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Conditions

SLEEP DISORDERS

Non-24-Hour Sleep-Wake Disorder in Hyderabad: When the Biological Clock Refuses to Follow the Sun — Cyclical Insomnia and Sleepiness With a Treatable Cause

Actigraphy diagnosis, tasimelteon, and melatonin treatment at KIMS Neuroscience, Secunderabad, Hyderabad.

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N24SWD is a circadian rhythm disorder in which the internal biological clock runs on a period longer than 24 hours (typically 24.2-25.5 hours), producing a daily progressive drift in sleep timing. The circadian clock (suprachiasmatic nucleus, SCN) is normally entrained to 24 hours by light signals via the retinohypothalamic tract. In totally blind individuals — who have no functional retinohypothalamic input — the clock free-runs at its natural period, causing a cyclical pattern: weeks of normal sleep when the clock is aligned with day-night, alternating with weeks of disabling insomnia (clock misaligned with night) and daytime sleepiness (clock misaligned with day). N24SWD affects approximately 50% of totally blind individuals. Melatonin and tasimelteon (Hetlioz) entrain the free-running clock.

What it is: Circadian clock runs longer than 24 hours (24.2-25.5h). Sleep timing drifts later each day. Produces cyclical insomnia (weeks) alternating with cyclical daytime sleepiness (weeks). Most common in totally blind individuals.
Why blindness causes N24SWD: The SCN circadian clock is entrained by light via the retinohypothalamic tract (RHT). Totally blind individuals have no RHT input — the clock free-runs at its natural period (>24h), progressively drifting later indefinitely.
Also occurs in sighted individuals: N24SWD is not exclusive to blindness. In sighted people it is commonly misdiagnosed as insomnia, depression, or DSPS — prospective actigraphy over 2-4 weeks reveals the progressive circadian drift and makes the distinction.
DLMO at KIMS: Dim Light Melatonin Onset: salivary melatonin sampled hourly from 5pm to midnight under dim light (<10 lux). Identifies exact circadian phase. Used to time melatonin/tasimelteon for maximum entrainment efficacy.
Actigraphy diagnosis: Wrist-worn accelerometer for 2-4 weeks. Produces a visual trace showing the sleep band rotating progressively later each day — the characteristic free-running pattern. Required for N24SWD diagnosis.
Treatment: totally blind: Tasimelteon (Hetlioz) — FDA/EMA approved for N24SWD in blind individuals. SET and RESET trials: ~20% complete entrainment, significant improvement in larger proportion. Taken at fixed clock time each evening.
Treatment: sighted: Low-dose melatonin (0.5-3mg) at precisely timed advancing intervals (not supraphysiological 5-10mg which reduces entrainment efficacy). Morning bright light therapy (10,000 lux). Strict temporal scheduling.
Contact KIMS: 040-44885000 · Actigraphy · DLMO assay · Melatonin prescription at physiological dose with precise timing · Tasimelteon access assisted at KIMS.

Fact	Details
Definition	Circadian clock period >24h (24.2-25.5h) · Free-running: progressive daily sleep timing drift · Cyclical insomnia alternating with daytime sleepiness over weeks
Who is affected	~50% of totally blind individuals (no light perception) · Also occurs in sighted individuals, where it is commonly misdiagnosed as insomnia, depression or DSPS · Any age; often lifelong in blind individuals
Diagnosis	Actigraphy (2-4 weeks) showing free-running sleep band · Sleep diary · DLMO salivary melatonin assay (hourly, 5pm-midnight, dim light) · Visual status assessment
DLMO	The most precise circadian phase marker · Melatonin onset ~2h before habitual sleep time · Used at KIMS to time treatment for maximum entrainment efficacy
Treatment (blind)	Tasimelteon (Hetlioz) — FDA/EMA approved · MT1/MT2 melatonin receptor agonist · SET + RESET trials: ~20% complete entrainment · Taken at same clock time each evening · Import access at KIMS
Treatment (sighted)	Melatonin 0.5-3mg (physiological dose, not 5-10mg) at precisely timed advancing intervals · Morning bright light (10,000 lux, 30-60 min) · Strict temporal scheduling of meals and social activities
N24SWD vs DSPD	DSPD: delayed but stable sleep phase · N24SWD: progressively rotating sleep phase, never stabilising · Actigraphy 3-4 weeks distinguishes definitively

About This Condition

What Is Non-24-Hour Sleep-Wake Disorder?

