Changes in nocturnal body temperature, sleep patterns, and blood variables with energy restriction (3347 kJ/d) were studied in nine overweight (body mass index 26.1 +/- 2.8) premenopausal women aged 20-36 y. Variables were measured both 2 wk before and in the final 2 wk of 4-wk dieting. Data collected 28 d apart were compared to attenuate menstrual cycle differences. Subjects lost 8 +/- 4% of their initial body mass after 4 wk of energy restriction and plasma triiodothyronine (T3) was significantly reduced from 5.9 +/- 0.7 to 5.1 +/- 0.6 pmol/L (P < 0.05). The implied suppression of heat production (metabolic rate) with reduced T3 may account for the observed decrease in minimum nocturnal rectal temperature (from 36.5 +/- 0.3 to 36.3 +/- 0.3 degree C, P < 0.05). Furthermore, dieting significantly altered sleep patterns; sleep onset latency was lengthened and slow-wave sleep decreased (P < 0.05). These changes may be indicative of reduced restorative/biosynthetic requirements. It thus appears that energy restriction results in a hypometabolic state that affects nocturnal body temperature and sleep patterns.