The effect of hypokinesis on plasma ACTH and cortisol concentrations.

The effects of hypokinesis on plasma ACTH and cortisol concentrations were studied in 5 healthy young males. The subjects submitted to 56 days of bed rest in a 14L:OD environment. There were marked daily fluctuations in circulating levels of both hormones with cortisol levels peaking in the early morning and ACTH in the late evening . After the 30th day of bed rest ACTH levels increased sharply and a threefold increase was observed by the 54th day. Mean cortisol levels rose during the early part of the study but had decreased markedly by the 54th day. These changes in hormone levels are concluded to reflect a change in the amplitude of their respective circadian rhythms.