Psychological studies have established that the central nervous system cannot sustain vigilance for an extended period of time. The consequent vigilance decrement implies a gradual reduction in the ability to process information effectively. This may result in a reduced ability to detect hidden predators, locate cryptic food items or make successful behavioural decisions. A model incorporating parameters of vigilance decrement during foraging and vigilance recovery during rest predicts first, the optimal allocation of time between foraging activity and rest, and second, the optimal length of each foraging episode. For a given rate of vigilance recovery, the model predicts that both the optimal proportion of time devoted for foraging activity and the optimal length of a foraging bout should be decreasing functions of the rate of vigilance decrement. Because the rate of vigilance decrement is larger for more difficult tasks, both the total time spent foraging and the duration of each foraging episode should be smaller for more demanding foraging activities. It is suggested that vigilance decrement is a dominant factor determining temporal patterns of animal behaviour.