DLMO stands for dim light melatonin onset: the time in the evening when your melatonin level starts to rise, measured in dim light so that light doesn't hold it back. It's the most widely used marker of where your body clock is set, and researchers and sleep clinicians use it to tell whether someone's clock runs early, late or on time.
Why Melatonin Makes a Good Marker#
Your master clock, the suprachiasmatic nucleus, tells the pineal gland when to release melatonin. Levels stay low through the day, start rising in the evening, peak in the middle of the night and fall toward morning. That rise is tightly controlled by the clock, so its timing is a good readout of the clock itself. For most people, DLMO arrives roughly two hours before they naturally fall asleep.
The catch is light. Bright light in the evening suppresses melatonin, so a measurement taken under normal room lighting can make your clock look later than it really is. That's why the test is done in dim light.
How It's Measured#
In a sleep lab, or at home following careful instructions, you sit in dim light for several evening hours and give a saliva (or blood) sample every 30 to 60 minutes. The samples go to a lab, and DLMO is the time your melatonin crosses a set threshold or rises clearly above its daytime level. It isn't something a phone or a wearable can measure; consumer devices don't measure melatonin.
What DLMO Tells You#
- Chronotype, measured rather than guessed. Evening types have later DLMOs and morning types earlier ones. Questionnaires such as the Munich ChronoType Questionnaire estimate the same thing from your sleep times.
- Delayed sleep phase. In delayed sleep-wake phase disorder, DLMO is much later than usual, which is why going to bed "early" doesn't work. In adults with ADHD and trouble falling asleep, DLMO has been found to be delayed by about an hour and a half on average (Van Veen et al., 2010, Biological Psychiatry). The ADHD sleep guide covers what helps.
- When to time light or melatonin. Clinicians time bright light therapy and melatonin relative to DLMO, because the direction of the shift depends on timing; see the phase response curve.
Moving Your DLMO#
You can't feel your DLMO, but you can move it:
- 1.Morning light moves it earlier. About 20 minutes outdoors soon after waking is a good daily habit; the morning sunlight guide explains how much you need.
- 2.Dim evenings stop it being pushed later. Lower the lights and screens in the last couple of hours before bed; the evening light guide has specifics.
- 3.Consistent timing keeps it stable. Big swings in wake time at weekends move it around; see what social jetlag is.
Melatonin supplements are sometimes used to shift DLMO, usually at low doses and at a specific time. Talk to a doctor or pharmacist before using them.
Where ARC Fits#
ARC doesn't measure melatonin. It works on the habits that move it: a 20-minute morning light timer that starts from your real sunrise, and a wind-down reminder before the bedtime you set, a cue to dim the lights while your melatonin is rising.
Educational content, not medical advice. ARC is a wellness app, not a medical device.