A set of data suggests that the thyroid gland plays a role in the bi-directional interaction between the electrophysiological and the endocrine components of sleep, e.g. the nonREM-REM-cycle and the patterns of nocturnal hormone secretion, respectively. In detail thyroid-stimulating hormone (TSH) and thyroxin (T4) show circadian rhythms. A specific relationship was observed between TSH and REM sleep. Blunted TSH levels were found in healthy elderly subjects and, probably due to overactivity of corticotropin-releasing hormone in patients with depression in comparison to young normal controls. Pulsatile administration of thyrotropin-releasing hormone induced a decrease of sleep efficiency and an earlier occurrence of the cortisol rise in normal controls. Slow wave sleep was reduced in patients with hypothyroidism in comparison to normal controls. The sleep EEG normalised after therapy.