Eli5: Why do our eyelids get heavy when we are tired/about to sleep?

Question

Eli5: Why do our eyelids get heavy when we are tired/about to sleep?

In: Biology

Answer 1

Basically, you use muscles to hold your eyes open, which is strange to think about, since you have your eyes open all the time. When you’re tired, your muscles get tired, including the muscles in your eyelids, so they feel heavy.