Restless legs syndrome (RLS) - a common sensorimotor disorder - and periodic limb movement disorder (PLMD) are currently treated with substances of four classes: dopaminergic agents, which are considered the drugs of choice, benzodiazepines, opioids and anticonvulsants. As their effects on sleep variables differ considerably, the aim of the present placebo-controlled sleep laboratory study was to measure the acute effects of 1 mg clonazepam on objective and subjective sleep and awakening quality in ten RLS and 16 PLMD patients, utilizing polysomnography (PSG) and psychometry. Descriptive data analysis demonstrated at the confirmatory level concerning three target variables that - as compared with placebo - clonazepam significantly improved objective sleep efficiency and subjective sleep quality in both patient groups, but failed to reduce the index PLM/h of sleep. At the descriptive level, in PLMD clonazepam improved PLM during time in bed, REM and wakefulness and showed more significant changes in various sleep and awakening measures than in RLS patients, though there were no significant inter-group differences. In conclusion, in both PLMD and RLS clonazepam exhibited acute therapeutic efficacy regarding insomnia, which is quite different from the mode of action of dopamine agonists.