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Periodic Limb Movement Disorder
Repetitive, rhythmic limb movements during sleep that trigger micro-arousals, often without the person noticing.
The general-population estimate is around 4%, but diagnosis requires polysomnography and the exclusion of other causes: true prevalence is uncertain.
The limbs move at regular intervals during sleep; each movement can set off a micro-arousal that is not remembered. The result is rest continuously interrupted below the threshold of awareness.
The person does not know why they are tired: they sleep enough by the clock but do not recover. Unexplained daytime sleepiness follows - and it is precisely when sleepiness has no explanation that a sleep assessment becomes useful.
Like apnoea and restless legs, it is a disorder of quality: sleep is there but crumbles. Drive Awake lines it up with the others to show how many different roads reach the same point.