People who dream or have dreams; people who are imaginative and think about possibilities rather than practical reality.
From Old English 'dream' (originally meaning joy or music, only later meaning a sleeping vision) plus '-er' (one who). The word shifted from meaning music to meaning sleep visions around 1300 CE.
During REM sleep, your brain is as active as when you're awake—dreamers' brains are literally hallucinating in full color with complex stories, and scientists still don't fully understand why we do this weird brain thing every night.
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