Category · F50-F59 block
F51 — Nonorganic sleep disorders
Terminology note: 'Nonorganic' means the sleep disturbance is not attributable to a known organic condition (such as a neurological disorder or sleep apnoea), a substance, or a medication, and is instead judged to have a primarily emotional or behavioural origin. Sleep terrors and nightmares are commonly confused despite being distinguished by clear, opposite features.
What is this category?
F51 covers primarily psychogenic disturbances of sleep quantity, quality, or timing, and abnormal behavioural events occurring during sleep (parasomnias), once a genuine organic or substance-related cause has been excluded. F51.0 (nonorganic insomnia) is difficulty initiating or maintaining sleep, or non-restorative sleep, causing distress or impairment. F51.4 (sleep terrors) and F51.5 (nightmares) are the classic parasomnia pair distinguished by sleep stage, recall, and arousal pattern.
Core diagnostic concept
Sleep terrors are an abrupt arousal from deep non-REM sleep, typically in the first third of the night, with marked autonomic arousal (screaming, sweating, a racing heart) and little or no recall of any dream content or of the episode itself afterward. Nightmares are vivid, well-recalled frightening dreams during REM sleep, typically later in the night, after which the person becomes rapidly alert and oriented and can describe the dream in detail. Confusing the two leads to the wrong subtype and the wrong reassurance to give a worried parent or partner.
Key diagnostic features
- Nonorganic insomnia: difficulty initiating or maintaining sleep
- Difficulty falling asleep, staying asleep, or non-restorative sleep quality, occurring at least several times a week, with daytime distress or impairment.
- Sleep terrors: abrupt NREM arousal with autonomic activation
- A sudden arousal from deep sleep, typically early in the night, with intense fear, screaming, and autonomic signs, and little or no recall of dream content.
- Sleep terrors: little or no recall the next morning
- The person typically has no memory of the episode itself when woken the next day, unlike a well-recalled nightmare.
- Nightmares: vivid, well-recalled frightening REM dreams
- A detailed, frightening dream during REM sleep, typically later in the night, with rapid full alertness and orientation on waking and clear recall of the dream content.
How to weigh evidence
These learning labels are teaching aids, not official ICD-10 terminology (see Sources and Method).
- supports Difficulty falling or staying asleep several nights a week with daytime impact and no identified organic cause (insomnia); or an early-night episode of screaming and apparent terror with no recall the next morning (sleep terrors); or a late-night vivid, well-recalled frightening dream with rapid full alertness on waking (nightmares).
- consistent not decisive One or two poor nights of sleep during a stressful week, without a recurrent pattern or daytime impairment.
- raises alternative Loud snoring with witnessed breathing pauses, or excessive daytime sleepiness suggesting an organic sleep-related breathing disorder rather than a nonorganic sleep disorder.
- contradicts or weakens The sleep disturbance is fully attributable to a diagnosed medical condition, medication, or substance.
- missing and necessary to ask Whether an organic cause (such as sleep apnoea) or a substance/medication has been excluded, and, for a parasomnia, whether the episode is recalled afterward and which part of the night it occurs in.
Diagnostic boundary
Nonorganic insomnia requires a recurrent, distressing difficulty initiating or maintaining sleep with an organic cause excluded; sleep terrors and nightmares are distinguished by sleep stage, timing in the night, autonomic activation, and — most decisively — whether the episode is recalled afterward.
Full comparisons:
Duration and course
Minimum duration: Nonorganic insomnia is generally expected to occur at least several nights a week for a sustained period with daytime impact; parasomnias are judged by recurrence and pattern rather than a fixed count.
Course: Can be persistent or intermittent; a single isolated poor night or a single bad dream is not, by itself, a disorder.
Included presentations
- Nonorganic insomnia: recurrent difficulty initiating or maintaining sleep, or non-restorative sleep, with daytime distress or impairment.
- Sleep terrors: recurrent abrupt NREM arousal with autonomic activation and little or no recall.
- Nightmares: recurrent vivid, well-recalled frightening REM dreams with rapid full alertness on waking.
Exclusions and distinguishing presentations
| If this is true instead… | Consider | Why |
|---|---|---|
| A sleep disturbance fully explained by a diagnosed organic sleep-related condition, such as obstructive sleep apnoea | organic medical cause behavioural syndrome capsule | F51 applies only once a genuine organic cause for the sleep disturbance has been excluded. |
| A sleep disturbance that is temporally limited to the puerperium and best understood as part of a broader postpartum presentation | F53 | New-parent sleep disruption tied to infant care and the puerperium is not automatically a stand-alone nonorganic sleep disorder. |
Four-character subtypes (orientation)
A learner should check sleep stage, timing in the night, and recall before choosing between sleep terrors and nightmares — an episode remembered in vivid detail the next morning weighs strongly against sleep terrors, not toward them.
