Regular pulses of glucocorticoids, mainly cortisol in the case of humans, are released regularly from the adrenal cortex following a circadian pattern in addition to their release as a part of the stress response.  Cortisol release follows a high frequency of pulses forming an ultradian rhythm, with amplitude being the primary variation in its release, so that the signal is amplitude modulated.  Glucocorticoid pulsatlity has been observed to follow a circadian rhythm, with highest levels observed before waking and before anticipated mealtimes. This pattern in amplitude of release is observed to be consistent across vertebrates.  Studies done in humans, rats, and sheep have also observed a similar circadian pattern of release of adrenocorticotropin (ACTH) shortly preceding the pulse in the resulting corticosteroid.  It is currently hypothesized that the observed pulsatility of ACTH and glucocorticoids is driven via pulsatility of corticotropin-releasing hormone (CRH), however there exists little data to support this due to difficulty in measuring CRH.