Effect of thyroxine on sleep in the rat

The relationships between daily sleep time and metabolic rate both in phylogeny and ontogeny have suggested that sleep functions in the regulation of an animal's energy expenditure. Smaller animals may sleep more to conserve more energy. The present study investigated whether, in individual animals, sleep increased in response to the administration of thyroxine, which increased metabolic rate. The sleep, motor activity, food intake, and weights of rats were sampled over a base-line period of 2--4 weeks, during 2--6 weeks of daily administration of thyroxine (150 or 300 microgram/kg), and during a recovery period of 2--4 weeks. In addition to comparisons among base-line, drug, and recovery periods, comparisons were also made with control animals recorded at the same time. Thyroxine produced the increases in food intake and decreases in weight gain which usually accompany increases in metabolic rate. In spite of this evidence of major systemic adjustments in energy input (increased food intake) and energy output (decreased growth) in response to the increased energy expenditure of a raised metabolic rate, there was no change in total sleep time. Therefore, the results do not support the theory that sleep adapts to prevailing energy demands. Although the rate of cycling through the sleep stages is also related to metabolic rate both in phylogeny and ontogeny, in this study thyroxine did not change the period of paradoxical sleep rhythm.