On three different occasions, six healthy young adult subjects ahd their body temperatures raised by an average of 2.0 degrees C for 30 min while sitting in baths of warm water. This was done once at 1700 h and on two occasions at 2100 h, once after the subjects had taken aspirin and once after a placebo. Nighttime sleep was recorded after each experimental condition and for baseline nights following nil heating. Records were scored both visually and by an automated sleep stager. Electroencephalographic (EEG) power was computed over the night. Results from the automated scoring were very similar to those of the visual method. While the early bath caused no changes in sleep, the late bath + placebo resulted in significant rises in stage 4 sleep and slow wave sleep (SWS) and significant falls in sleep onset and in REM sleep. Aspirin mostly counteracted these effects and, in particular, left stage 4 sleep and SWS at baseline levels. EEG power was significantly increased only after the late bath plus placebo, supporting the SWS outcome. These findings were assessed in light of other comparable results from our laboratory. It seems that as the time of the day of heating recedes from nighttime sleep, a larger "dose" of heating is required to produce the same effect.