Patients’ environmental light exposure in the intensive care unit is consistently low and follows a diurnal pattern. No effect of nighttime light exposure was observed on melatonin secretion. Inadequate daytime light exposure in the ICU may contribute to abnormal circadian rhythms. A total of 67,324 30-second epochs of light data were collected from 17 subjects. Light intensity peaked in the late morning, median 64.1 (interquartile range 19.7–138.7) lux. The 75th percentile of light intensity exceeded 100 lx only between 9 AM and noon, and never exceeded 150 lx. A light monitor was affixed to subjects’ bed at eye level in a modern intensive care unit and continuously recorded luminescence for at least 24 h per subject. Blood was sampled hourly and measured for plasma melatonin.

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