Why do your eyelids feel heavy when you get sleepy?

Blame it on a long day of exercise! That’s the word from Mark Andrews, a professor of physiology at Seton Hill University in Pennsylvania. He says even though the muscles that power your eyelids are very small, they’re no different from the ones that power your arms or legs, and just as your biceps need a break after a workout at the gym, your eyelids need a “time out” after a full day of holding your eyes open.

Andrews says the “heaviness” you feel comes from tiny muscle fibers in your upper eyelids, which do most of the work reading, scanning, blinking, or even watching TV. He says you’ll feel that heaviness faster if you’re in a dark environment, bbecause the muscles have to work harder to open your eyes wider, to let in more light. Also, as you get older, muscle tissue weakens, so your eyelids may have to work harder just to stay open!

As you’ve probably noticed, the most common way to relieve that droopy-eyelid feeling is to rub your eyes. Andrews says that has the same effect of massaging your calves after a long run, because rubbing increases blood flow to the area, which gives your eyelids a temporary jolt of energy.

Also, when you use your eyes a lot, waste materials build up around your eyelid muscles. That’s why your eyes tend to look puffy when you’re tired. So, rubbing helps move some of that waste matter into the lymph system.

However, as you might guess, there’s only one sure way to stop your eyelids from feeling so heavy: Get some sleep!