Why We Lose Weight When We Sleep

Jul 15, 2013
Originally published on July 15, 2013 3:02 pm

Why do we weigh less in the morning than we do at night?

NPR’s Robert Krulwich decided to find out and he shares his findings with us.

It turns out that much of it has to do with breathing.


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It's HERE AND NOW. I'm Jeremy Hobson.

And I want you to do a mini experiment tonight. Weigh yourself before you go to bed and then first thing in the morning, right after you wake up, weigh yourself again. And you will probably find that you lose a pound during that beauty sleep. So how is that possible?

Well, Robert Krulwich of WNYC's RadioLab has been asking that very question, and he writes about it on his blog Krulwich Wonders. He joins us from New York with more. And, Robert, let's start with breathing. How does that factor into this?

ROBERT KRULWICH, BYLINE: I think people don't quite realize how voluminous a breath is if you're an atom because we don't ever think of ourselves as atom.


KRULWICH: But here's a story. So Julius Caesar is in the Roman Forum in 44, and he's going for, you know, to say hi to the other senators. And Brutus and the other ones come up and they plunge a knife in his chest.


KRULWICH: And then he goes, oh, and then, you know, has a very unfortunate next minute. At which point, he manages to say, you too, Brutus? And then exhales and dies. So you'll just close your eyes and hear Caesar go, oh. Now the exhalation of Caesar's breath on that occasion, so many years ago, released 10 billion trillion atoms or some crazy numbers. So really...

HOBSON: That's an exact number?

KRULWICH: Well, I think, it's a grossly exact number.



KRULWICH: Now here's the interesting thing. I mean, there are so many atoms in that breath that if you - go ahead, Jeremy, and just suck in your breath right now, just suck it in and hold it just for a second.


KRULWICH: All right. I am not making this up and this is not an analogy. In Jeremy's lung, at this moment, at this moment is not just poetically but actually one of the atoms that was exhaled by Julius Caesar in 44. You can release your breath.


KRULWICH: That means that also, of course, would mean that if he farted or something that might also be...


HOBSON: Great.

KRULWICH: And it also has to do a little bit with sea levels. Your being in Boston, now you're closer to the sea level. So, I think, it's fair to say that literally, an atom in you was in Julius. And that means that - among other things, it means that, that there's a lot of atoms in a breath. Now, if you...

HOBSON: I'd probably add a few other historical figures in there as well.

KRULWICH: Well, that's true. You have Shakespeare in you.


KRULWICH: And you have, you know, and you have also Joe the Phoenician. I mean, anybody you want to name is in you.


KRULWICH: It's not a very hygienic image, really, when you think about it.


KRULWICH: But now you go to sleep. So when you sleep, you also breathe in and out. I think the numbers would be - you're going to breathe, oh, 16 breaths a minute. And we'll say you go to sleep for eight hours.


KRULWICH: So you just posed the question, like how could you - if you go to sleep weighing - and by the way, there's this fellow, Derek Muller, who's an Australian, works in Perth, who did a video about this which just fascinated me. He literally - just wanted to be sure about this - he would weigh himself in the exact, same clothing on the exact, same scale just before he went to bed and before he pooped or ate breakfast in the morning. And in his case, he lost - I think it was a little bit more than you said. He lost, I think, it's 1.5 kilos, which, I guess, it means about 3.3 pounds or something like that. So he...

HOBSON: Just from sleeping?

KRULWICH: Just from sleeping as far as he knows. So he wondered, like, what happened? And then he proceeded to investigate. Now, the obvious thing you'd think is, oh, I had food yesterday, I digested it overnight, it turned into energy or something, and so I'm lighter. That's what you'd think.

HOBSON: Or I got up and did some sleepwalking or sleep running, maybe.

KRULWICH: Yes, a little bit of (unintelligible). So it turns out he says, well, that doesn't really resolve the matter because even if you can turn matter into energy, you still have to get rid of the matter. So he said, now, how does a body get rid of the matter? And he really - he thought about this, and he came up with two answers. The first thing is you sweat, right?


KRULWICH: So - and we all know that if you put a lot of water in a bucket, that weighs a lot. So the water has some weight. If you're in bed and it's - your breathing in cool air, it's going to probably be cooler than 98.7 or whatever it is our body temperature is. It goes into your body, and then your body warms it up and it will carry off some water when you exhale.

