The effects of six graded and measured exercise activities on sleep patterns were investigated in two healthy young men. Electrophysiological recording were made continuously throughout the night to distinguish sleep states. This experiment was designed to test the hypothesis of a relation between physical activity and slow-wave sleep (SWS: stages 3 and 4 of non-REM sleep). A progressive increase in SWS over the whole-night sleep record was found with progressively increasing physical fatigue. A fall in rapid-eye-movement (REM) sleep and at higher exercise levels, of stage 2 sleep, was found. The results support the hypothesis that SWS is involved in the recovery process from fatigue.