Heat production rates of sleeping honeybees were determined at ambient temperatures between 20 and 35 degreesC. They increased with temperature from 4.7 mW g(-1) (S.D. 2.2 mW g(-1)) at 20 degreesC (n = 18) up to a value of 12.3 mW g(-1) (S.D. 7.6 mW g(-1) n = 12) at 35 degreesC. This indicates that honeybees behave ectothermicly during sleep. The preferred ambient temperatures for sleep were investigated in a temperature choice experiment. The highest number of sleeping bees were found at 28 degreesC. Evaluation of sleep behaviour in an observation hive revealed that bees prefer the same ambient temperature of about 28 degreesC under natural conditions. Honeybees save energy during sleep with an ectothermic behaviour, but do not reduce their metabolic rates as much as possible by choosing places in the beehive with the lowest temperature. Instead, they prefer places with moderate intermediate temperatures, probably in order to promote regenerative processes during sleep.