Spatial sensitivity of human circadian response: Melatonin suppression from on-axis and off-axis light exposures

Abstract

A better understanding of the spatial sensitivity of the human circadian system to photic stimulation can provide practical solutions for optimized circadian light exposures. Two psychophysical experiments, involving 25 adult participants in Experiment 1 (mean age = 34.0 years [SD 15.5]; 13 females) and 15 adult participants in Experiment 2 (mean age = 43.0 years [SD 12.6]; 12 females), were designed to investigate whether varying only the spatial distribution of luminous stimuli in the environment while maintaining a constant spectrally weighted irradiance at the eye could influence nocturnal melatonin suppression. Two spatial distributions were employed, one where the luminous stimulus was presented On-axis (along the line of sight) and one where two luminous stimuli were both presented Off-axis (laterally displaced at center by 14°). Two narrowband LED light sources, blue (λ_max = 451 nm) for first experiment and green (λ_max = 522 nm) for second experiment, were used in both the On-axis and the Off-axis spatial distributions. The blue luminous stimulus targeting the fovea and parafovea (On-axis) was about three times more effective for suppressing melatonin than the photometrically and spectrally matched stimulus targeting the more peripheral retina (Off-axis). The green luminous stimulus targeting the fovea and parafovea (On-axis) was about two times more effective for suppressing melatonin than the photometrically and spectrally matched stimulus targeting the more peripheral retina (Off-axis).

Keywords: Circadian phototransduction; Light-at-night; Lighting distribution; Melatonin suppression; Peripheral light.

Fig. 1

Schematic projection of the off-axis (shown in light blue) and on-axis (shown in dark blue) distributions with respect to the macula and its subareas. (Adapted from an illustration by Zyxwv99, Wikimedia Commons, distributed under a CC-BY 4.0 license.) (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

Relative spectral power distributions of the narrowband blue light source (λ_max = 451 nm) and the green light source (λ_max = 522 nm) employed for the two spatial distributions, On-axis and Off-axis. Note that the relative spectral power distribution of the blue light source was not determined by a single LED type (B); rather, the control system only minimized flux from the other two LED types (R and G) in the luminaire. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

The layout of the desktop luminaires in both Experiment 1 (blue) and Experiment 2 (green) with respect to the participants' eyes during the experiment (left), and the two spatial distributions of the luminous stimuli when delivering the On-axis and Off-axis interventions (right). The regions highlighted in blue and green depict the luminous areas. The circle shown in the center of the luminaires represents the suspended, dynamic low-power LEDs that were used to maintain fixation (see Protocol). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

Experimental protocol for both Experiment 1 (blue light) and Experiment 2 (green light), bold, showing the salivary melatonin sample times (S1, S2, and S3). The order of presentation for the On-axis, Off-axis and Dim-light control conditions was counterbalanced across three groups.

Fig. 5

Mean melatonin suppression, compared to the dim-light control condition, for (a) the two blue light spatial distribution profiles combined across duration, (b) the two blue light duration profiles combined across distributions, (c) the two green light spatial distribution profiles combined across duration, (d) the two green light duration profiles combined across distributions. The error bars represent standard error of the mean (SEM) and the asterisks represent statistical significance (p < 0.001). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)