Entrainment of rhythmic melatonin secretion in man to a 12-hour phase shift in the light/dark cycle

Daily rhythms in melatonin secretion were monitored in four healthy adult males by measuring the melatonin contents of sequential 4-hour urine specimens and of plasma samples collected at 12-hour intervals, or, in one subject, continuously for 24 hours. All subjects exhibited similar diurnal rhythms, with peak urinary melatonin excretion rates and blood melatonin levels occurring during the daily period of darkness and sleep. When the daily light/dark regimen was phase-shifted by 180°, the plasma and urinary melatonin rhythms required 5–7 days (depending on the subject) to re-entrain to the new schedule. Simultaneous measurements of plasma melatonin levels and melatonin excretion rates indicate that urinary melatonin reflects, with remarkable fidelity, circulating melatonin levels.