High-resolution monitoring reveals fragmented 24-hour light exposure in intensive care units

Light is a key regulator of circadian rhythms, sleep, and neuroendocrine function. Abnormal light exposure has been associated with delirium and adverse outcomes in critically ill patients. While previous ICU studies have generally reported low daytime illuminance and intermittent nocturnal light exposure, most relied on averaged measurements or low temporal resolution monitoring, potentially obscuring clinically relevant exposure patterns. In routine ICU care, light exposure often occurs as brief but frequent events related to necessary interventions, yet the extent and temporal structure of such light fragmentation remain poorly characterized. This study therefore aimed to characterize high-resolution 24-hour light exposure patterns and nocturnal fragmentation in ICU patients.

In this observational study, ambient illuminance was continuously recorded every 5 s in adult ICU patients over a one-year period. A total of 222 patients were monitored for up to 7 consecutive days, yielding more than 14 million individual measurements. Light exposure was analysed across complete 24-hour cycles and stratified into daytime (07:00-20:59) and nighttime (21:00-06:59) periods. Outcomes included median illuminance, frequency of light events, extreme illuminance (99th percentile), and duration of uninterrupted nocturnal darkness. Mixed-effects models and negative binomial regression were used to account for repeated measurements and to assess associations with room configuration.

Across the 24-hour cycle, median illuminance demonstrated a flattened diurnal pattern, with daytime light levels remaining markedly below those typically encountered in indoor working environments and limited sustained darkness at night. Nighttime exposure was dominated by frequent short-duration light events, resulting in pronounced fragmentation of nocturnal darkness. Fragmentation patterns were largely independent of room configuration and treatment intensity, as ambient illuminance did not differ according to organ support modality once time of day was considered. Extreme illuminance values occurred intermittently, predominantly during daytime, and varied according to room type.

ICU light exposure is characterized primarily by temporal fragmentation rather than sustained brightness. Repeated interruptions of darkness may represent a plausible environmental mechanism contributing to circadian dysregulation. These findings highlight care processes as a key target for interventions and support the use of fragmentation-based metrics in future circadian research in critical care.