REM

Rapid Eye Movement in sleep was first documented in 1952 by Nathaniel Kleitman and Gene Aserinsky.  According to Dement, they discovered it by accident, while studying slow eye movements at sleep onset.  Rapid eye movement is highly coordinated, and appears identical to a waking person's eye movements.  Kleitman, Aserinksy, and Dement would later conclude that REM is associated with dreaming.

Sources:

Dement, The Promise of Sleep