Unanticipated daytime melatonin secretion on a simulated night shift schedule generates a distinctive 24‐h melatonin rhythm with antiphasic daytime and nighttime peaks

Circulating melatonin levels were significantly increased during daytime sleep (p < .0001).

• A secondary peak in melatonin was observed during the circadian day, alongside the primary peak at night, following night shift conditions.
• This unusual diurnal rhythm with peaks at opposite times challenges existing models of melatonin secretion regulation.
• Increased daytime melatonin concentrations were positively correlated with shifts in the timing of the nighttime melatonin peak (p = .002).
• Repeated exposure to simulated night shifts intensified both daytime melatonin increases and shifts in nighttime peak timing (p = .002 and p = .006).
• Melatonin secretion during daytime sleep was positively associated with increased 24-hour glucose and insulin levels during the night shift protocol (p = .014 and p = .027).

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