Non-24-hour sleep-wake disorder (N24SWD) is a circadian rhythm disorder in which the internal biological clock runs on a period longer than 24 hours — typically 24.2 to 25.5 hours in affected individuals — instead of the normal period of approximately 24.1 to 24.2 hours that is entrained to 24 hours by light.

In N24SWD, the clock progressively drifts later each day: if the person falls asleep at 11pm on Monday, they may not feel sleepy until 11:30pm on Tuesday, midnight on Wednesday, 12:30am on Thursday, and so on.

Over weeks, the sleep period cycles through all times of day and night, producing periods of severe insomnia (when the internal clock is misaligned with the desired sleep time) alternating with periods of extreme daytime sleepiness (when the clock is aligned with the daytime).

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040-44885000ICD-10: G47.24 — Circadian rhythm sleep disorder, free-running type

N24SWD in Context

Who Is Affected and How the Disorder Presents

N24SWD has a distinctive epidemiology and a clinical pattern that differs fundamentally from ordinary insomnia or a late chronotype.

Who is most affected

N24SWD is most common in totally blind individuals, affecting approximately 50% of people with no light perception. The reason: the circadian clock (located in the suprachiasmatic nucleus, SCN, of the hypothalamus) is normally entrained to the 24-hour light-dark cycle by light signals transmitted from the retina via the retinohypothalamic tract. Totally blind individuals have no functional retinohypothalamic input, so the SCN clock free-runs at its natural (longer than 24-hour) period. N24SWD is most prevalent in individuals with total blindness, where the mechanism is clear: absence of light-induced entrainment via the retinohypothalamic tract to the SCN. However, N24SWD also occurs in sighted individuals, where circadian pacemaker sensitivity or tau abnormalities may underlie the free-running cycle.

The cyclical pattern

The cyclical drifting of the sleep phase in N24SWD produces a predictable clinical pattern: periods of 2–4 weeks when the internal clock is aligned with the conventional day-night cycle, during which the person sleeps normally at night and feels alert during the day; followed by periods of progressive misalignment, during which insomnia at the desired bedtime and daytime sleepiness worsen; reaching a peak of misalignment (when sleep is desired during the day) that produces maximum disability; then gradually improving again as the clock cycles back into alignment. This cyclical pattern — weeks of normal sleep followed by weeks of disabling insomnia and sleepiness — is the hallmark of N24SWD.

N24SWD in sighted individuals

While total blindness is the most common setting for N24SWD, sighted individuals can also develop a free-running circadian rhythm: the pacemaker does not entrain reliably to the 24-hour light-dark cycle despite functional vision. Contributing factors include irregular sleep schedules, social isolation with minimal time cues, and an atypically long endogenous period. Sighted N24SWD is frequently misdiagnosed because it presents intermittently — when the drifting sleep phase aligns with conventional sleep times the patient appears to sleep normally, and as the phase continues to drift, insomnia and excessive daytime sleepiness recur cyclically. Three misdiagnoses are common: severe insomnia (when the drifted phase falls in the daytime and sleep cannot be initiated at a conventional hour); depression (social desynchrony, inability to meet daytime obligations, and chronic sleep deprivation produce mood and functional impairment resembling depressive illness); and delayed sleep phase syndrome (DSPS), which is clinically indistinguishable when the patient is assessed during the delayed phase of the drift. The consequence of misdiagnosis is treatment with antidepressants or CBT-I while the circadian disorder itself goes untreated. Correct diagnosis requires prospective actigraphy over 2–4 weeks with a detailed sleep diary, demonstrating progressive drift rather than a fixed delay. Management in sighted individuals differs from blind individuals: bright light therapy and strict social scheduling are the primary tools.

