Mailbox: Why we rub our eyes when we're tired

28 January 2020

Interview with 

Chris Smith, The Naked Scienitsts

TIRED_YAWN

A tired man covering a yawn with his hand.

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It's the part of the show where we open our mailbox and see what messages you’ve been sending our way! This week, listener Gal asks us: “Why does one rub one's eyes when one is tired?” Phil Sansom put the question to Chris Smith...

Phil - Gal, as far as I understand it, rubbing your eyes can stimulate the glands that make tears in there, which would help with dry, tired eyes. And apparently it might also help you to relax by slowing your heart rate. Chris, any insight? Am I right?

Chris - Yeah, when we get tired, basically you're fighting against your body clock which is instructing you to go to sleep. And when we go to sleep, our nervous system shuts off the supply of various secretions, including saliva, but also critically tears. Now when you rub your eyes, what you're doing is a) distributing the smaller amount of tears that you have over the eye more efficiently, and b) by stimulating the front of the eye, actually the eye responds to direct stimulation via a nervous reflex to produce more tears. And it's there because if you get dirt or muck in your eye, it triggers reflex eye watering to flush out the muck. So by rubbing your eye, you're basically fooling your eye into thinking you've got muck in it, so you increase and augment the tear flow and that helps to make your eyes feel a bit more comfortable when you're tired.

Phil - I was going to ask just about the opposite scenario because sometimes when I get into bed I find my eyes weirdly water quite a lot.

Chris - Well, it's possible because of your posture; when you're standing up, the area that the tears drain into is this thing called a punctum. And if you look on your lower eyelid, right where the lower eyelid meets your nose, you'll see a tiny black dot. And that's your plug hole for tears. And they drain down there into your nasolacrimal duct and then into your nose. When you lie flat, of course, it's harder for the tears to run across your eye and then down the drain. So I'm not so surprised because your tears are still being produced and you haven't fallen asleep yet and dialed down the tear production. So they have to go somewhere, and they're finding it harder to get down the hole, so they just run down your face.

Phil - That's fascinating. I had no idea. I didn't know that existed on my face.

Chris - Have a look in the mirror and you'll see this tiny black dot. And I've had people come and they say, "I've got this thing in my eye and I've been picking at it trying to get it out," and it's actually their tear duct.

Phil - Oh no!

Chris - Yeah. So don't pick at that.

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