To test the hypothesis that nocturnal body temperature (Tb) and metabolic rate (MR) in the pigeon are regulated during sleep at levels proportional to energy reserves, continuous recordings of Tb, oxygen consumption ( $$\dot V$$ O2), carbon dioxide production, and electrophysiological measures were taken from five pigeons subjected to two separate 4-day fasts. Energy reserves were depleted differentially during the fasts by 12-h diurnal infusions of either saline or isosmotic glucose solutions. Although Tb and $$\dot V$$ O2 were closely correlated, $$\dot V$$ O2 declined throughout the fast during diurnal and nocturnal phases of the 12:12 light-dark cycle whereas significant declines in Tb were restricted to the night. Diurnal thermal conductance declined over days of fasting, especially during saline infusions, and was reduced to minimal levels each night. The durations and distributions of arousal states did not change during the fast or differ between conditions. The results were consistent with the hypothesis of a nocturnal regulation of Tb and metabolic rate proportional to energy reserves.