Circadian Rhythms and Time Perception in Healthy Adults During Constant Wakefulness

Inclusion Criteria: * 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 Criteria: * 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).