Circadian timing does not modulate human chromatic temporal contrast sensitivity

IntroductionPrevious studies have reported diurnal variations in colour and brightness perception, suggesting modulation of visual processing by the circadian pacemaker. Visual processing of image features such as colour and brightness begins with photoreceptor activity in the retina. To clarify potential time-of-day effects, we examined whether photoreceptor activation--the earliest stage of visual processing--is modulated by circadian timing. MethodsWe disentangled circadian and homeostatic influences on human visual performance using a 40-hour forced desynchrony protocol. Twelve participants (5 female, 24.4 {+/-} 2.9 years, range 19-30) followed a 2:1 sleep-wake cycle (3.75 h each) under constant environmental conditions. Circadian phase was determined from 46 salivary melatonin samples collected per participant. During wake periods, participants repeatedly completed temporal contrast sensitivity tasks at 2 Hz and 8 Hz, assessing luminance (L+M+S) and chromatic (L-M, S) mechanisms. We tested for circadian modulation of visual performance by comparing multi-level cosinor and uninformative Bayesian models. ResultsAll participants showed robust melatonin rhythms (mean{tau} = 24.18 {+/-} 0.39 h). Bayesian inference provided decisive evidence against circadian variation in luminance or chromatic contrast sensitivity. DiscussionThese findings challenge previous reports of circadian changes in visual sensitivity, indicating that earlier effects likely reflected uncontrolled homeostatic factors or mechanisms beyond cone-mediated vision. SignificanceThrough precise control of environmental and physiological factors, this study offers new insights into how biological timing interfaces with human visual processing.