Play Tetris for hours, and falling blocks invade your mind as you fall asleep

Robert Stickgold's 2000 study had people play Tetris for hours across three days, then tracked what they saw drifting through their minds at sleep onset — most reported vivid, intrusive images of tumbling geometric pieces. Strikingly, three amnesiac patients who couldn't consciously recall playing the game at all reported the same falling-block imagery, showing the effect runs on a memory system separate from ordinary recall.

— Robert Stickgold et al., Replaying the Game: Hypnagogic Images in Normals and Amnesics — Science, vol. 290, 2000