Evidence of circalunar rhythmicity in young children's evening melatonin levels

In adults, recent evidence demonstrates that sleep and circadian physiology change across lunar phases, including findings that endogenous melatonin levels are lower near the full moon compared to the new moon. Here, we extend these results to early childhood by examining circalunar fluctuations in children's evening melatonin levels. We analysed extant data on young children's circadian rhythms (n = 46, aged 3.0-5.9 years, 59% female). After following a strict sleep schedule for 5-7 days, children completed an in-home, dim-light circadian assessment (<10 lux). Salivary melatonin was assessed at regular 20- to 30-min intervals until 1 h past each child's scheduled bedtime. Melatonin levels varied significantly across lunar phases, such that melatonin was lower in participants assessed near the full moon as compared to near the new moon. Significant differences were observed at 50 min (mean_full  = 2.5 pg/ml; mean_new  = 5.4 pg/ml) and 10 min (mean_full  = 7.3 pg/ml; mean_new  = 15.8 pg/ml) before children's scheduled bedtime, as well as at 20 min (mean_full  = 15.5 pg/ml; mean_new  = 26.1 pg/ml) and 50 min (mean_full  = 19.9 pg/ml; mean_new  = 34.3 pg/ml) after bedtime. To our knowledge, these are the first data demonstrating that melatonin secretion, a process regulated by the human circadian system, is sensitive to changes in lunar phase at an early age. Future research is needed to understand the mechanisms underlying this association (e.g., an endogenous circalunar rhythm) and its potential influence on children's sleep and circadian health.