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.

Inclusion criteria

• Aged 23 to 45 years
• Able to understand and communicate in Danish
• Completed at least upper secondary education (e.g., gymnasium)
• Categorized as either a Morning Type (MT) or Evening Type (ET) based on standardized chronotype assessments:

• Munich Chronotype Questionnaire (MCTQ):

• MT: sleep midpoint on free days (MSFsc) ≤ 03:00
• ET: sleep midpoint on free days (MSFsc) ≥ 05:00
• Morningness-Eveningness Questionnaire (MEQ):

• One of four defined chronotype categories (definitely morning, moderately morning, moderately evening, definitely evening).

Exclusion criteria

• Current engagement in shift work, rotating schedules, or other forms of irregular sleep-wake timing
• Diagnosis of any neurological, psychiatric, or sleep-related disorder (e.g., insomnia, narcolepsy, sleep apnea)
• Use of medications that affect sleep, alertness, or circadian functioning (e.g., melatonin, stimulants, antidepressants)
• International travel across time zones within the four weeks preceding study participation
• Are classified as having an intermediate chronotype based on validated chronotype assessments (MCTQ or MEQ), as this group does not provide the necessary contrast to evaluate circadian alignment effects.