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Summary This study characterizes hypnagogic hallucinations reported during a polygraphically recorded 90‐min daytime nap following or preceding practice of the computer game Tetris. In the experimental group (N = 16),
participants played Tetris in the morning for 2 h during three consecutive days, while in a first control group (N = 13, controlling the effect of experience) participants did not play any game, and in a second control group (N = 14, controlling the effect
of anticipation) participants played Tetris after the nap. During afternoon naps, participants were repetitively awakened 15, 45, 75, 120 or 180 s after the onset of S1, and were asked to report their mental content. Reports content was scored by three judges (inter‐rater reliability
85%). In the experimental group, 48 out of 485 (10%) sleep‐onset reports were Tetris‐related. They mostly consisted of images and sounds with very little emotional content. They exactly reproduced Tetris elements or mixed them with other mnemonic components. By contrast, in the
first control group, only one report out of 107 was scored as Tetris‐related (1%), and in the second control group only three reports out of 112 were scored as Tetris‐related (3%; between‐groups comparison; P = 0.006). Hypnagogic hallucinations were more
consistently induced by experience than by anticipation (P = 0.039), and they were predominantly observed during the transition of wakefulness to sleep. The observed attributes of experience‐related hypnagogic hallucinations are consistent with the particular organization
of regional brain activity at sleep onset, characterized by high activity in sensory cortices and in the default‐mode network.