- F51.0 — Nonorganic insomnia
- Recurrent difficulty initiating or maintaining sleep, organic cause excluded. (in_depth)
- F51.1 — Nonorganic hypersomnia
- Excessive daytime sleepiness or prolonged sleep, organic cause excluded. (taught for orientation only, not separately assessed in this pilot)
- F51.2 — Nonorganic disorder of the sleep-wake schedule
- Sleep-wake timing out of alignment with the desired or required schedule. (taught for orientation only, not separately assessed in this pilot)
- F51.3 — Sleepwalking (somnambulism)
- Complex motor behaviour arising from deep NREM sleep, with little or no recall. (taught for orientation only, not separately assessed in this pilot)
- F51.4 — Sleep terrors
- Abrupt NREM arousal, early night, autonomic activation, little or no recall. (in_depth)
- F51.5 — Nightmares
- Vivid, well-recalled frightening REM dream, later night, rapid full alertness on waking. (in_depth)
- F51.8 — Other nonorganic sleep disorders
- A nonorganic sleep disturbance not matching the named subtypes. (taught for orientation only, not separately assessed in this pilot)
- F51.9 — Nonorganic sleep disorder, unspecified
- A nonorganic sleep disorder identified but without enough information to specify further. (taught for orientation only, not separately assessed in this pilot)
External causes to check first
Categories in this block require that the disturbance is not better explained by another external cause listed below. These short capsules are reminders, not full teaching modules.
External cause capsule — Organic or medical presentations resembling F50-F53
A reminder that an eating change, sleep disturbance, sexual dysfunction, or postpartum mental/behavioural change can instead be caused by a genuine physiological or medical condition (e.g. a gastrointestinal or endocrine disorder producing weight and appetite change, obstructive sleep apnoea producing a sleep complaint, diabetes or a medication side effect producing sexual dysfunction, postpartum thyroiditis producing a mood or behavioural change) rather than being a primary F50-F53 disorder, and that this should be actively considered before a primary F50-F53 classification is settled.
What to check:
- Timing: did the change begin during or shortly after a newly diagnosed medical condition, a new medication, or a physiological event (such as childbirth) with a recognised association with the symptoms described?
- Medical context: is there a known medical, endocrine, gastrointestinal, or neurological condition with a recognised association with the presentation?
- Adequacy of investigation: has an appropriate medical work-up actually been completed, rather than simply not yet undertaken?
- Course after the cause is addressed: a presentation that substantially improves once the medical condition is treated or the medication is changed weighs toward a secondary cause rather than a primary F50-F53 disorder.
A full teaching module on the relevant medical specialty's own diagnostic criteria, or a general medical work-up, both of which are outside this capsule's scope.
Worked example — sleep terrors, not nightmares
A father describes his 7-year-old waking around 90 minutes after falling asleep with a piercing scream, sitting bolt upright, sweating, and appearing terrified, but not responding when spoken to. The episode lasts a few minutes and the child then settles back to sleep. The next morning, the child has no memory of the episode at all and is confused when told about it.
- Step 1 — Note the timing: early in the night, consistent with deep NREM sleep.
- Step 2 — Note the presentation: intense autonomic arousal (screaming, sweating) without responsiveness during the episode.
- Step 3 — Note recall: no memory of the episode the next morning, unlike a well-recalled nightmare.
- Conclusion — F51.4, Sleep terrors — the early-night timing, autonomic activation, and absent recall together point away from a nightmare.
Interactive practice
- Type A Wide awake at 3am, most nights Recognition / application · Foundation
- Type B Screaming at midnight, gone by morning Boundary discrimination · Developing
Retrieval prompts
Try answering each of these from memory before revealing where to check. Nothing you think or type here is sent or saved anywhere.
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State the three features that distinguish sleep terrors from nightmares.
Reveal answer location
Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.
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Explain why an organic cause must be excluded before diagnosing a nonorganic sleep disorder.
Reveal answer location
Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.
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Explain why new-parent sleep disruption is not automatically classified as F51.
Reveal answer location
Check Duration and course, Diagnostic boundary, or the rapid revision card below — the cue you need is one of those, in this category's own already-authored wording.
These self-checks are a personal study aid only, visible to you on this page for this visit — they are not stored and do not affect your mastery dimensions.
Rapid revision card
F51: Nonorganic insomnia is a recurrent, impairing difficulty with sleep once an organic cause is excluded; sleep terrors (NREM, early night, no recall) and nightmares (REM, later night, vivid recall) are distinguished by stage, timing, and recall.
Decisive boundary cues:
- Recurrent difficulty falling or staying asleep, daytime impact, organic cause excluded -> weighs toward F51.0.
- Early-night arousal, autonomic activation, no recall next morning -> weighs toward F51.4.
- Later-night vivid dream, well recalled, rapid full alertness on waking -> weighs toward F51.5.
Duration/course: Recurrence over time distinguishes a disorder from an isolated poor night or single bad dream.
Nearest alternatives: f53, f41
Common mistake: Treating any frightening night-time episode as a 'nightmare' without checking recall and timing, which can instead point to sleep terrors.
Retrieval prompt: Is the episode recalled in vivid detail the next morning, or not recalled at all?
(or use your browser's print function) Open full-page printable revision view →
Related glossary terms
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Parasomnia
An abnormal behavioural or physiological event occurring during sleep or the sleep-wake transition, such as sleepwalking, sleep terrors, or nightmares. Sleep terrors (an abrupt NREM arousal with little or no recall) and nightmares (a vivid, well-recalled REM dream) are the two parasomnias distinguished in F51.
Source basis
WHO ICD-10 international classification structure and code/title (ICD-10 Browser, 2019 English release).
Original educational synthesis, informed by general published clinical-education understanding of ICD-10 F50-F59; no verbatim CDDG or DCR text is reproduced. See Sources and Method for the full policy.
Last reviewed: 2026-09-16. Full policy: Sources and Method.
Educational resource only. This page and its cases teach ICD-10 classification reasoning using fictional scenarios. It does not diagnose real people and should not be used to assess yourself or anyone else. If you or someone you know is struggling with eating, sleep, sexual, postpartum, or substance-use difficulties, please seek prompt professional evaluation.