Non-24-Hour Sleep-Wake Rhythm Disorder is characterized by

• A. Alignment with a 24-hour day.
• B. A fixed 24-hour schedule.
• C. A drift in sleep-wake times later each day, not aligned to a 24-hour day.
• D. An absence of daytime sleepiness.

Answer: (D)

Non-24-Hour Sleep-Wake Rhythm Disorder reflects a circadian rhythm that fails to entrain to the 24-hour light-dark cycle. The internal clock runs longer than 24 hours or otherwise doesn’t lock onto daily cues, so sleep-wake times drift later and later each day. Because the schedule isn’t aligned with the day, there isn’t a stable 24-hour pattern or fixed sleep-wake times. Daytime functioning is often impaired as wakefulness and sleep occur at odd, shifting times, leading to daytime sleepiness. This contrast with a normal 24-hour alignment or a fixed schedule helps explain why the option describing a drift not aligned to the 24-hour day is the correct characterization. Absence of daytime sleepiness wouldn’t fit the typical impact of this disorder, since misalignment usually produces daytime impairment. Treatments aim to re-entrain the clock to a 24-hour cycle using timed light exposure or melatonin.