<prompt> Plasma ACTH levels are high between these following hours4 PM and 8 PM4 AM and 10 AM10 AM and 3 PM10 AM and 2 PM  <response>: B 4 AM and 10 AM ACTH the primary stimulator of adrenal cortisol production is synthesized by pituitary in response to CRH, which is released by hypothalamus. Plasma ACTH and cortisol levels exhibit peaks at 6-8 AM and nadirs at 11 PM. Referene values: 10-60 pg for AM values. No established reference values for PM. 