Seasonal variation in hair cortisol concentration: A systematic review

Hair cortisol concentration (HCC) is a crucial biomarker in psychoneuroendocrinological research, offering unique insights into long-term hypothalamic–pituitary–adrenal axis activity. Season has repeatedly shown associations with HCC. However, as of yet, no systematic attempt at quantifying season’s influence on HCC has been undertaken. We conducted a systematic search of the bibliographic databases PubMed and PsycINFO. Twenty-nine between- and within-person studies fulfilled all eligibility criteria (N = 10,520 participants in total). Overall, 22 studies (76%) reported significant differences in HCC across seasons. Most between-person studies reported lower HCC in winter/spring than in summer/autumn (10/15). This pattern was supported by 2/14 within-person studies, whereas others reported lower HCC in summer than in autumn (6/14). The remaining studies reported other patterns or no seasonal variations in HCC. In conclusion, there is accumulating evidence for seasonal variations in HCC, highlighting the need to consider the seasons in future research on HCC and health. Mechanisms related to meteorological, ecological, sociocultural, and lifestyle factors may underlie seasonal rhythmicity in cortisol secretion and accumulation in hair.