I sleep with my eyes open, well slightly cracked and one of them is half way open. It can be for a few reasons in which mostly all of them have to do with your facial muscles being weak. Sometimes it’s cause by facial nerve issues.
Edit: but I’m assuming that they don’t send visual stimuli to the brain unless you’re in a light sleep phase in which the brain is still slightly active. Like phase 1 when you can be easily woken but don’t remember waking.
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