ACTH secretion reaches its peak level at-

Ans. is 'd' i.e., Early morning Regulation of glucocorticoid secretion* The adrenal cortex is under the control of hypothalamo-hypophysial axis. The secretion of glucocorticoids by adrenal cortex is regulated by anterior pituitary gland through ACTH (corticotropin). ACTH secretion itself is regulated by hypothalamus through corticotropin releasing hormone (CRH). The chain is called hypothalamus- pituitary-adrenal-axis.* The secretion of ACTH and consequently that of cortisol follows a circadian rhythm due to hypothalamic (Suprachiasmatic nucleus) control. ACTH secretion is minimum during night and maximum early in the morning (6-8 am).* The hypothalamo-hypophyseal axis is susceptible to feedback control by cortisol. The secretion of ACTH and CRH is subjected to negative feedback (inhibition) by glucocorticoids which keeps glucocorticoids secretion within normal limits. Stress can stimulate the hypothalamus to release more CRH that in turn increases ACTH secretion. Consequently glucorticoid secretion is elevated in stress.