LIGHT SPECTRA: AN IMPORTANT CONSIDERATION FOR CIRCADIAN ALIGNMENT IN THE MEDICAL ICU

Abstract

TOPIC: Sleep Disorders TYPE: Original Investigations PURPOSE: Human circadian rhythms are regulated by a central ‘master clock’ which is entrained (i.e., aligned) to the 24-hour day primarily by light. Bright daytime light and lack of nighttime light are key entrainment signals. Light is detected via retinal photoreceptors, of which melanopsin is the primary circadian receptor. Circadian rhythms are best entrained by light wavelengths between 446 and 477 nanometers (nm), which are dominant in sunlight. Light wavelengths outside of this range will fail to optimally activate circadian photoreceptors and thus will not promote circadian alignment. A lack of activating wavelengths in hospital settings may potentiate circadian misalignment; circadian misalignment is associated with poor outcomes in critically ill patients.Light spectra measurements provide the intensity, or relative presence, of wavelengths within a defined range. Our study aims to determine the intensity of wavelengths within the approximate visible light range (380-780 nm) in patient rooms in two medical intensive care unit (MICU) campuses in a tertiary academic hospital system. We hypothesized that, during the day, MICU room lighting lacks significant presence of wavelengths between 446 and 477 nm. METHODS: To test this, we conducted an observational study of the visible spectra in 13 MICU rooms. Spectra were captured at 09:00, 13:00, and 17:00 on a sunny and cloudy day each month for 3 months, with a plan to continue for a full year. To test the contribution of artificial light, the spectra of each light source in the room was measured at night. All readings were taken from a standard location at the head of the patient bed. RESULTS: Rooms in both MICUs revealed common peak light intensities at 545 nm and 611 nm at all time points. These wavelengths align with the two wavelengths of greatest intensity emitted by artificial lights in the MICU. On average, 40% of measured intensities came from artificial room lighting, with MICU 1 having a higher percentage (46%) than MICU 2 (31%). Within the total visible spectrum, wavelengths between 446 and 477 nm had a greater daily contribution in MICU 2 (12%) than MICU 1 (8%). MICU rooms with west facing windows had a higher percentage of artificial light present in their spectra (52%) compared to rooms with east facing windows (35%). Similarly, rooms with east-facing windows demonstrated greater relative intensity of wavelengths between 446 and 477 nm (11%) compared to west-facing windows (7%). Over the day, as expected, room light spectra progressively shifted away from natural light, with late afternoon having a strong percentage of artificial spectra. CONCLUSIONS: The wavelengths of greatest intensity present during the day in MICU rooms are outside of the range known to optimally activate retinal photoreceptors that signal day-night orientation. Patients receive insufficient light reflecting natural sunlight, thus, our MICU room lighting likely fails to entrain patient circadian rhythms. CLINICAL IMPLICATIONS: Circadian misalignment has been associated with cardiovascular, immune, metabolic, and neurologic dysfunction, and in critically ill patients, death. ICUs contain myriad factors that disrupt circadian rhythms. The particular wavelengths of light patients are exposed to must be closely considered in the environment of care so that patients have the opportunity to remain entrained in order to heal. DISCLOSURES: No relevant relationships by Taylor Intihar, source=Web Response No relevant relationships by Melissa Knauert, source=Web Response No relevant relationships by Katherine Wasden, source=Web Response