DST in depression is unaffected by altering the clock time of its administration.

Circadian oscillators in major depressive illness may be phase advanced by several hours. We attempted to determine whether phase advance of the oscillator responsible for hypothalamic-pituitary-adrenal (HPA) function in depressives might influence the outcome of the overnight dexamethasone suppression test (DST). Six major depressives underwent DST with dexamethasone doses administered in a randomized fashion at 1900h and 2300h on separate evenings. Twenty-four hour cortisol secretory patterns basally and postdexamethasone were obtained for each subject. Postdexamethasone cortisol responses were similar for both the 1900h and 2300h dosage schedules in suppressors, nonsuppressors, and an early escape responder. We conclude that failure of the HPA axis to suppress normally with DST in major-depressive illness is a primary feature of neuroendocrine regulatory mechanisms rather than secondary to a posited phase advance of the related circadian oscillator.