Sixty subjects were tested five times per waking day on two occasions for accuracy and reliability in throwing 20 darts at a target. Two experimental conditions were investigated: following a normal nocturnal sleep (7-8 h sleep, normal) and after having retired to bed 4 h later than normal the previous night but rising at the normal time (3-4 h sleep, sleep deprivation). Sublingual (core) temperature and subjective estimates of alertness and fatigue were measured in all sessions. Performance at throwing darts was assessed by three methods: mean distance of the dart from the bulls-eye; number of times the target was missed; and variability of the scores from the darts thrown. There was no evidence that performance was affected by physical fatigue arising during the course of throwing the 20 darts. All variables showed significant diurnal rhythms, those of alertness and performance being phased over 1 h earlier than core temperature, and that of fatigue over 1 h earlier than the inverse of temperature. Core temperature was not affected by sleep deprivation, but all other variables showed significant changes, indicative of mood and performance decrement. Increasing time awake was associated with decreased alertness and increased fatigue, as well as slight negative effects upon performance. We conclude that the simple task of throwing darts at a target provides information about chronobiological changes in circumstances where time awake and sleep loss might affect psychomotor performance.