Environmental lighting conditions in neonatal intensive care units and their potential relevance for circadian entrainment: A multicenter observational study

BACKGROUND: Light is the primary environmental cue synchronizing circadian rhythms, yet the lighting conditions in neonatal intensive care units (NICUs) often deviate from natural light-dark cycles. This study aimed to quantify circadian-relevant light exposure in NICUs and examine its potential association with neonatal melatonin production. METHODS: In this prospective multicenter study, ambient and procedural illuminance were measured at patient and caregiver eye level using a portable light meter across eight NICUs in four tertiary hospitals. In a single-center substudy, a dosimeter continuously recorded illuminance and correlated color temperature (CCT). Urinary 6-sulfatoxymelatonin (aMT6s) was collected every four hours on days 1, 2, 3, 5, and 7 from 37 preterm and term neonates. RESULTS: Among 1022 measurements, 82.3% (neonates) and 81.8% (caregivers) were exposed to <100 lx at eye-level. Examination lamps displayed significant variability, with maximum intensities ranging from 3747 to 25,720 lx. Across 314 patient-days, median daytime illuminations never exceeded 100 lx, whereas nighttime exposure exceeding 100 lx for more than 10 min occurred on 24.4% of days. CCT values remained near 4000 K throughout the 24-h cycle, showing minimal spectral variation. Urinary aMT6s showed no diurnal rhythm and did not differ between term and preterm infants. CONCLUSION: NICU lighting conditions were characterized by low daytime illuminance, limited day-night spectral variation, and recurrent nocturnal exposure to artificial light with higher correlated color temperatures. These environmental lighting patterns may be potentially unfavorable for circadian entrainment in neonates and warrant further investigation regarding their biological relevance for hospitalized infants and healthcare staff within the same environmental setting. IMPLICATIONS FOR CLINICAL PRACTICE: Integrating circadian-aware lighting concepts into daily nursing workflows, including optimized daytime illumination and reduced nocturnal light exposure during care activities, may represent a practical strategy to support neonatal adaptation and improve NICU lighting environments.