The Effect of a Continuous 1-Hour Time Delay on Circadian Rhythms

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