Patient Story

The Student Whose Sleep Was On a Different Calendar

A 23-year-old student in Hyderabad who has been blind since age 8 comes to KIMS with a complaint he has struggled to explain to physicians for years: his sleep problems cycle. For about 2–3 weeks at a time, he sleeps normally at night and attends his accessible university programme without difficulty. Then, gradually, he begins finding it harder to fall asleep at his normal bedtime. Over the next 2 weeks, his bedtime drifts later and later — from 11pm to midnight to 1am to 2am — and his waking time drifts correspondingly. Eventually he finds himself sleeping from 5am to 1pm, completely unable to maintain any normal schedule. Then, over the following 2 weeks, the cycle reverses and his sleep gradually drifts back toward a normal night-time schedule, before the cycle begins again. He has tried every sedative and every piece of sleep hygiene advice, with no lasting benefit.

At KIMS, the diagnosis is made from the history alone, confirmed by 4 weeks of actigraphy (a wrist-worn accelerometer that records activity and rest patterns continuously). The actigraph trace shows a classic N24SWD pattern: the sleep onset time advances by approximately 45–60 minutes per day consistently over the 4-week recording, with the sleep band clearly rotating across all times of day in a predictable free-running cycle.

Treatment with melatonin (0.5mg taken at the same time each evening, timed to be given approximately 6 hours before the current sleep onset time, then gradually advancing the timing) is initiated. The goal is to use exogenous melatonin as a zeitgeber (time-giver) to provide the missing light-entrainment signal to the SCN. At 3-month review: the actigraphy shows significantly stabilised sleep timing, with the sleep period remaining within a 2-hour window of the target bedtime rather than free-running across all clock times. He is attending his university programme consistently for the first time in years.

Understanding the Disease

The Circadian Clock — How N24SWD Develops

The mechanism behind the symptoms — and why standard insomnia treatments do not work for N24SWD.

The suprachiasmatic nucleus (SCN) — the master clock

The SCN is a small paired nucleus in the hypothalamus containing approximately 20,000 neurons that generate a self-sustaining circadian oscillation with a period of approximately 24.1–24.2 hours. This intrinsic period is slightly longer than 24 hours, so the clock must be reset (entrained) slightly earlier each day to stay synchronised with the 24-hour day. The primary entrainment signal is light, transmitted from the retina to the SCN via the retinohypothalamic tract (RHT). In totally blind individuals, this RHT input is absent, removing the primary mechanism for daily resetting. The clock then free-runs at its natural period, producing N24SWD.

Dim Light Melatonin Onset (DLMO) — the circadian marker

Melatonin is secreted by the pineal gland starting approximately 2 hours before the habitual sleep time. This onset of melatonin secretion — the Dim Light Melatonin Onset (DLMO) — is the most reliable measurable marker of the internal circadian phase. In N24SWD, the DLMO drifts progressively later (in blind individuals) or may occur at abnormal times. Measuring the DLMO (from salivary melatonin samples collected hourly in the evening under dim light conditions) at KIMS allows precise characterisation of the circadian phase and optimises the timing of melatonin or tasimelteon treatment.

The weekly insomnia-sleepiness cycle

The practical consequence of free-running sleep in N24SWD: as the internal clock drifts through the day, there are windows when it is aligned with the desired night-time sleep period (weeks of good sleep) and windows when it is misaligned (weeks of intractable insomnia at night and irresistible sleepiness during the day). During misaligned weeks, the person has involuntary sleep propensity during the day and inability to sleep at the desired time — not from behavioural choice but from the misalignment between the internal clock and the conventional schedule. Standard insomnia treatments (sleep hygiene, CBT-I, sedatives) do not address this circadian misalignment and therefore do not produce lasting benefit.

