Circadian Timing and Time Perception in Healthy Adults

Background/Rationale. Circadian rhythms vary across individuals (chronotypes) and can be misaligned with social schedules. Both circadian timing and time perception influence cognition and behavior, yet within-person effects of circadian alignment on temporal judgments and related functions are not well characterized. Objectives/Hypotheses. Primary: test whether circadian misalignment (testing at a non-preferred time) increases time-estimation/production bias and reduces vigilance versus aligned testing. Secondary: assess effects on temporal variability, duration discrimination, decision-making strategies, social conformity, and simple color-perception measures (white-point/unique-hue). Design. Randomized, two-condition crossover trial with within-subject comparison of circadian-congruent vs incongruent sessions (order counterbalanced). Sessions occur on the same day (morning and evening) with a substantial break; no visible clocks; caffeine/exercise restrictions prior to testing. A subset (n≈60) completes 7-day actigraphy and sleep diary. Participants. Healthy adults, 23-45 years, Danish-speaking, clear morning or evening chronotype (MEQ/MCTQ). Key exclusions: shift work, recent trans-meridian travel, sleep/neurologic/psychiatric disorders, medications affecting sleep/cognition, intermediate chronotype. Intervention. Behavioral manipulation of testing time relative to chronotype (congruent vs incongruent) in a crossover design. Outcomes. Primary: (1) Time-estimation/production bias across 10-60 s targets; (2) Psychomotor vigilance task (lapses, RT). Secondary: Time-estimation variability (CV), duration discrimination threshold, passage-of-time ratings, KSS sleepiness, mood/fatigue scales, information-conformity effect, decision-making metrics, color-perception indices (white-point/unique-hue). Outcome time frame: per session; within-subject contrast (incongruent-congruent). Sample size/Analysis. Target N≈128. Within-subject models test condition effects, with chronotype and session order as factors; missing data handled with mixed-effects models. Alpha=0.05 (two-sided). Data management/Sharing. Pseudonymized data stored per AU policy/GDPR. De-identified datasets and analysis code will be shared in an open repository within 12 months of primary publication. Risk/Benefit. Minimal risk (temporary tiredness). Potential benefits include improved understanding of circadian influences on cognition with implications for scheduling and performance.