Something that prevents or counteracts sleep; causing wakefulness rather than drowsiness.
From Greek 'anti-' (against) + 'hypnos' (sleep) + '-ic' (adjective suffix), formed in medical terminology in the 19th century as the opposite of 'hypnotic'.
While hypnotics put you to sleep, anthypnotics keep you awake—but paradoxically, some sleeping pills can have anthypnotic effects in certain people, which is why your friend might get hyper after taking something supposed to relax them.
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