Diagnosis at KIMS

Diagnosis of N24SWD requires objective demonstration of the free-running sleep-wake pattern over weeks — not a single night's recording. At KIMS, the workup combines actigraphy, sleep diary, DLMO measurement, and visual status assessment to confirm the diagnosis and guide treatment.

Actigraphy — the essential diagnostic tool for N24SWD

A wrist-worn accelerometer worn continuously for a minimum of 2–4 weeks. Records periods of activity and rest, allowing visual identification of the sleep-wake pattern over time. In N24SWD, the actigraph shows the characteristic free-running pattern: the sleep band gradually rotating clockwise (later each day) across all times of day over the recording period. This visual pattern is diagnostic of N24SWD and is far more informative than any single night's recording. Prospective actigraphy over 2–4 weeks reveals the progressive circadian drift, distinguishing N24SWD from DSPS, where the sleep phase is delayed but fixed.

Sleep diary — alongside actigraphy

A prospective sleep diary (recording bedtime, rise time, and sleep quality daily) completed simultaneously with actigraphy. Provides subjective confirmation of the objective actigraphy pattern and documents the cyclical insomnia-sleepiness pattern.

DLMO (Dim Light Melatonin Onset) — for precise phase characterisation

Salivary melatonin samples collected every 30–60 minutes from 5pm to midnight (or later if the sleep phase is significantly delayed), under dim light conditions (<10 lux). The onset of melatonin secretion above baseline (typically >4 pg/mL rise) identifies the DLMO, which marks the circadian phase. At KIMS, DLMO measurement is used to precisely time melatonin or tasimelteon treatment for maximum entrainment efficacy.

Visual status assessment

Formal ophthalmological and neurological assessment of visual function. Total blindness (no light perception) greatly increases the probability of N24SWD and changes the treatment approach. Light perception intact despite reduced visual acuity suggests that some RHT input may be intact and that light therapy could be part of the treatment plan.

Treatment

Treatment at KIMS

The goal of treatment is to entrain the free-running circadian clock to a stable 24-hour schedule using precisely timed zeitgebers — melatonin, tasimelteon, light therapy, and temporal structure.

Approved Therapy

Tasimelteon (Hetlioz) — Approved for N24SWD in Blind Individuals

Tasimelteon (Hetlioz) is a melatonin receptor agonist (MT1 and MT2) approved by the FDA (2014) and EMA (2015) for N24SWD in totally blind individuals. It is taken at the same clock time each evening (regardless of the current circadian phase), and acts as an artificial zeitgeber to entrain the free-running clock. Clinical trials (SET and RESET studies) showed that tasimelteon entrains approximately 20% of totally blind N24SWD patients to the 24-hour day, with significant improvement in night-time sleep and daytime wake in a larger proportion. Once treatment stops, the clock returns to free-running. Tasimelteon is not widely available in India; KIMS assists with imported prescription access for appropriate patients.

Primary Therapy

Melatonin — The Primary Treatment Available in India

Low-dose melatonin (0.5–3mg) taken at a precise, advancing time each evening can entrain the free-running clock in many N24SWD patients. The critical principle: the timing and dose of melatonin determine its phase-shifting effect. At KIMS, the DLMO is used to determine the optimal melatonin administration time, and the dose is titrated at the lowest effective level (supraphysiological doses of 5–10mg may actually shorten the duration of melatonin action and reduce entrainment efficacy). Melatonin must be taken at a consistent time daily — missing doses allows the clock to drift.

Adjunctive Therapy

Light Therapy — For Sighted Individuals with N24SWD

In sighted individuals with N24SWD, bright light therapy (10,000 lux light box, 30–60 minutes in the morning at the desired wake time) provides the entraining light signal that is failing to reset the clock. Combined with melatonin in the evening and strict social scheduling (consistent meal times, social interaction, exercise at fixed times), light therapy can entrain the free-running clock in sighted N24SWD. Success requires near-perfect compliance with the timing protocol.

Supportive Care

Strict Temporal Scheduling

Regardless of the primary treatment, strict temporal scheduling is essential: fixed wake time each morning; consistent meal times; consistent social activities and light exposure. These social and photic zeitgebers (time-givers) provide supplementary entrainment signals alongside melatonin or tasimelteon.

