During both the cool and hot periods of the dry season of the Sahelian climate, sleep patterns of four African sportsmen from Niger (23 +/- 0.8 years old) were studied after square-wave cycle ergometer exercise. The sleep patterns were studied in the sedentary (Baseline) condition and under two exercise conditions, with and without rehydration. All conditions were randomly assigned. The exercise programme consisted of 3 sequences of cycling, beginning with 10 min at 30% of maximal aerobic power (MAP), followed by an exhaustive supramaximal effort (130% of MAP), and ending with a 10 min recovery at 30% of MAP. Polysomnography was recorded for 2 consecutive nights in each condition. One adaptation night, to familiarize the subjects to the polysomnography protocol, preceded the first session. Baseline slow-wave sleep (SWS) and REM sleep were high. In the hot season, there was an overall increase in SWS (p < 0.02), due primarily to an increase in Stage 4 (p < 0.01). Stage 2 sleep decreased (p < 0.01). This effect was also observed after exercise with rehydration, but was absent in the non-hydration condition. This distinction between the rehydrated and non-rehydrated condition is thought to be due to the greater stress which may accompany an exercise-induced relative dehydration. REM sleep decreased after exercise in the cool season (p < 0.001), but equalled baseline levels during the hot season. The results suggest a combined action of hyperthermia, water balance and stress, and are further supportive of an underlying hypothalamic control of human sleep.