Evening Light Intensity and Phase Delay of the Circadian Clock in Early Childhood

Abstract

Late sleep timing is prevalent in early childhood and a risk factor for poor behavioral and health outcomes. Sleep timing is influenced by the phase of the circadian clock, with later circadian timing linked to delayed sleep onset in young children. Light is the strongest zeitgeber of circadian timing and, in adults, evening light produces circadian phase delay in an intensity-dependent manner. The intensity-dependent circadian phase-shifting response to evening light in children, however, is currently unknown. In the present study, 33 healthy, good-sleeping children aged 3.0 to 4.9 years (M = 4.14 years, 39% male) completed a 10-day between-subjects protocol. Following 7 days of a stable sleep schedule, an in-home dim-light circadian assessment was performed. Children remained in dim-light across 3 days (55 h), with salivary melatonin collected in regular intervals throughout each evening. Phase-shifting effects of light exposure were determined via changes in the timing of the dim-light melatonin onset (DLMO) prior to (Day 8) and following (Day 10) a light exposure stimulus. On Day 9, children were exposed to a 1 h light stimulus in the hour before their habitual bedtime. Each child was randomly assigned to one intensity between 5 and 5000 lux (4.5-3276 melanopic EDI). Across light intensities, children showed significant circadian phase delays, with an average phase delay of 56.1 min (SD = 33.6 min), and large inter-individual variability. No relationship between light intensity and magnitude of the phase shift was observed. However, a greater percentage of melatonin suppression during the light exposure was associated with a greater phase delay (r = -0.73, p < 0.01). These findings demonstrate that some young children may be highly sensitive to light exposure in the hour before bedtime and suggest that the home lighting environment and its impact on circadian timing should be considered a possible contributor to behavioral sleep difficulties.

Keywords: child development; circadian rhythm; light; phase delay; preschool; sleep.

Fig 1:. 10-day study protocol.

Children followed a strict, parent-selected sleep schedule for 7 days, followed by a 3-day circadian protocol. On Days 8 and 10, DLMO was assessed in order to quantify circadian phase. On Day 9, children were exposed to light for 1 h in the hour before scheduled bedtime. The sleep schedule depicted is an example; actual bedtimes and wake times differed across individual participants.

Fig 2:. Distribution of the circadian timing of light onset across participants.

Light onset ranged from 68 min before to 68 min after children’s baseline dim-light melatonin onset.

Fig 3:. Group averages of salivary melatonin levels.

Closed circles represent saliva samples collected during the Baseline night (Day 8) and open circles represent those collected on the Final night of the study protocol (Day 10). Error bars denote standard error. Melatonin levels on Day 10 were significantly lower than at the same clock time on Day 8 at 50 min, 30 min, and 10 min before habitual bedtime, as well as 20 min and 50 min after habitual bedtime.

Fig 4:. Circadian phase shift as a function of light intensity.

Negative numbers (below the dashed line) indicate a phase delay, whereas positive numbers (above the line) indicate a phase advance.

Fig 5:. Melatonin profiles of 6 participants.

Orange lines represent melatonin on the baseline night and blue lines represent melatonin on the final night of the assessment. The dotted line at 4 pg/ml depicts the threshold for melatonin onset. The timing of the 1h light exposure is represented by the yellow shaded area. A and B depict the melatonin profiles of children who received low intensities of light at close circadian times. Similarly, participants C and D received a medium intensity of light, and participants E and F received a high intensity. In each pair, despite similarities in the circadian timing and intensity of the experimental light exposure, large individual differences in circadian phase shift were observed.

Fig 6:. Scatterplot illustrating association between acute percent melatonin suppression (Day 9) and subsequent phase shift (Day 10).

We observed a significant negative association between the magnitude of the melatonin suppression and phase shift (r = −0.73, p < 0.01).