Related Conditions

Non-24-Hour Sleep-Wake Disorder belongs to the family of sleep disorders managed at KIMS Neuroscience:

Narcolepsy Kleine-Levin Syndrome Obstructive Sleep Apnoea REM Sleep Behaviour Disorder

FAQs

Frequently Asked Questions

Yes — the pattern you are describing (cyclical alternation between normal sleep and severely disrupted sleep, repeating over weeks) in a totally blind individual is the classic presentation of N24SWD. The free-running circadian clock in blindness produces exactly this pattern: weeks of alignment with the conventional day-night cycle (normal sleep), alternating with weeks of progressive misalignment (insomnia, daytime sleepiness), as the clock cycles through all times of day. This is not a psychiatric problem, a drug problem, or poor sleep hygiene — it is a direct consequence of the absence of retinal light input to the SCN. At KIMS, actigraphy over 3–4 weeks confirms the diagnosis by visualising the free-running sleep pattern. Melatonin and potentially tasimelteon can stabilise the sleep pattern. Please call 040-44885000.

Melatonin is generally very well tolerated for long-term use at the doses used for N24SWD (0.5–3mg). It does not cause the dependence, tolerance, or withdrawal seen with sedative-hypnotics (benzodiazepines, Z-drugs). Long-term use studies in blind individuals with N24SWD show no significant adverse effects at physiological doses. The main precautions: melatonin has mild interactions with anticoagulants (warfarin) and may lower blood pressure; it should be taken at a consistent time (not at different times each night, which would provide a confusing zeitgeber signal); and it should be taken at a physiological dose (0.5–3mg) rather than the supraphysiological doses (5–10mg) commonly available in pharmacy supplements, which can paradoxically reduce entrainment efficacy. At KIMS, the melatonin dose and timing are specifically prescribed for each patient based on their DLMO. Call 040-44885000.

Sighted N24SWD is rare but real. The key question is whether your sleep period is truly free-running (progressively drifting later, cycling through all times of day over weeks) or whether you have severe delayed sleep phase disorder (DSPD) with a fixed late sleep time that does not progress further. Sighted N24SWD typically shows the full free-running pattern on actigraphy — the sleep period continues to advance later every few days indefinitely, not stabilising at a fixed late time. If your pattern fits this (rather than a stable late sleep phase), then the investigation and treatment approach for N24SWD applies. At KIMS, 3–4 weeks of actigraphy distinguishes these conditions definitively. Sighted N24SWD is treated with morning bright light therapy combined with precisely timed evening melatonin. Call 040-44885000.

Being a night owl — the common description of a late chronotype — usually refers to delayed sleep phase disorder (DSPD), in which the sleep period is consistently shifted later than conventional (e.g., unable to fall asleep before 2am and unable to wake before 10am) but is stable at that later timing. In DSPD, the sleep phase is delayed but does not progressively drift further; the person sleeps well during their delayed window and is alert during their waking hours within that window. N24SWD is fundamentally different: the sleep phase is not fixed at a later time but continues to drift later each day (in a totally blind individual) without stabilising, cycling through all times of day over weeks and producing cyclical periods of complete misalignment with any fixed schedule. Actigraphy over 3–4 weeks reveals the distinguishing pattern: DSPD shows a consistently late but stable sleep band; N24SWD shows a progressively rotating sleep band. At KIMS, the treatment differs: DSPD is treated with chronotherapy and morning light; N24SWD requires entrainment with melatonin or tasimelteon. Call 040-44885000.

The blind student whose sleep cycles through every time of day. The sighted professional whose bedtime drifts later every week without stopping.

Non-24-hour sleep-wake disorder is diagnosable at KIMS with 4 weeks of actigraphy and a DLMO salivary melatonin assay, and is treatable with precisely timed melatonin or tasimelteon that entrains the free-running circadian clock. Call 040-44885000.

040-44885000

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