Phasic pupil responses to melanopsin- and cone-directed stimulation remain stable across circadian time
Year: 2026
Type: preprint
Abstract: Human pupil responses vary with time of day, but whether this reflects circadian modulation of specific photoreceptor pathways is unclear. Twelve healthy adults completed repeated pupillometry during a 40-hour ultra-short sleep-wake protocol that limited sustained wakefulness; data from eight met the quality-control criteria and were analysed. This design was intended to limit progressive accumulation of homeostatic sleep pressure across sampled circadian phase. During wake episodes, 0.4-Hz silent-substitution stimuli targeted broadband flux, melanopsin, luminance, L-M cone-opponent, or S-cone pathways. Salivary melatonin provided participant-specific period estimates. We separated slow tonic constriction from phasic pupil tracking and compared hierarchical circadian cosinor models with constant-level models. Constant-level models were favoured for phasic amplitude in all five conditions (log 10 BF 10 = −3.36 to −1.10). They were also favoured for tonic minimum in four conditions; L-M favoured the circadian model (log 10 BF 10 = 0.67), but the evidence was imprecise. Under the tested contrast, adaptation, and frequency, these data do not support circadian modulation of phasic photoreceptor-directed pupil tracking. Other metrics and smaller or non-sinusoidal effects remain possible.
Institution: Max Planck Institute for Biological Cybernetics
Language: English
Subfield: Endocrine and Autonomic Systems
Field: Neuroscience
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W7222938847