Variation of ACTH Secretion	

ACTH is secreted episodically throughout a twenty-four-hour period. There are between seven and thirteen episodes, most of which occur during the second half of the normal sleep period and which are largely reproducible from one twenty-four-hour period to another. Since cortisol and all of the adrenal androgens, with the exception of DHEA-sulphate, are responsive to ACTH, there are also episodic changes in the peripheral plasma levels of these steroid hormones throughout the day (Fig. 7.9). It is not known what factors cause this pattern of ACTH release, and the physiological advantage, if any, is also not clear. The rhythm is shifted by an alteration in the sleep-wake pattern, such as that which occurs after travel across time zones, and five to seven days are required to complete the shift. Alterations of circadian rhythm also occur in disease states, particularly in adrenal hyperfunction (Cushing's syndrome) and in depressive illness.