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