Circadian Rhythm Sleep-Wake Disorders
The clock and its inputs
The suprachiasmatic nucleus drives circadian timing, entrained primarily by light via the retinohypothalamic tract. Light in the early biological morning advances phase; light in the late evening delays it. Melatonin has the opposite phase-response relationship: evening melatonin advances, morning melatonin delays. Core body temperature minimum and dim-light melatonin onset (DLMO) are phase markers.
The disorders
Delayed sleep-wake phase disorder (DSWPD)-the most common in adolescents-features sleep and wake times later than desired with difficulty advancing. Advanced sleep-wake phase disorder is earlier than desired, more common in older adults. Irregular sleep-wake rhythm shows fragmented, non-24-hour patterns (e.g., dementia, institutional care). Non-24-hour sleep-wake rhythm is free-running, classically in totally blind individuals. Shift work disorder and jet lag disorder are exogenous. Familial advanced sleep phase can involve clock-gene mutations (e.g., PER2).
Treatment logic: timed morning bright light and evening melatonin for DSWPD; timed evening light and morning melatonin for advanced phase; carefully timed melatonin or light for non-24-hour rhythm.
Traps: treating DSWPD with sedatives instead of phase-shifting strategies; giving melatonin at the wrong time (it can worsen the problem); and confusing DSWPD with insomnia or with insufficient sleep. Also remember that shift work disorder requires sleep disturbance or sleepiness temporally linked to the work schedule.
Rule of thumb: Advance phase with morning light and evening melatonin; delay phase with evening light and morning melatonin.
Worked example: A teenager sleeping 2 AM–10 AM with normal sleep quality on weekends likely has DSWPD; gradual advancement with morning bright light and low-dose evening melatonin is first-line.