When your head hits the pillow, your eyes still function. "But they can only sense light versus dark," says physician Michael Breus, a clinical psychologist who founded SoundSleepSolutions.com, a sleep-information Web site. This explains why a bright light or the sunrise often wakes a person up.
Still, the eyes don't send image information to the brain during sleep. In fact, the connection between your eyes and your brain needs to reboot when you wake up. "It takes 30 seconds for the brain to recognize that you are awake," Breus says. "That's why you don't see complete images right away."
Your eyes do zip around during rapid-eye movement (REM) sleep, but they aren't sending any visual data to the brain, even though studies have shown that the visual cortex — the part of the brain that processes images — is active. Scientists believe that this activity marks a memory-forming or memory-reinforcing process, perhaps locking in your recollection of what happened during the day.
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