system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why eyes roll back into the head as a pleasure reflex

Why do our eyes roll back in our heads when experiencing extreme pleasure?
gpt
Contrary to the ~~above~~ below answer, here's the real one. (It's partially correct, as you'll see, but the mechanism is all wrong.)

When you faint, your body experiences a loss of muscle "tone," also known as continuous postural tension. Your eyes are naturally rolled up, and it's just the tone of the oculomotor muscles that keeps them down. Yes, this happens even while sleeping. (Same for your eyelids -- fully relaxed = 90% closed, not 100% closed. That's why syncope caused by drugs has the distinct half-lidded look rather than other forms of fainting, which reflexively tighten the eyelid muscles to keep them shut.)

Fainting and experiencing extreme pleasure activates similar pathways, as the body starts to ignore or shut down certain parts of the body to focus on  pleasurable sensation. Your eye muscles lose tension (since you don't need your eyes to orgasm), so your eye naturally rolls back.

Watch someone blink in slow motion when they're not focusing intensely on something. You'll notice their eyes roll back slightly as the oculumotor muscles relax during the blink. (Intense focusing can overcome this reflex.)

Sidenote: parasympathetic innervation is involved with sexual intercourse (it's nicknamed "feed and breed," after all) but it doesn't cause the eye roll reflex since it doesn't innervate the oculomotor muscles. The parasympathetic response is in full force on the initiation of coitus anyway...

**Edit:** change "above" to "below" since the other answer got downvoted below mine.