Smooth pursuit and saccadic eye movements as possible indicators of nighttime sleepiness

Sleepiness is associated with specific variations of spontaneous oculomotor activity. During nocturnal sleep onset periods and also during the Multiple Sleep Latency Test (MSLT) a reduction of both rapid eye movements and blinks are recorded. In many operational contexts it might be even more relevant to assess whether and to what extent voluntary visual ocular control is affected by sleepiness due to sleep deprivation and time-of-day effects. In this study we evaluated, in a laboratory simulation of a sudden inversion of the sleep-wake cycle, the nocturnal modifications of smooth pursuit (SP) and saccadic (SAC) eye movements as possible indicators of sleepiness. Levels of sleepiness were objectively measured by means of MSLT and Maintenance of Wakefulness Test (MWT); subjective ratings of sleepiness were also obtained. After a diurnal sleep, five subjects underwent four nocturnal test sessions, each one comprising an SP and a SAC trial. Both the SP variables considered (velocity gain and phase) showed a trend similar to that one of MWT latencies, being significantly impaired only in the last nocturnal trial, when levels of sleepiness were maximal. Saccadic accuracy showed the same trend, being negatively affected by sleepiness only in the last nocturnal session. In addition, percentage of rejected (inappropriate) saccades showed a linear increase during the night, paralleling the shortening of sleep latency at MSLT and the linear increase of subjective ratings of sleepiness. These results, suggesting that saccadic performance, unlike SP, seems to be more sensitive to increasing levels of sleepiness, encourage further research on this topic.