Decreased human circadian pacemaker influence after 100 days in space: A case study

Objective: The objectives of this study were (1) to assess the circadian rh ythms and sleep of a healthy, 42-year-old male astronaut experiencing micro gravity (weightlessness) for nearly 5 months while living aboard Space Stat ion Mir as it orbited Earth and (2) to determine the effects of prolonged s pace flight on the endogenous circadian pacemaker, as indicated by oral tem perature and subjective alertness rhythms, and their ramifications for slee p, alertness, and performance. Methods: For three 12- to 14-day blocks of t ime (spread throughout the mission), oral temperatures were taken and subje ctive alertness was self-rated five times per day. Sleep diaries and perfor mance tests were also completed daily during each block. Results: Examinati on of the subject's circadian alertness and oral temperature rhythms sugges ted that the endogenous circadian pacemaker seemed to function quite well u p to 90 days in space. Thereafter (on days 110-122), the influence of the e ndogenous circadian pacemaker on oral temperature and subjective alertness circadian rhythms was considerably weakened, with consequent disruptions in sleep. Conclusions: Space missions lasting more than 3 months might result in diminished circadian pacemaker influence in astronauts, leading to even tual sleep problems.