Interpreting thoughts during sleep

Rapid eye movements during sleep are a readout of thoughts during mouse dreams Have you ever noticed someone’s eyes move rapidly under their eyelids during sleep? Owing to technical challenges in measuring eye movements in freely moving animals with their eyes half closed, this phenomenon has remained enigmatic. Sleep-related rapid eye movements occur during a specific sleep phase called rapid eye movement (REM) sleep, which is associated with vivid dreams ( 1 ). If rapid eye movements reflect thoughts during sleep, reading the eye movements of others, while observing them sleep, would open a window for reading and potentially manipulating their thoughts during dreams. On page 999 of this issue, Senzai and Scanziani ( 2 ) reveal that in mice, rapid eye movements during sleep are a readout of the internal sense of direction.