Which hormone is primarily responsible for the nocturnal rise in cortisol levels?

The hormone primarily responsible for the nocturnal rise in cortisol levels is adrenocorticotropic hormone (ACTH). ACTH is secreted by the anterior pituitary gland in response to corticotropin-releasing hormone (CRH) released from the hypothalamus. This release of ACTH typically occurs in a pulsatile manner, with increased secretion during the early morning hours.

Cortisol, though the hormone we are focusing on, is not the one that triggers its own increase in levels. Instead, the secretion of ACTH regulates cortisol production in the adrenal cortex. The circadian rhythm of ACTH and cortisol secretion allows cortisol levels to peak in the morning, corresponding to the body's need for increased energy and metabolism during wakefulness.

Adrenaline plays a role in the body's immediate stress response, but it is not responsible for the gradual increase of cortisol levels throughout the night. Similarly, thyroid hormones are involved in metabolism and energy regulation, but they do not influence the diurnal pattern of cortisol secretion directly.

Therefore, ACTH is essential in stimulating cortisol production in a rhythmic pattern, thus leading to the observed nocturnal rise in cortisol levels.