During the usual waking day there is a circadian propensity for sleepiness in the early afternoon. This contrasts with the circadian peak of alertness in the early evening. The former is apt to be masked by various exogenous factors. Alertness was compared at these times of day ('afternoon': 1200-1600 hours and 'evening': 1800-2200 hours) under contrasting environmental conditions: boring vs. stimulating ('interest') x warm vs. cool ambient environment ('temperature'); making four combinations, with two times of day--i.e. eight independent conditions, each containing six subjects (N = 48). The emphasis of the study was to make the conditions 'natural' and not unpleasant--e.g. the environmental temperatures were not extreme and kept within a comfortable range. All conditions were run in a climatic chamber. Alertness was measured by reaction times and subjective sleepiness scales. Heart rate and body temperature were monitored continuously. Apart from time of day, 'interest' exerted a powerful effect that was significant for all variables, and was particularly potent in the afternoon. By comparison, the 'temperature' effects were minor. For our subjects the afternoon 'dip' centred between 1500 and 1530 hours, and displayed bicircadian characteristics.