DLMO

DLMO, or Dim Light Melatonin Onset, is a widely used marker of circadian timing in humans. It refers to the time in the evening when melatonin production begins under dim lighting, reflecting the internal circadian phase independent of bedtime.

Measurement involves placing the subject in dim light, typically less than 5–10 lux, for several hours in the early evening and collecting saliva (or plasma) samples at 30 to 60 minute intervals. Melatonin concentration is plotted over time, and DLMO is defined as the point at which levels rise above a predetermined threshold, commonly 3 pg/mL for saliva, or by the initial rise using interpolation between samples. Some laboratories use 4 pg/mL or alternative criteria; definitions can vary.

DLMO timing is typically several hours before habitual bedtime and serves as a robust index of circadian phase. Shifts in DLMO accompany behavioral and environmental changes: bright evening light tends to delay DLMO, while morning light can advance it. Melatonin administration can also shift timing when applied relative to the DLMO.

Applications include assessment of circadian rhythm sleep-wake disorders such as delayed sleep phase syndrome, jet lag, and shift-work disorder, as well as guiding chronotherapy with light exposure and/or melatonin. It is also used in research to study chronotypes and the dynamics of circadian entrainment.

Limitations include variability due to age, health, medications, and lifestyle; the need for controlled lighting and repeated sampling; and a lack of universal standardization for the threshold, which can affect cross-study comparisons.