'REM' originates from the English phrase 'rapid eye movement', where 'rapid' meant 'fast', 'eye' referred to the organ of sight, and 'movement' meant 'motion'.

'REM' was coined in the 1950s to describe a specific phase of sleep characterized by eye movements.

Initially, it referred specifically to the eye movements observed during this sleep phase, but it has come to encompass the entire sleep stage.