So half of the weight you lose overnight is just water leaving you, either because you sweat or because you are just, basically, lifting water off of your lungs and off of your insides and taking them outside. So that's half, that's about 60 percent of it. But now here's the surprise - and this I just found so interesting - when you breathe in air, you know you're breathing in oxygen.


KRULWICH: When you breathe out air, some of that oxygen becomes CO2, carbon dioxide. That's just the basic metabolism.

HOBSON: Right.

KRULWICH: So he decided to look at the C's. He says, so in comes the O2s and they pick up a C, a little carbon atom, and then out you go. So you now lost a C and an O and a two. So he said, could it be that just carbon leaving your body is causing you to lose weight? What you multiply by 10 billion, trillion atoms per breath times 16 breath per minute times 18 hours of sleep, it turns out that the carbon leaving your body is about 100 grams, which is - that's about another - about a pound and a half, something like that.

HOBSON: So just the carbon leaving your body causes you to lose some weight while you sleep?

KRULWICH: Yeah. And how much does a carbon atom weigh? Hardly anything. Hardly anything. But when you multiply by the amount of atoms in a breath, you're up to a hundredths of a gram, then you multiply and you multiply and you multiply. And then it just that all that carbon adds up. So he said, wow. So then just the act of exhaling lifts carbons out of you and makes them go elsewhere, and that accounts for about a half of your weight loss overnight.

HOBSON: But, Robert, wouldn't you be releasing these carbon atoms while you're awake as well?

KRULWICH: You would. You would. But here's the - so you would be losing weight all the time, every time you'd be - a little bit. However, in the day, you're doing other things like eating. I actually wondered, and so did Derek, like, what, you know, what is a big fat sandwich mean? And a big fat sandwich would be about - can be upwards of two pounds, you know, something like that. If you eat food - over the course of the day, you might be eating three, four, five pounds of matter.

Your poop - he weighed his poop too. And the poop is roughly a touch under the weight of your intake, but sometimes it could be a little bit excessive, so a little bit excess. So food in, poop out is your basic business of trading with the atmosphere.


KRULWICH: Overnight, you have this little extra little...

HOBSON: It's a nice way to put it.

KRULWICH: ...extra little zing, like you are fearlessly, quietly, subtly and constantly losing carbon atoms, and that's like the little bit of extra. Now, if you wonder, and I wonder, I said, when do I weigh the least?


KRULWICH: I think you weigh the least after your big poop, whenever that comes. When do you weigh the most? After your heaviest meal. So all we're really saying is that when you lie down, everything else being equal, you will wake up lighter. You will not wake up lightest. You have to wait for your poop, and you will - when you go to bed, you will not be heaviest because that will be right after your heaviest meal. So meals and poop dominate the scene, but breathing is the hidden little secret.

HOBSON: Yeah. Because I remember even at the gym, going to the gym, there was a white board on the wall that said, make sure you do three things to stay in shape: workout regularly, eat healthy and make sure you get your eight hours of sleep.

KRULWICH: And so I guess they were referring to the tiny - it's not much, but it's - yes, it makes a little bit of difference. It's also free, which makes it feel like you're stealing it from the gods. You just lie down, and then you wake up lighter. You go woo.

HOBSON: So have you lost weight since you figured this out, Robert?

KRULWICH: No. I've just thought about it. If thinking can lose me weight, yes, I've lost a little bit of thinking weight. But as far as I know, there's no such thing as losing thinking weight. So, no, I haven't corrected anything about myself.

HOBSON: Robert Krulwich, co-host of WNYC's RadioLab, who blogs at Krulwich Wonders. Robert, thank you so much.

KRULWICH: You're welcome.


So last week, when we said that we were doing a story on how diet soda might make you gain weight, someone wrote us to say, this is The Onion, right?


YOUNG: Please tell me this is The Onion. This is satire. But, you know, maybe there was more poop conversation than some people might want. Our apologies. But when Robert gets through, it makes sense.

HOBSON: He really makes it sound like it's for real, and maybe it really is. But maybe better, Robin, than on a diet soda would be just to sleep more.

YOUNG: Sleep more.

HOBSON: You lose weight every time.

YOUNG: That's our conclusion. This is HERE AND NOW from NPR and WBUR Boston. I'm Robin Young.

HOBSON: And I'm Jeremy Hobson.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.