RecruitingNot ApplicableNCT07294781

Circadian Rhythms and Time Perception in Healthy Adults During Constant Wakefulness

CircaTime: Effects of Circadian Rhythms on Time Perception During a 36-Hour Constant Routine in Healthy Adults

Sponsor

University of Aarhus

Enrollment

30 participants

Start Date

Dec 5, 2025

Study Type

INTERVENTIONAL

Conditions

Healthy Volunteers Circadian Rhythm Sleep Deprivation Time Perception

Summary

This study examines how the internal body clock (circadian rhythms) influences the way healthy adults experience time, think, and feel when they stay awake for an extended period. Participants will spend about 36 hours in a controlled sleep laboratory while remaining awake the entire time. Light, posture, food intake, and activity are kept as constant as possible (a "constant routine") so that changes over time mainly reflect the body's internal clock and increasing sleepiness, rather than changes in the environment. Every two hours, participants complete a brief test battery that includes ratings of sleepiness and mood, a reaction-time task, and short tasks that assess how fast or slow time seems to pass, how accurately they can estimate time intervals, how they respond to simple decisions, and how they judge colours. Saliva samples are collected repeatedly to measure melatonin, a hormone that indicates circadian phase. By comparing changes in behaviour, perception, and melatonin levels across the 36-hour wake period, the study aims to identify when during the circadian cycle people are most vulnerable to distortions in time perception and reduced alertness. The findings may help improve scheduling of shift work and other activities that require sustained wakefulness.

Eligibility

Min Age: 23 YearsMax Age: 45 Years

Inclusion Criteria10

Age 23 to 45 years.
Able and willing to provide written informed consent.
Fluent in Danish and able to understand study procedures and instructions.
Generally healthy, as assessed by medical history, screening questionnaires, and basic clinical measures (e.g., blood pressure, heart rate).
Self-reported regular sleep-wake schedule for at least 4 weeks prior to the laboratory visit (typically 6.5-9 hours of sleep per night, with usual sleep period between approximately 22:00-01:00 and 06:00-09:00).
Body mass index (BMI) within a non-extreme range (for example, approximately 18.5-30 kg/m²), if required by the study physician.
No regular night work or rotating shift work during the 3 months before participation.
No travel across more than 2 time zones in the 2 months before the constant-routine session.
Willing to abstain from caffeine, nicotine, alcohol, and recreational drugs for the specified washout periods before and during the 36-hour constant-routine session.
For participants who can become pregnant: negative pregnancy test at screening/arrival and agreement to use reliable contraception for the duration of participation.

Exclusion Criteria10

Any known or suspected major sleep disorder (e.g., insomnia disorder, obstructive sleep apnoea, restless legs syndrome, narcolepsy), based on self-report or prior diagnosis.
Current or past major psychiatric or neurological disorders (e.g., major depressive disorder, bipolar disorder, psychotic disorders, epilepsy), unless considered mild and stable and explicitly approved by the study physician.
Chronic medical conditions that could be worsened by prolonged wakefulness or that might confound outcome measures, such as significant cardiovascular disease, uncontrolled hypertension, diabetes mellitus, severe respiratory disease, or other serious systemic illness.
Regular use of medications or supplements that may affect sleep, circadian rhythms, melatonin secretion, alertness, or mood (e.g., hypnotics, sedative-hypnotics, melatonin, stimulants, certain antidepressants or beta-blockers), unless a safe washout is possible and approved by the study physician.
High habitual caffeine intake (for example, \>400 mg/day) or nicotine dependence if the participant is unable or unwilling to abstain for the required washout periods.
Current harmful alcohol use or substance use disorder, or frequent use of recreational drugs.
Pregnancy or breastfeeding.
Previous severe adverse reaction to sleep deprivation, extended wakefulness, or similar laboratory protocols.
Claustrophobia or inability to tolerate prolonged stays in a controlled laboratory environment.
Any condition or circumstance that, in the judgement of the investigators, would make participation unsafe, interfere with the 36-hour wakefulness protocol, or compromise data quality (e.g., inability to remain awake despite support, strong fear of needles or saliva sampling, or inability to comply with study restrictions).

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Interventions

BEHAVIORALConstant Routine Wakefulness

Behavioral intervention consisting of approximately 36 hours of continuous wakefulness under constant-routine conditions. Environmental factors (light level, temperature, noise) are held as constant as feasible; posture and activity are standardised; and participants receive small, isocaloric snacks at regular intervals. A repeated cognitive and perceptual test battery is administered every two hours to assess time perception, vigilance, mood, and related functions across the circadian cycle.