep (microsleep) as a sleep onset criterion. Subjects underwent MSLTs unde r two conditions: after normal (baseline) nighttime sleep, and after n ighttime sleep extension (creating a 'floor effect' of minimal daytime sleepiness). MSLT sleep onset thresholds of 5 s (microsleeps), 30 a ( the norm) and 90 s of sustained sleep gave 3 separate sleep latency sc ores for 240 MSLT trials derived from 10 subjects. With low daytime sl eepiness, whether this be in the morning after baseline sleep or throu ghout the day after sleep extension, the 5 s sleep onset criterion was a more sensitive measure of sleepiness than the established 30 s crit erion. This was the case both for sleep onset latency and for the freq uency of sleep onsets. Spectral analyses of the EEG indicated that suc cessive microsleep episodes generally became more substantial, and, de pending on the level of sleepiness, culminated in more overt signs of sleep. There was little difference between the 30 s and 90 s criteria for sleep onset latency scores, although there was a small but signifi cant difference between them in the frequency of sleep onsets. As dayt ime sleepiness increased, particularly in the afternoon and under base line, the 5 s criterion reached a ceiling, with the 30 s criterion bec oming more sensitive.