Sleep and body temperature in "morning" and "evening" people.

Three groups of young, normal sleepers were selected as morning types (MTs), evening types (ETs), and neither types (NTs) as determined by the Horne and Ostberg questionnaire. Sleep and rectal temperatures were recorded under three conditions: baseline nights (Cond. 1), sleep on the recovery day after 1 night of sleep deprivation (Cond. 2), and sleep on the recovery night after 1 night and 1 day of sleep deprivation (Cond. 3). During Conds. 1 and 3, when sleep schedules were self-determined, sleep structure and body temperature were similar in MTs, and ETs, and NTs. During Cond. 2, however, MTs had poorer sleep, i.e., a smaller percentage of REM sleep and more awakenings, than ETs. This difference can be related to the evolution of temperature during Cond. 2; i.e., a temperature increase in the MT and NT and a decrease in the ET.