Paul Reading. ABC of Sleep Medicine (ABC Series). BMJ Books (2013)
It is normal to exhibit some movement even during deep sleep. Involuntary simple shifts of body position every 20 minutes or so reﬂect the commonest phenomenon. However, vigorous socalled hypnic jerks at the point of sleep onset are also common, particularly in young or sleep deprived subjects. Occasionally, such jerks occur in association with alarming sensory symptoms, such as ‘ﬂashes’ or ‘bangs’. The extreme variety of this generally benign phenomenon has been called ‘exploding head syndrome’ and can be a potent cause for sleep-onset insomnia.
During REM sleep, all voluntary movements except those involving the diaphragm and oculomotor muscles are actively inhibited by descending pathways from the brainstem to the motor neurones. However, minor intermittent (myoclonic) jerks of limbs or facial muscles are frequently seen. Arousals to full wakefulness from REM sleep are also very common, especially late in the night, usually in association with vivid dreams or even disturbing nightmares.
Leg movements resembling a slow withdrawal reﬂex may recur in bursts every 20 seconds or so during all stages of sleep. Such periodic limb movements (PLMs) may be observed in up to a third of the normal elderly population and are most often a benign epiphenomenon. However, if PLMs are particularly violent or observed in association with signiﬁcant restless legs syndrome, sleep may be disturbed. Drug therapy may be warranted, if only to improve the sleep quality of the bed partner.