The purpose of this study is to investigate whether the experience of a daily time delay can affect our internal circadian rhythm.

Inclusion Criteria

• Healthy adults aged 23-45 years
• Part of a heterosexual, cohabiting couple willing to participate together in a five-day in-laboratory study
• Both partners meet all inclusion criteria
• Completion of at least upper-secondary education
• Maintain a regular sleep-wake schedule
• Habitual sleep timing within a normative range (non-extreme chronotype), assessed using the Morningness-Eveningness Questionnaire (MEQ) or the Munich Chronotype Questionnaire (MCTQ)
• Both partners fall within the acceptable chronotype range to ensure aligned sleep-wake patterns
• Low seasonality scores on the Seasonal Pattern Assessment Questionnaire (SPAQ)
• Free from underlying sleep or mood disorders

Exclusion Criteria

• Engages in night-shift work or maintains an irregular work or sleep schedule
• International travel involving a time-zone change of more than two hours within the past two months, or anticipated travel before study completion
• Diagnosed neurological, psychiatric, or sleep disorder (e.g., insomnia, sleep apnea, bipolar disorder)
• High risk of sleep apnea, defined as a Berlin Questionnaire score >2 (Lauritzen et al., 2018)
• Use of medications known to affect sleep, alertness, melatonin secretion, or circadian timing
• Unable or unwilling to comply with behavioral restrictions, including refraining from electronic devices displaying time cues unless clocks are removed and devices are disconnected from Wi-Fi
• Unable or unwilling to comply with consumption restrictions, including abstaining from caffeine, alcohol, and melatonin-rich foods during the study
• Extreme chronotype, defined as a habitual midsleep time outside 03:00-05:00 on the MCTQ or classification as an extreme morning or extreme evening type on the MEQ
• Daily caffeine consumption exceeding 400 mg (approximately 4-5 cups of coffee)
• Current smoker or smoking within the past six months

De-identified individual participant data (IPD) underlying the published results will be shared with other researchers upon reasonable request, in accordance with Danish data protection regulations, institutional policies, and ethics approval. Data will be pseudonymised before sharing and no direct identifiers (e.g., name, contact details, personal ID numbers) will be included. Any data elements that could reasonably lead to re-identification in combination with other information will be removed or aggregated where necessary.

De-identified individual participant data and supporting documents will be made available beginning within 12 months after publication of the main results article and will remain available for at least 5 years thereafter.

De-identified individual participant data (IPD), together with the study protocol, statistical analysis plan, and analytic code, will be available to qualified researchers affiliated with recognised research institutions who submit a methodologically sound proposal and obtain any required ethical or institutional approvals. Requests should describe the planned analyses and data needed. Approved requesters will sign a data use agreement prohibiting re-identification of participants and requiring appropriate data security. Data will be shared via secure transfer or a controlled-access repository approved by Aarhus University.

Open-label, single-group basic science study; no masking used.