Epidemiological studies suggest that cardiorespiratory fitness (CRfitness) is associated with reduced risk of depression and anxiety in women, however, the mechanisms by which CRfitness may be protective against the development of these disorders are less clear. Because sleep problems are associated with both a higher risk for mental illness and altered physiological responses to stress, this study investigated whether sleep quality might influence the relationship between CRfitness and physiological stress responses in women. Thirty healthy women (18-45 y) who were medication-free, with regular menstrual cycles completed: (1) enrolment visit [including the assessment of CRfitness via maximal oxygen consumption during exercise]; (2) one-week sleep monitoring period including subjective (daily sleep diaries) and objective (wrist actigraphy) sleep measures; and (3) psychosocial stressor protocol (the Trier Social Stress Test; TSST) for the collection of heart rate (HR), blood pressure (BP), and salivary cortisol stress responses. Higher CRfitness was associated with reduced wake after sleep onset (WASO) duration (r = -.38, p = 0.04), higher self-reported sleep quality (higher scores reflect poorer sleep quality; r = -.37, p = 0.05), and lower HR (r = -.43, p = 0.02) during the stressor. Higher sleep quality was associated with a lower HR during the stressor (r = .44, p = 0.01). Increased WASO duration and WASO number were associated with blunted cortisol output during the stressor (r = -.44, p = 0.02, and r = -.46, p = 0.02, respectively). Results suggest that, in women, CRfitness may be protective against the deleterious effects of stress via improved sleep quality.

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