The present study examines whether monitoring and control behaviour ar
e affected by sleep loss and fatigue. The effects of one night of slee
p loss are investigated in a dual-task consisting of a monotonous trac
king task that requires continuous control behaviour and a time estima
tion task that requires monitoring with an 'open-loop' character. In t
he tracking task subjects had to keep a square cursor in the centre of
a U-shaped target. In the time estimation task subjects had to estima
te consecutive time intervals and to respond when an interval had ende
d. There were three experimental conditions. In the first condition a
clock was presented continuously on the screen; thus, subjects had a c
lear notion of the time course. In the second condition subjects could
obtain time information by pressing a key. In the third condition sub
jects could not obtain time information, but the status of the time in
terval could be obtained by pressing a key; that is, information was g
iven whether a time interval had ended or not. The present study confi
rms earlier findings that tracking performance is sensitive to sleep l
oss particularly towards the end of the half hour working session. In
contrast, time estimation performance was not impaired, although subje
cts tended to make more observations when deprived of sleep. This resu
lt would not have been expected on the basis of a decreased level of a
ctivation due to sleep loss. It appears, however, that checking the ti
me course with a motor response requires less effort than continuously
updating an internal clock, which puts heavy demands on working memor
y. The observed difference between the tasks with respect to the vulne
rability to sleep loss is explained in terms of their motivating prope
rties.