The circadian system is a network of molecular clocks in virtually every cell, coordinated by a master pacemaker in the suprachiasmatic nucleus of the hypothalamus. When the timing of the internal clock is misaligned with the external light-dark cycle or social schedule, circadian rhythm disorders result. These are distinct from insomnia disorder and require specific interventions targeting the clock itself rather than sleep pressure.
Advanced Sleep Phase Syndrome
Advanced sleep phase syndrome (ASPS) involves a circadian clock running earlier than the environmental cycle: affected individuals become sleepy in the early evening (7 to 9pm), fall asleep, and wake fully alert in the very early morning (2 to 5am) without being able to return to sleep. This is more prevalent in adults over 60 due to age-related shortening of the intrinsic period of the biological clock. ASPS is frequently misidentified as early-morning insomnia or depression.
Delayed Sleep Phase Syndrome
Delayed sleep phase syndrome runs in the opposite direction: the clock is phase-shifted later, making sleep onset before 1 to 3am physiologically difficult. This is most prevalent in adolescents and young adults but can persist into adulthood or develop de novo. The result is difficulty waking at socially required times, severe morning sleepiness, and adequate sleep quality if allowed to sleep on the delayed schedule.
The Ageing Clock
The amplitude of circadian rhythms reduces with age, producing less robust entrainment to light-dark cycles. Older adults show reduced melatonin production, reduced sensitivity to phase-shifting stimuli, and more fragmented circadian rhythms. These changes are associated with increased sleep fragmentation, earlier wake times, and reduced total sleep time in the elderly. They are partly reversible through bright light exposure and regular physical activity.
Evidence-Based Interventions
- Morning bright light therapy: 10,000 lux light box for 20 to 30 minutes within one hour of waking; most effective entrainment stimulus for advancing or stabilising the circadian phase
- Melatonin timing: low-dose melatonin (0.5mg) taken 5 to 6 hours before habitual sleep onset advances the circadian phase; evening melatonin (2 to 5mg) taken 90 minutes before target sleep onset is appropriate for DSPS
- Exercise timing: morning or midday exercise advances the circadian phase; evening exercise (particularly high intensity) delays it
- Consistent wake time: the most powerful behavioural zeitgeber (time cue) is a fixed wake time, maintained even on weekends
- Darkness in the evening: avoiding bright light and blue light for 2 hours before bed reduces the light-induced phase delay
Sources
- Sack RL, et al. (2007). Circadian rhythm sleep disorders: Part I, basic principles, shift work, and jet lag disorders. Sleep, 30(11), 1460-1483.
- Cajochen C, et al. (2006). Role of melatonin in the regulation of human circadian rhythms and sleep. Journal of Neuroendocrinology, 15(4), 432-437.
- Lewy AJ, et al. (2006). The circadian basis of winter depression. Proceedings of the National Academy of Sciences, 103(19), 7414-7419.