Every night you lose more than a pound while you're asleep (for the oddest reason)

Jun 21, 2013
Originally published on June 24, 2013 7:21 am

Editor's Note: Robert has added an update to this post. Scroll down to read it.

Here's a simple question: Why do you weigh more when you go to sleep than when you wake up? Because you do. In the video below, you'll see the evidence. You can check this yourself. Somehow, while doing absolutely nothing all night but sleep, you will wake up lighter.

This is not about bathroom stuff. If you awaken and weigh yourself even before going to the toilet, you will still be lighter than when you went to bed. Why?

Where Does The Weight Go?

My first thought was "sweat." Maybe you sweat when you sleep, so some of your water weight disappears as water vapor. Turns out, that's true. That's part of the explanation — but not the fascinating part.

Derek Muller, a physics teacher in Perth, Australia, and host of one of my favorite science blogs, Veritasium, came up with the full answer, and it's so surprising, so simple, it feels like one of those No Fuss, No Muss, Miracle Cures they talk about on late night television.

This is like the Sting song, "Every breath you take ... " All night long, every time you breathe out, a bunch of carbon atoms, formerly inside your body, leave your insides and take off into the night air. You breathe in oxygen, O2. You breathe out carbon dioxide, (two oxygen atoms with a carbon atom attached), so there's an extra carbon atom leaving in every round trip.

Each of those carbon atoms weighs almost nothing, a fraction of a fraction of a gram. But every breath expels roughly 10,000,000,000,000,000,000,000,000,000,000, or ten billion trillion atoms, so add up all the atoms coming from all the breaths you take all night long ... and — could it be this simple? — you wake up carbon-depleted, more than a pound lighter.


Yes, We Lose Carbon At Night, Says This Reader. But Water Too! Remember The H2O!

By Robert Krulwich

Several of our readers have written in to say Derek's explanation for overnight weight loss needs one important amendment. They say Derek's right that we lose carbon atoms when we sleep. Nobody argues with that. "Yes, you are technically losing carbons," writes "Bologna Vest," "but that's not all you're losing. That's not even MOST of what you're losing."

Here's "Bologna's" version: When we sleep we also exhale water vapor. On most nights, the room we're in is cooler than we are. Our throat, our lungs, the inside of us, is roughly 98 degrees. The bedroom might be 75 degrees. When you breathe in, cool air enters your body. Then, when it's time to exhale, says Bologna Vest, "our body moistens the surfaces of our lungs and the air we exhale, now warmed to approximately 90F has a relative humidity of almost 100%" — which means, I think, that when you breathe out, your breath pulls water from inside you and "whoosh!" — once it goes, you lose a little bit of water-weight. Anyone who's had to lug a pail of water knows that H2O has mass. Now, multiply by a night of breaths and gazillions of atoms, and there's a second explanation for weight loss: disappearing water vapor.

"Bologna" says, (and I can't verify his/her figures; I don't know him/her) while we do lose weight from carbon depletion all night, we lose MORE weight from water depletion.

Here's what "Bologna" says. If anybody thinks he/she's wrong, let us know.

Yes, you are technically losing carbons, but that's not all you're losing. That's not even MOST of what you're losing.

Assume you live in an air conditioned house. Assume that you keep the AC set at 70F. Because of the AC, the relative humidity inside your house is approximately 50% (in addition to cooling, AC adjusts humidity to between 40 and 60%). So what you breath in is approximately 70F at 50% humidity. At that temperature there are 0.0079 grams of water vapor in each gram of dry air. Meanwhile, the dry air in our atmosphere contains 0.00046g of Carbon Dioxide in each gram of dry air. But don't forget that CO2 is not all carbon, so there's only about 0.00013 g of Carbon in each gram of dry air.

As we exhale, our body moistens the surfaces of our lungs and the air we exhale, now warmed to approximately 90F has a relative humidity of almost 100%. At 90F and 90%, each gram of dry air we exhale (that is, the air we exhale minus the weight of the water vapor in it) also carries with it 0.0272 grams of water vapor. We also exchange oxygen for carbon dioxide. The dry air we exhale contains approximately 0.046 g of Carbon Dioxide in each gram of dry air exhaled. Again, though, CO2 is not all carbon. There's only about 0.013 g of C in each gram of dry air exhaled.

So for every gram of air we breathe, we lose less than 0.013 g of carbon and more than 0.019 g of water vapor.

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