system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does it feel so good to rub your eyes when you're tired\sleepy?
gpt
Your eyes tend to dry out when you get sleepy because of reduced tone in the sympathetic nervous system. Normally, your eyelids are subconsciously kept open by the superior tarsal muscle, which is an involuntary smooth muscle activated by your sympathetic nervous system. However, when you get tired and sleepy, reduced sympathetic tone causes this muscle to relax, making it more difficult to keep your eyes open. You are still able to open your eyes but this requires a concerted effort and voluntary muscle contraction, as anyone who's ever been very drowsy can attest to. In any case, when it becomes difficult to keep your eyes open and the muscles keeping your eyes open begin to fatigue, your blinking frequency decreases correspondingly. The decrease in blinking reduces the distribution of tears over the eye surface, which is the reason your eyes also become dry when you are sleepy. This mild irritation is alleviated by rubbing your eyes because rubbing reflexively stimulates the release of tears from your lacrimal gland. The rubbing also stimulates a small amount of local sympathetic tone, making it temporarily easier to keep your eyes open, which means you blink more frequently and keep your eyes moist more easily.

coluv101 has a really good point though...