DLMO (Dim Light Melatonin Onset)

Dim Light Melatonin Onset (DLMO) is the time of evening at which endogenous melatonin in saliva or plasma rises above a defined threshold under dim ambient light (typically below 50 lux) which would otherwise suppress secretion. Standard thresholds are around 3 pg/mL in saliva or 10 pg/mL in plasma, sampled at 30-minute intervals. Formalised by Alfred Lewy and colleagues, DLMO is widely regarded as the gold-standard phase marker of the human circadian system because melatonin secretion is tightly controlled by the suprachiasmatic nucleus and is minimally confounded by sleep, posture, or activity. Clinically, DLMO is used to diagnose delayed and advanced sleep-wake phase disorders (DSPS/ASPS), to time exogenous melatonin and light therapy, to manage shift work and jet lag, and as a research outcome in chronobiology trials.