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

The daily rhythm of plasma melatonin concentrations is typically unimodal, with one broad peak during the circadian night and near‐undetectable levels during the circadian day. Light at night acutely suppresses melatonin secretion and phase shifts its endogenous circadian rhythm. In contrast, exposure to darkness during the circadian day has not generally been reported to increase circulating melatonin concentrations acutely. Here, in a highly‐controlled simulated night shift protocol with 12‐h inverted behavioral/environmental cycles, we unexpectedly found that circulating melatonin levels were significantly increased during daytime sleep (p