Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Distinguish circadian misalignment from insomnia and insufficient sleep, document timing across work and free days, and use carefully timed behavioural, light and specialist treatments without worsening the phase.
Human sleep timing reflects interaction between the circadian clock and homeostatic sleep pressure. A circadian disorder is more than preferring late nights: the mismatch is persistent, causes insomnia or excessive sleepiness at required times and impairs function. A key diagnostic clue is substantially better sleep when the person can follow their internal timing, unlike insomnia that persists despite a suitable preferred schedule.
Delayed sleep-wake phase disorder commonly presents as difficulty sleeping and waking early for school or work, with catch-up sleep on free days. Advanced sleep-wake phase disorder causes unintended early evening sleep and very early waking. Non-24-hour sleep-wake disorder involves progressive drift, particularly in totally blind people, while irregular sleep-wake rhythm shows fragmented episodes without a consolidated main sleep, often with neurological or environmental contributors.
A two-week diary records intended and actual sleep, wake time, naps, work shifts, free days, light, caffeine and medicines. Actigraphy can support longitudinal timing. Polysomnography is not routine solely to confirm circadian misalignment but may be appropriate for suspected apnoea, movement disorder or another sleep condition. Laboratory phase markers are specialist tools rather than usual primary-care tests.
Genetic, developmental and age-related differences influence circadian period and preferred timing, with delayed patterns especially common in adolescence.
Late light, weak morning cues, shift work, travel, irregular meals and variable recovery sleep can push internal timing away from social demands.
Internal circadian promotion of wake or sleep occurs at odds with the required external schedule, creating insomnia and sleepiness at different times.
Light or melatonin produces different phase shifts according to biological timing, so identical exposure can advance, delay or have little effect.
Naps, catch-up sleep and variable rising alter sleep pressure, interacting with circadian phase to reinforce difficulty at the desired bedtime.
Sleep onset and natural waking are consistently late, required early rising is difficult, and sleep improves when late timing is permitted.
Sleepiness arrives unusually early in the evening with correspondingly early waking, despite acceptable sleep when this schedule is followed.
Sleep and wake times move progressively around the clock, producing alternating periods of alignment and severe mismatch with social time.
Sleep is divided into several episodes across day and night without a stable main period, often amid weak environmental time cues.
Insomnia occurs during attempted daytime or early sleep and excessive sleepiness during shifts, with impairment linked to the duty schedule.
Voluntary or constrained short sleep opportunity causes daytime sleepiness without an intrinsic inability to sleep at an appropriate available time.
Sleep remains difficult or unrefreshing even when the person can choose a preferred schedule, often with conditioned arousal and extended wakefulness in bed.
Narcolepsy and idiopathic hypersomnia produce pathological sleepiness not fully corrected by schedule alignment or adequate sleep opportunity.
Sleepiness during commuting, night work or early duties increases driving collisions, clinical errors and machinery incidents.
Persistent misalignment and sleep loss can worsen depression, while aggressive schedule or light changes may precipitate elevation in vulnerable people.
Repeated lateness, absence and schedule conflict can impair education, employment, relationships and access to daytime healthcare.
A naturally late or early preference needs clinical treatment only when persistent mismatch causes distress, impairment or inability to meet necessary schedules.
The same light source can advance or delay the clock depending on when it reaches the biological system, so clock-time advice must fit the phenotype.
Normal consolidated late sleep on free days supports delayed phase, whereas fragmented poor sleep at all clock times points back towards insomnia or comorbidity.
Housing, childcare, school and inflexible work can prevent ideal timing; a useful plan is feasible and does not frame structural constraints as non-adherence.
Naps and caffeine cannot fully neutralise long or rapidly rotating duties; fatigue risk assessment and roster design remain employer responsibilities.
Calling delayed sleep insomnia after asking only about required bedtime and not sleep quality on the person's preferred schedule.
Giving bright-light advice without specifying timing, phase direction, eye safety or the risk of mood elevation.
Using bedtime melatonin generically and escalating dose when the actual problem is incorrect circadian timing.
Advancing wake time while allowing inadequate total sleep, then misreading predictable deprivation as treatment failure.
Diagnosing shift-work disorder without considering unsafe rosters, apnoea, sedatives, long commutes and cumulative sleep debt.
Using unsupervised chronotherapy that progressively delays sleep around the clock and may destabilise routine or mood.
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A 19-year-old cannot sleep before 03:00 or wake for college, but on holidays sleeps continuously from 03:30 to 11:30 and feels refreshed. Which interpretation best fits this pattern?
National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes
Treatment aligns multiple time cues. A stable wake time, appropriately timed light, reduced opposing light, regular activity and meals, and gradual schedule change are planned around the specific phase direction. Morning light can advance a delayed pattern; evening light can delay an advanced pattern. Generic advice to use bright light at bedtime is therefore potentially wrong. Eye disease, photosensitising medicines, migraine and bipolar vulnerability require individual consideration.
Melatonin can shift circadian phase when timed correctly, but formulations, licences and evidence differ by indication and age. It should not be presented as a universal bedtime sleeping tablet, and dose escalation cannot correct wrong timing. Shift-work disorder also requires analysis of duty sequences, breaks, commuting, naps and recovery sleep. Employers retain responsibility for fatigue risk; workers should not be expected to compensate indefinitely for hazardous rostering.