Altitude training is widely used in elite swimming, although its effects on sleep and acclimatisation remain insufficiently understood. The aim of this study was to describe changes in sleep difficulty, physiological acclimatisation, athlete-reported outcomes, and haematological markers during a 21-day altitude training camp (2320 m), and to explore the association between chronotype and sleep difficulty. Seven world-class swimmers (4 men, 3 women) were monitored before, during, and after altitude exposure. Sleep difficulty was assessed using the Athlete Sleep Screening Questionnaire, while daily monitoring included resting oxygen saturation, resting heart rate, acute mountain sickness symptoms, fatigue, recovery, hydration status, and training load. Chronotype was determined using the Morningness-Eveningness Questionnaire. Sleep difficulty was descriptively higher during the first week at altitude (6.14 ± 1.07 vs. 4.14 ± 1.21 at baseline) and returned towards baseline by week 3 (4.29 ± 1.50), although pairwise comparisons did not remain significant after multiplicity correction. Strong inverse associations between chronotype and sleep difficulty were observed at baseline (rho = -0.87, p = 0.014) and week 1 (rho = -0.85, p = 0.019), but neither met the Bonferroni-adjusted significance threshold (α = 0.010). SpO2 increased across the camp, with week 3 significantly higher than weeks 1 and 2 after Bonferroni correction, whereas RHR was significantly lower in week 3 than in week 1, but not week 2, after correction. Athlete-reported outcomes remained stable, acute mountain sickness symptoms were minimal, and haematological and iron-status markers showed no significant pre-to-post changes. Sleep difficulty showed a transient descriptive increase during the initial phase of the altitude camp, while SpO2 and RHR changes were consistent with physiological acclimatisation. Chronotype-related differences in sleep difficulty appeared to be present before altitude exposure rather than emerging specifically during the camp. Given the small sample size, these findings should be considered exploratory and hypothesis-generating.
