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Abstract

Background: Daily physiological and behavioural patterns are governed by two independent processes: an endogenous circadian rhythm and a sleep homeostat. The synchronisation of the circadian rhythm with the external environment depends on light cues that are processed by the retinohypothalamic pathway connecting the retina to the internal clock in the hypothalamus. Thus, visual input is a critical circadian signal. Based on evidence for time-of-day dependency in image-forming (e.g., luminance perception) and non image-forming functions (e.g., pupillary light response), we hypothesise that the sensitivity of the visual system may also be modulated by the circadian clock.MethodsIn a 16-hour protocol, we measured variation in early stages of visual processing in two healthy participants. Whilst staying in constant conditions, they provided repeated measures of ocular structures and functional mechanisms in 3-hour intervals. Pupillometry during exposure to chromatic light was used to address photoreceptor sensitivity, and psychophysical thresholds were determined with silent substitution stimuli to characterise variations in post-receptoral retinal mechanisms underlying luminance and colour perception. We collected salivary melatonin and core body temperature as biomarkers of circadian phase.Results DiscussionOur results suggest considerable intra- and inter-individual variability in eye physiology and visual function. Along with the procedural insights from the pilot study, these can inform experimental design and analysis of future studies aiming to disentangle circadian and homeostatic influences on retinal mechanisms. Thoroughly characterising circadian influences on visual functions has important implications for our understanding of the complex interplay between internal physiological processes and external environmental cues.