Characterizing light-dark cycles in the Neonatal Intensive Care Unit: a retrospective observational study

Abstract

Objectives: To characterize bedside 24-h patterns in light exposure in the Neonatal Intensive Care Unit (NICU) and to explore the environmental and individual patient characteristics that influence these patterns in this clinical setting. Methods: We conducted a retrospective cohort study that included 79 very preterm infants who stayed in an incubator with a built-in light sensor. Bedside light exposure was measured continuously (one value per minute). Based on these data, various metrics (including relative amplitude, intradaily variability, and interdaily stability) were calculated to characterize the 24-h patterns of light exposure. Next, we determined the association between these metrics and various environmental and individual patient characteristics. Results: A 24-h light-dark cycle was apparent in the NICU with significant differences in light exposure between the three nurse shifts (p < 0.001), with the highest values in the morning and the lowest values at night. Light exposure was generally low, with illuminances rarely surpassing 75 lux, and highly variable between patients and across days within a single patient. Furthermore, the season of birth and phototherapy had a significant effect on 24-h light-dark cycles, whereas no effect of bed location and illness severity were observed. Conclusion: Even without an official lighting regime set, a 24-h light-dark cycle was observed in the NICU. Various rhythmicity metrics can be used to characterize 24-h light-dark cycles in a clinical setting and to study the relationship between light patterns and health outcomes.

Keywords: NICU; chronobiology; circadian rhythms; light; light-dark cycles; neonatology; phototherapy; preterm birth.

FIGURE 1

Overview of 24-h light exposure in the NICU. (A) Examples of light profiles. On the left an example of a patient with a clear 24-h light-dark cycle. This light profile illustrates the light exposure in lux (logarithmic scale) over 24 h. Grey boxes indicate the day with a total of 5 days. On the right an example of a patient without a clear 24-h light-dark cycle due to phototherapy. The grey dotted lines indicate light intensities measured during phototherapy. As the infants’ eyes were covered during phototherapy, values were set to 3 lux for further analyses (indicated by the black lines below the grey dotted lines). (B) Average light distribution over 24-h across all NICU days and patients (N = 79), subdivided in light exposure categories (<5 lux, 5–20 lux, 20–50 lux, and >50 lux) and presented as percentages. (C) Average light exposure per nurse shift comprising morning (from 07:30–15:30), afternoon (from 15:30–23:15) and night (from 23:15–07:30). ***: p < 0.001 (posthoc comparisons following linear mixed effects model with patientID as random effect). Data points represent the mean light exposure per patient across multiple days. Horizontal lines and error bars indicate the mean and standard deviation of each nurse shift. (D) Average time in hours per day above a certain light exposure threshold. Grey lines indicate the individual patients, and the yellow line illustrates the mean.

FIGURE 2

Distribution of light exposure patterns including the individual values. (A) Boxplot indicating the median relative amplitude (RA) with the interquartile range (IQR). RA ranges from 0 (no contrast between the darkest 5 h and the brightest 10 h) and 1 (maximum contrast between the darkest 5 h and the brightest 10 h). (B) Boxplot indicating the median interdaily stability (IS) with the IQR. IS ranges from 0 for a complete lack of consistency between days to 1 for complete day-to-day similarity (C) Boxplot indicating the median intradaily variability (IV) with the IQR. IV ranges from 0 for low fragmentation and 2 for high fragmentation. (D, E) Circular plot showing the start time of the 5 darkest hours (D5) (panel D) and of the brightest 10 h (B10) (panel E). Data points represent individual patients, the direction of the arrow represents the circular mean across patients, and its length represents the mean resultant length (a measure of the spread of circular data).

FIGURE 3

Associations of (A–D) relative amplitude, (E–H) interdaily stability, and (I–L) intradaily variability with environmental and individual patient characteristics. Horizontal lines indicate significant (p < 0.05) posthoc comparisons in case main effects were significant. See Supplementary Table S2 and Supplementary Table S1 for full details and statistics.