What it isThe time in the evening when your body begins producing the sleep hormone melatonin, measured under dim lighting conditions to avoid light suppression. DLMO typically occurs 2-3 hours before your natural sleep time.

Why it mattersDLMO is considered the gold standard for measuring circadian phase in clinical settings. It tells clinicians exactly where your biological clock is set, which is essential for diagnosing and treating circadian rhythm disorders.

Think of it like thisDLMO is like catching the exact moment your body starts dimming its own lights for the evening. It marks the official beginning of your biological night.

Dim-light melatonin onset (DLMO) is the time at which endogenous melatonin concentration rises above a defined threshold under dim lighting conditions (typically <30 lux), serving as the gold-standard biomarker of circadian phase in humans. The threshold is commonly set at 3 pg/mL for salivary samples or 10 pg/mL for plasma. DLMO typically occurs 2 to 3 hours before habitual sleep onset in normally entrained individuals.

MechanismThe SCN drives melatonin synthesis via a multisynaptic pathway: SCN → paraventricular nucleus → intermediolateral cell column → superior cervical ganglion → pineal gland. Norepinephrine activates AANAT (the rate-limiting enzyme), converting serotonin to melatonin. This pathway is acutely suppressed by blue light (~480nm) via melanopsin-expressing ipRGCs. DLMO is measured under dim light specifically to reveal the underlying endogenous circadian signal without masking.