Circadian rhythms have been shown to regulate sleep-wake timing across the lifespan, yet many questions remain about early childhood circadian physiology. Understanding dim-light melatonin onset (DLMO) and offset (DLMOff), established markers of circadian phase, is essential for characterizing circadian rhythms in early childhood. We examined the distribution of salivary DLMO and DLMOff and their relationship with actigraphic sleep timing across 20 healthy preschoolers (M = 4.31 ± 0.34 years, 45% female). After maintaining a consistent sleep schedule for 7 days, children completed an in-home circadian assessment. Children were awoken 1.5 h before habitual wake time, and saliva samples were collected in 20- to 30-min intervals throughout the morning to determine DLMOff, then in the evening until 50 min past habitual bedtime to assess DLMO. A 4-pg/ml threshold was used to calculate each phase marker. DLMO ranged from 17:22 to 20:40 (M = 18:55 ± 0:54) and was positively associated with bedtime, sleep onset, and midsleep. In contrast, morning melatonin levels were highly variable, allowing DLMOff calculation in only 8 participants. Within this small subsample, later DLMOff was associated with later chronotype (r = 0.81), sleep offset (r = 0.84), and midsleep (r = 0.80). Across the full sample, interpolated melatonin levels at habitual wake remained ≥ 4 pg/ml for 45% of children, a pattern broadly consistent with findings in adults, in which a majority of participants exhibit DLMOff after habitual wake time. These findings indicate that although evening melatonin profiles were consistently well-defined, permitting reliable calculations of DLMO across all participants, morning melatonin patterns were often irregular in young children. When able to be calculated, DLMOff showed strong associations with sleep timing, suggesting it could be a reliable marker of circadian phase. However, high variability and fluctuating morning melatonin patterns make DLMOff difficult to determine in many preschool-aged children.
