Introduction: The diurnal rhythm of saliva cortisol and its association to adaptation, performance and health were examined in personnel over-wintering at two British Antarctic stations. Methods: In total, 55 healthy individuals (49 males, 6 females) participated in the study. Cortisol in saliva was sampled on 3 consecutive days (at awakening, 15 and 45 min after waking, at 15.00 h, and 22.00 h) immediately after arrival at the station, midwinter, and the last week before departure. Subjective health complaints were also measured at arrival, midwinter, and the last week before departure, while depression (Burnam screen for depression) and positive and negative affect (PANAS) were measured at midwinter only. At the end of the winter appointment, base commanders evaluated the performance of all personnel. Results: The variations in external light (darkness during winter, midnight sun during arrival and departure) did not influence the diurnal rhythms. The normal peak level in the morning, and the normal and gradual fall towards the evening were observed at arrival, midwinter, and before departure. Immediately after arrival the cortisol values were relatively high and correlated positively with base commander’s evaluation of performance. During midwinter, approximately 58% scored for depression on the Burnam scale. However, when examining these data more closely, only 4 participants (7%) reported depression, the main reason for the high score on the depression scale was related to sleep problems and tiredness. Conclusions: There was no indication that over-wintering led to any disturbance in the diurnal rhythm of cortisol in British Antarctic personnel. There were no other indications of any ‘over-wintering syndrome’ than reports of subjective sleep problems and tiredness.