Hydrocortisone and ACTH levels in manned spaceflight

The plasma hydrocortisone, plasma ACTH, and urinary hydrocortisone values were recorded for each man of the crews of Apollo flights eight through fifteen, 30, 14, and 5 days before flight, immediately after spaceflight recovery, and on future days until the return of most variables to preflight values. The plasma and urinary preflight hydrocortisone values were significantly higher than the postflight values. This result is discussed in terms of three possible explanations: (1) the adrenal-cortical function is suppressed during spaceflight; (2) the activity in flight may amount to stressful exercise, which tests have shown can cause a decrease in plasma adrenocortical hormones; and (3) the in-flight work-rest cycles may be such as to affect the circadian periodicity of the pituitary-adrenal function.