Bayesian inference provided decisive evidence against circadian variation in luminance or chromatic contrast sensitivity, suggesting that previously reported time-of-day effects on visual performance may reflect homeostatic influences rather than circadian modulation.
Key Findings
Results
No detectable near-24-hour circadian modulation of cone-mediated temporal contrast sensitivity was found in any of the conditions tested.
Testing was conducted using a 40-hour forced desynchrony protocol to disentangle circadian from homeostatic influences
Temporal contrast sensitivity was measured at 2 Hz and 8 Hz temporal frequencies
Both luminance (L+M+S) and chromatic (L-M and S-cone-directed) modulations were probed
Bayesian inference provided 'decisive evidence against circadian variation in luminance or chromatic contrast sensitivity'
Hierarchical cosinor models were compared against intercept-only null models using Bayes factors
Results
All twelve participants demonstrated robust melatonin rhythms, confirming that the forced desynchrony protocol successfully characterized individual circadian periods.
Mean circadian period (τ) was 24.18 ± 0.39 hours across participants
Up to 46 salivary melatonin samples were collected per participant to estimate circadian period
The sample consisted of twelve participants (five female), aged 24.4 ± 2.9 years (range 19–30)
The forced desynchrony protocol used a repeating 3.75-hour cycle comprising 2 hours 30 minutes of wakefulness and 1 hour 15 minutes of sleep under constant environmental conditions
Discussion
Previously reported diurnal variations in visual contrast sensitivity may reflect homeostatic influences, pupil-linked changes in retinal illuminance, or mechanisms outside the cone-mediated pathways rather than circadian modulation.
The authors note that prior studies did not disentangle circadian and homeostatic influences on visual performance
Proposed alternative explanations include 'homeostatic influences, pupil-linked changes in retinal illuminance, or mechanisms outside the cone-mediated pathways probed here'
The study design controlled for environmental conditions throughout the protocol
The forced desynchrony approach allowed circadian phase to be separated from sleep-wake-related influences
Methods
The forced desynchrony protocol with a 3.75-hour cycle was used to separate circadian phase from sleep-wake (homeostatic) influences on visual performance.
The 3.75-hour cycle (2 hours 30 minutes wake, 1 hour 15 minutes sleep) is incommensurate with the ~24-hour circadian period, allowing independent assessment of circadian and homeostatic contributions
The total protocol duration was 40 hours
Constant environmental conditions were maintained throughout
Temporal contrast sensitivity tasks were completed repeatedly during wake periods across the protocol
What This Means
This research suggests that the time of day, as controlled by the body's internal ~24-hour clock (the circadian pacemaker), does not meaningfully affect how well people detect flickering or changing visual patterns (temporal contrast sensitivity). The study used a clever 'forced desynchrony' protocol in which 12 participants followed an unusual 3.75-hour sleep-wake schedule over 40 hours. Because this schedule is out of sync with the body's natural clock, it allowed researchers to mathematically separate the influence of the circadian clock from the influence of how long someone has been awake (the homeostatic or sleep-pressure effect). Participants' internal clock timing was tracked with up to 46 saliva samples each, measuring the hormone melatonin.
Participants repeatedly completed visual tests measuring their ability to detect luminance (brightness) and color flickering at two different speeds (2 Hz and 8 Hz). Using Bayesian statistical methods, the researchers found strong evidence that the circadian clock does not drive changes in any of these visual abilities. This held true for all three types of visual pathways tested (brightness, red-green color, and blue-yellow color contrast).
This research suggests that earlier studies reporting that vision changes throughout the day were likely capturing other influences—such as how alert or sleepy someone is, or changes in pupil size that affect how much light reaches the retina—rather than true circadian clock effects. By carefully isolating the circadian clock's contribution, this study clarifies that the body's timekeeping system may not directly regulate this aspect of cone-based vision, which has implications for how we interpret time-of-day effects in visual and broader neuroscience research.
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Heinrichs H, Spitschan M. (2026). Circadian timing does not modulate human temporal contrast sensitivity.. Journal of vision. https://doi.org/10.1167/jov.26.10.14