Summary: Previous studies have inferred a relationship between core temperature and sleep disruption from manipulations of core temperature such as heating prior to sleep or administration of hyperthermic substances. To examine the relationship more directly, this study aimed to produce a direct increase in core temperature during the sleep period. Following an adaptation night, 16 subjects underwent counter-balanced baseline and experimental conditions, on non-consecutive nights between 1900 and 0800h. In the experimental condition, subjects were heated between 0230h and wake up, which significantly increased mean core temperature from baseline levels between 0400 and 0700h by 0.18 ± 0.03 °C (mean ± SEM, p < 0.05). This increase in core temperature was associated with a significant decrease in sleep efficiency between 0330 and 0730h of 5.5 ± 0.9% (mean ± SD, p < 0.05). Polysomnographic measures indicated a significant increase in the number of stage changes and the amounts of stage 0 and stage 1 sleep (p < 0.05). Other stages of sleep and the number and duration of arousals were not significantly effected by heating. There was a strong trend toward and increase in the number of arousals (p = 0.054), however, core body temperature did not increase across arousals. Also, melatonin output was not effected by heating. Taken together, these results suggest that increased nocturnal core temperature alone may disrupt sleep. Additionally, the results support evidence suggesting that the circadian regulation of the sleep/wake cycle may be mediated via core temperature.