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The transduction of light energy at the retina goes on to affect a range of non-image forming processes, most notably circadian photoentrainment, mood, and the pupillary light reflex. Our lab has previously demonstrated that retinal phototransduction can also modulate an animal’s breathing. However, it remains unknown how the timing of a light stimulus alters phototransduction and resulting behavior. Here, we investigate how unpredictable light stimuli affect respiratory frequency and tidal volume in mice. Male C57BL/6J mice (n = 8) were maintained on a 12:12 light–dark cycle, and breathing was assessed using whole-body plethysmography. All light stimuli were presented during the animals’ dark phase, either 1 hour after lights off (“early” dark phase, ZT13 to ZT16) or 5 hours after lights off (“late” dark phase, ZT17 to ZT20). As supported by our prior research, early light stimuli immediately suppressed breathing and, later, led to an increase in breathing after stimuli offset. However, late dark phase stimuli failed to affect respiration, even when spectral composition of the light was modified. These data demonstrate that the timing of light has differential effects on breathing. These data may implicate time-dependent differences in phototransduction and/or time-dependent differences in signal processing which go on to affect fundamental physiological processes. 

Respiratory frequency and tidal volume exhibit daily, 24-hr rhythms in human and rodent models. Environmental light has emerged as a potential modulator of ventilatory rhythmicity, as mice lacking intrinsically photosensitive retinal ganglion cells fail to alter their breathing in response to light. Despite this evidence, it remains unknown how the duration of light exposure influences breathing in mice. To assess the effects of light exposure on breathing, male wild-type mice (n = 8) were exposed to broad-spectrum white light (~450 lux) in the standard dark phase, either for 3h (ZT 13-16) or for 5 minutes (ZT13-13.05). Respiratory measures were assessed for 36 hours using whole-body plethysmography. To determine whether the light manipulation produced significant deviations from expected respiratory patterns, a nonparametric, within-subjects bootstrapping approach was conducted in R. This compared parallel time points between a predefined test period and the equivalent control period without a light manipulation. We found that a 3h light stimulus administered during the standard dark phase reduced tidal volume and respiratory frequency for ~80 minutes during light exposure. Immediately following the offset of the 3h stimulus, respiratory frequency was increased for 2h compared to control. While statistical analysis is currently ongoing, a 5-minute light stimulus appeared to decrease both tidal volume and respiratory frequency during light exposure. In contrast to a 3hr stimulus, both tidal volume and respiratory frequency were increased at 90 minutes following the offset of the 5-minute light stimulus. While preliminary, these data suggest that tidal volume and respiratory frequency differentially respond to the duration of environmental light exposure. Subsequent research is required to determine if tidal volume is specifically responsive to shorter light durations and the extent to which a 3h light stimulus may be “masking” the tidal volume response. This work expands upon our current understanding of respiratory physiology to include light duration as a key variable affecting daily breathing