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Parasomnias matter because their timing within the night points straight to the diagnosis, since each type has a fixed pattern. Sleep cycles between NREM (non-rapid eye movement) sleep, including deep stage N3 sleep, and REM (rapid eye movement) sleep, when most dreaming occurs. For example, imagine a child who sits up screaming an hour after falling asleep but remembers nothing the next morning. This points to an NREM parasomnia, not a nightmare, before any further history is taken.

NREM parasomnias arise from deep stage N3 sleep, concentrated in the first third of the night, and include sleepwalking and night terrors, sudden screaming with autonomic (heart rate and breathing) arousal. Both are followed by amnesia, no memory of the event. REM parasomnias occur later in the night and include nightmares, vivid frightening dreams that are usually recalled in detail on waking. REM sleep behaviour disorder is different again: the muscle paralysis that normally accompanies REM sleep fails, so the person physically acts out their dreams. Picture an older man who starts punching and shouting in his sleep while acting out a vivid dream. In practice, this pattern of REM sleep behaviour disorder often precedes a diagnosis of Parkinson's disease or Lewy body dementia (a dementia with fluctuating cognition and visual hallucinationsSeeing, hearing, or otherwise sensing things that are not really there.Read more →, which are false perceptions of things not really there) by several years.

• A man in his late sixties repeatedly punches and kicks during sleep, apparently acting out vivid dreams, and can describe them on waking. Which diagnosis best fits?
• A man newly diagnosed with dream-enactment behaviour during sleep asks about the long-term outlook. Which condition is he at substantially increased risk of developing?
• A student wakes unable to move for about a minute, fully aware, sometimes seeing a figure in the room. Which phenomenon does this describe?
• Sleepwalking and night terrors both arise from the same sleep stage, which explains their timing early in the night. From which stage do they arise?

• Sleepwalking and night terrors, NREM parasomnias with amnesia for the event, unlike REM nightmares which are usually recalled in detail.
• Nightmares, occur in REM sleep, late in the night, whereas night terrors occur in NREM sleep, early in the night.
• REM sleep behaviour disorder, dream enactment from lost REM paralysis, unlike ordinary nightmares which cause no physical movement.
• REM sleep behaviour disorder, an early warning sign of Parkinson's disease or Lewy body dementia, rather than a purely benign sleep phenomenon.

• That condition produces brief stereotyped movements rather than dream enactment.
• That condition produces repetitive leg jerks rather than dream enactment.
• That condition arises from deep sleep with no dream recall afterwards.
• That condition involves screaming with no recall of any dream.
• That inherited condition is not linked to this sleep disorder.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.