DLMO (Dim Light Melatonin Onset)
The gold-standard marker for reading the exact time on your internal body clock
Definition
DLMO (Dim Light Melatonin Onset) is the evening moment at which the pineal gland begins secreting melatonin, measured under dim light (below 30-50 lux) so the hormone is not suppressed. It is regarded as the most reliable marker of central circadian clock phase in humans: it pinpoints what "internal time" the body is keeping, independent of the clock on the wall. DLMO is determined by sampling saliva or blood at 30-minute intervals through the evening and detecting when melatonin crosses a fixed threshold (typically 3-4 pg/mL in saliva).
Detailed explanation
DLMO is the gold standard of chronobiology because melatonin secretion is driven directly by the suprachiasmatic nucleus (the hypothalamic circadian pacemaker) via the pineal gland, and unlike core body temperature or cortisol it is largely unaffected by activity, posture or sleep itself. The methodological key is dim light: even moderate household lighting suppresses melatonin, so samples are collected in near-darkness in the hours before habitual bedtime.
In a person with an intermediate chronotype, DLMO typically occurs 2-3 hours before sleep onset. A delayed DLMO is the physiological signature of delayed sleep-wake phase disorder, and an advanced DLMO that of advanced sleep-wake phase disorder.
Its clinical value is twofold: it phase-types the circadian clock in rhythm disorders (jet lag, shift work, circadian insomnia, seasonal depression) and it precisely times therapies such as bright light or exogenous melatonin. Taking melatonin a few hours before DLMO produces the largest phase advance, whereas dosing it at random can be ineffective or even counterproductive.
Scientific sources
- PubMed — The dim light melatonin onset as a marker for circadian phase position (Lewy & Sack)
- PubMed — Dim light melatonin onset (DLMO): a tool for the analysis of circadian phase in human sleep and chronobiological disorders (Pandi-Perumal)
- PubMed — The dim light melatonin onset, melatonin assays and biological rhythm research in humans (Lewy)
- PubMed — Calculating the dim light melatonin onset: the impact of threshold and sampling rate (Molina & Burgess)
Related terms
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