The Multiple Sleep Latency Test (MSLT) is widely believed to offer an
objective, physiological measure of sleepiness. The speed with which a
person falls asleep throughout the day is understood to be related sy
stematically to sleep need and circadian phase. This study examined wh
ether normal subjects (n = 14 young female adults) could achieve faste
r MSLT sleep onsets if they were given the incentive to do so. During
week 1 baseline MSLTs were determined over 1 day for all subjects. In
week 2 they were randomly assigned to two groups. Control subjects und
erwent a second MSLT testing day identical to that of week 1, whereas
Incentive subjects had an additional financial incentive to sleep. The
re was a significant reduction in sleep onset latency (indicating incr
eased sleepiness) during the 1500 h trial following the incentive, whe
n subjects also reported a significantly greater increase in sleepines
s over the trial. These findings suggest that when coupled with a mid-
afternoon increase in sleepiness, increased motivation to sleep can re
duce sleep onset latency.