Plasma Melatonin Levels, Seasonal Variation, and Sleep Quality: A Stratified Analysis of Adults Aged 25–65 Years Using the Pittsburgh Sleep Quality Index and Actigraphy

This study aimed to examine plasma melatonin levels, seasonal variation, and their association with sleep quality in adults aged 25–65 years using both the Pittsburgh Sleep Quality Index and actigraphy. A cross-sectional study was conducted in a stratified random sample of 500 adults from Salamanca and Ávila (Spain). Plasma melatonin was quantified by HPLC-DAD-MS/MS, and sleep quality was assessed using the Pittsburgh Sleep Quality Index and a 5-day wrist actigraphy protocol (ActiGraph GT3X+). Participants were stratified by age and sex, and seasonal variation was evaluated. Multivariable analyses were adjusted for relevant confounders. A total of 334 participants with valid melatonin measurements were included (mean age 43 years). Mean plasma melatonin concentration was 14.5 ± 15.9 pg/mL, with higher levels observed in men than women and a decreasing trend with age. A trend toward higher melatonin levels during winter was observed, although the differences were not statistically significant. No significant association was found between plasma melatonin levels and Pittsburgh Sleep Quality Index scores. However, higher melatonin levels were associated with slightly greater sleep efficiency measured by actigraphy. Participants reporting melatonin supplement use showed higher Pittsburgh Sleep Quality Index scores and lower total sleep time compared with non-users. Overall, plasma melatonin levels varied by sex, age, and season but showed no association with subjective sleep quality, while a modest relationship was observed with objective sleep efficiency, suggesting that melatonin alone is not a strong determinant of perceived sleep quality and highlighting the multifactorial nature of sleep regulation in adults.