Weekly and seasonal changes in human melatonin rhythms linked to local clock, social schedules, light, or sun time?

A one hour earlier timing of melatonin onset in summer compared to winter was observed.

• Melatonin rhythm changes are influenced by both weekly and seasonal factors.
• A 20-minute delay in melatonin onset occurs on work-free days compared to work days.
• Findings suggest that melatonin timing is more closely linked to local clock time and light exposure rather than solely to 'sun time.'
• Data are reported in standard time, providing a framework for understanding circadian timing variations.

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