Sleepiness is associated with specific variations of spontaneous oculomotor
activity. During nocturnal sleep onset periods and also during the Multipl
e Sleep Latency Test (MSLT) a reduction of both rapid eye movements and bli
nks are recorded. In many operational contexts it might be even more releva
nt 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. Lev
els of sleepiness were objectively measured by means of MSLT and Mainteinan
ce of Wakefulness Test (MWT); subjective ratings of sleepiness were also ob
tained. 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 tria
l, when levels of sleepiness were maximal. Saccadic accuracy showed the sam
e trend, being negatively affected by sleepiness only in the last nocturnal
session. In addition, percentage of rejected (inappropriate) saccades show
ed 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 res
earch on this topic. (C) 1998 Elsevier Science Inc.