Field assessment of melatonin timing reveals circadian misalignment beyond chronotype in elite female football players.

Melatonin production is the gold-standard marker of the physiological "biological night". Assessing it in athletes outside the laboratory enables individualized circadian interventions. This study quantified morning misalignment in elite athletes and examined its association with subjective well-being. Twenty-two elite female athletes (mean age = 26.8 ± 4.3 years) from the same national squad were monitored during a one-week training camp with standardized meal, training and sleep opportunities. Athletes wore actimetry devices to measure sleep and light exposure and completed daily sleep diaries with well-being ratings. On one day, salivary melatonin was self-sampled hourly from four hours before habitual sleep onset to one hour after, and again at wake time and one hour after waking. A longer phase angle between melatonin onset and sleep onset was associated with longer total sleep duration, whereas being an evening chronotype and sharing a room with a dissimilar chronotype were both independently associated with shorter total sleep duration. Elevated melatonin one hour after waking, indicative of morning circadian misalignment, was associated with lower subjective well-being. These findings demonstrate that real-life circadian timing influences both sleep and well-being in elite athletes and support the value of individualized circadian-informed strategies to optimize health and recovery.