Effect of exposure duration and light spectra on nighttime melatonin suppression in adolescents and adults

Abstract

This study investigated how light exposure duration affects melatonin suppression, a well-established marker of circadian phase, and whether adolescents (13-18 years) are more sensitive to short-wavelength (blue) light than adults (32-51 years). Twenty-four participants (12 adolescents, 12 adults) were exposed to three lighting conditions during successive 4-h study nights that were separated by at least one week. In addition to a dim light (<5 lux) control, participants were exposed to two light spectra (warm (2700 K) and cool (5600 K)) delivering a circadian stimulus of 0.25 at eye level. Repeated measures analysis of variance revealed a significant main effect of exposure duration, indicating that a longer duration exposure suppressed melatonin to a greater degree. The analysis further revealed a significant main effect of spectrum and a significant interaction between spectrum and participant age. For the adolescents, but not the adults, melatonin suppression was significantly greater after exposure to the 5600 K intervention (43%) compared to the 2700 K intervention (29%), suggesting an increased sensitivity to short-wavelength radiation. These results will be used to extend the model of human circadian phototransduction to incorporate factors such as exposure duration and participant age to better predict effective circadian stimulus.

Figure 1

The relative spectral power distributions for the two lighting interventions used in this study

Figure 2

The protocol used in this study, showing the relationship between the successive study nights (listed in order of occurrence), lighting conditions and saliva sample times

Figure 3

The significant main effect of duration of exposure. The bars show the combined mean melatonin suppression (T2–T5) for both age groups and the two intervention spectra (both providing CS=0.25). The error bars represent SEM, and the asterisks denote statistical significance

Figure 4

The significant interaction between spectrum and participant age. The bars show mean melatonin suppression over the entire 4-h light exposure, by age group and intervention spectrum (both providing CS=0.25). The error bars represent SEM, and the asterisk denotes statistical significance

Figure 5

Mean melatonin suppression recorded for the adults and adolescents during exposure to each of the two intervention spectra at saliva sample times. The error bars represent SEM, and the asterisks denote statistical significance