system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 “ What is happening when your eyes get heavy from lack of sleep “?
gpt
Muscles you use the most tend to get tired first, like for example,  if you walk all day your legs will become fatigued. This also applies to the muscles around your eyes - your ocular and brow muscles become tired because you’re using them for most of your waking hours. Looking at a computer screen for a long time or having chronic allergies can compound the heaviness feeling in your eyes.