Abstract: The seasonal variations of psychological reactions in isolated and confined environments have been studied and theorized in terms of the third-quarter phenomenon; the third quarter of the isolation period is the moment when the most discomfort is reported by the winterers. Referring to Rivolier's comparative study, this article examines the manifestations of the third-quarter phenomenon by analyzing the data collected with an observation grid completed by the mission's doctor. The stress reactions of 27 winterers are observed during a 50-week period. The results show that the third-quarter phenomenon does not appear after the middle of the stay but more precisely after the middle of the isolation period. Changes in moods and personal reactions are reported but also in social and physical reactions. The results are discussed in terms of externalization of stress reactions and the influence of the data collection method emphasized.