Circadian Rhythm

The circadian rhythm is an approximately 24-hour biological cycle regulated by the suprachiasmatic nucleus in the hypothalamus, with peripheral tissue clocks in virtually every organ. It coordinates sleep-wake timing, hormone secretion (cortisol peaking at waking, melatonin rising in evening), metabolism, immune function, and cognitive performance. Disruption of circadian alignment contributes to insomnia, depression, metabolic dysfunction, and reduced cognitive performance, making it a foundational concern for sleep and mental health.

The dim-light melatonin onset (DLMO) is the most reliable phase marker, identified by measuring salivary melatonin in dim-light conditions 5-7 hours before habitual sleep. In healthy adults, DLMO occurs approximately 2 hours before habitual bedtime. In delayed sleep phase syndrome and in adults with ADHD, DLMO is delayed by 90 minutes or more, producing difficulty falling asleep at conventional times and daytime dysfunction. Genetic variants in clock genes (PER3, CLOCK, BMAL1) contribute to chronotype variance but are modifiable through zeitgeber manipulation.

Therapeutic circadian interventions follow established principles. For phase advance (shifting earlier): morning bright light exposure within 30 minutes of waking (10,000 lux for 15-30 minutes), low-dose melatonin (0.3-0.5mg) taken 3-5 hours before habitual sleep, and consistent sleep-wake timing. For phase delay: evening light exposure and avoiding morning light. Chronotherapy protocols (Asherson et al., 2025 for ADHD-DSPS) advance the schedule by 15-30 minutes daily over 3-4 weeks, reaching desired sleep time without abrupt shifts. Meal timing (time-restricted eating with 12-14 hour overnight fasts) and exercise timing contribute as secondary zeitgebers.