How does ActiGraphy (activity monitoring) differ between individuals with Non-24 Hour Sleep-Wake Disorder (Non-24) and a circadian cycle less than 24 hours versus those with a cycle more than 24 hours?

Actigraphy Differences Between Non-24-Hour Sleep-Wake Disorder Subtypes

Actigraphy patterns in Non-24-Hour Sleep-Wake Disorder show distinctly different patterns depending on whether the individual's circadian cycle is shorter or longer than 24 hours, with the key difference being the direction of daily phase shifts observed in sleep-wake patterns over extended monitoring periods. 1

Key Actigraphic Differences

Longer Than 24-Hour Cycle (Most Common)

• Progressive daily delays in sleep onset and wake times
• Sleep onset and offset times shift progressively later each day
• Complete cycle typically repeats every 24.5-25.5 hours
• Pattern shows a rightward diagonal drift on actigraphy plots over multiple weeks
• Most common pattern, especially in blind individuals with Non-24

Shorter Than 24-Hour Cycle (Less Common)

• Progressive daily advances in sleep onset and wake times
• Sleep onset and offset times shift progressively earlier each day
• Complete cycle typically repeats every 23-23.9 hours
• Pattern shows a leftward diagonal drift on actigraphy plots over multiple weeks
• Less common pattern, more frequently seen in females and African-Americans 2

Diagnostic Considerations

Duration of Monitoring

• Minimum of 7 days of actigraphy is required per AASM guidelines 1
• Extended monitoring (2-3 weeks) is strongly recommended to properly identify the free-running pattern 1
• Berger et al. suggest up to 3 weeks of monitoring to obtain valid patterns 1

Technical Requirements

• Continuous 24-hour recording is essential to capture the progressive phase shifts
• Concurrent sleep logs enhance interpretation accuracy
• Raw data inspection is required for proper interpretation 3

Pattern Recognition

• The hallmark finding is the progressive daily shift in sleep-wake times
• The shift typically ranges from 30-60 minutes per day in either direction
• Complete cycle of symptoms occurs when sleep times have shifted through the entire 24-hour day
• Periods of relative symptom improvement occur when the free-running rhythm temporarily aligns with conventional sleep-wake times 1

Clinical Implications

Differential Diagnostic Value

• The direction of phase shift (advance vs. delay) helps determine optimal treatment timing
• For longer cycles (>24h): melatonin administration should be approximately 6 hours before desired bedtime
• For shorter cycles (<24h): melatonin administration should be at the desired wake time 2

Common Pitfalls

• Mistaking Non-24 for insomnia with variable sleep times
• Insufficient monitoring duration failing to capture the free-running pattern
• Misinterpreting temporary symptom improvements when the free-running rhythm temporarily aligns with conventional times
• Failing to differentiate between Non-24 and other circadian rhythm disorders like DSPD or ASPD 4

Special Considerations

Blind vs. Sighted Patients

• Non-24 is most common in totally blind individuals (55-70%) 2
• In sighted individuals with Non-24, actigraphy is particularly valuable for diagnosis as the condition is rare and often misdiagnosed 5
• Actigraphy is superior to sleep logs alone in documenting the progressive phase shifts 6

Treatment Monitoring

• Actigraphy is valuable for assessing treatment response to chronotherapeutic interventions
• Successful entrainment shows stabilization of sleep-wake times rather than continued free-running pattern
• Actigraphy can document whether entrainment occurs at the appropriate circadian phase 1, 7

By capturing the direction and magnitude of daily phase shifts over extended periods, actigraphy provides essential diagnostic information that distinguishes between these two subtypes of Non-24-Hour Sleep-Wake Disorder, directly informing treatment approaches and timing of interventions.