A friend with a Ph.D. in Biology once offhandedly remarked to me that children grow while sleeping. I was surprised to hear this, and asked if she could cite a source for that statement. She said it was something she had always believed, and she had never thought about where she had first come across the idea. I might note that she had two very tall children.

We all know that people (adults and children) get longer overnight because the discs between their vertebrae expand while they are sleeping in a horizontal position. But that's not what she was talking about. She was suggesting that growing bodies create new tissue (muscles, fat, bone, and so on) largely while a person is sleeping, and less so during the day.

I thought about this a bit. If you're growing bodily tissues, could you actually measure an overnight weight gain? Well, probably not, because there's a very fundamental law of Physics called "Conservation of Mass". Your body can't build tissue out of nothing. Any muscles, fat, or bone that you add has to be constructed out of the contents of your digestive tract. The law of "Conservation of Mass" basically says that "stuff" can't come out of nowhere. Barring a nuclear reaction, as long as you're not taking stuff in or putting stuff out, your mass (essentially, your weight) can't change.

I never did settle the question of whether or not people add bodily tissue largely during the night. But we can't measure the weight of just our muscles, fat, bone, blood, lymph, and other tissues that comprise our body. We can only measure our total weight, which includes the contents of our digestive system, and for that matter, any urine in our bladders.

For that matter, as any dieter knows, our bodies hold a variable amount of water in our circulatory and lymphatic systems. If I eat a lot of salt, my body can retain a great deal of water, which is needed to dilute that salt. This water can easily amount to a kilogram (over 2 pounds). I sometimes try to estimate how much water I'm holding by seeing how easy it is to remove my wedding ring. After a day with a lot of salt, I sometimes have trouble taking it off at all.

This blog entry is about our total weight, the weight we can read off a scale. Is there any possibility that we could weigh ourselves before going to bed, and then the next morning find we had gained some weight overnight?

At first glance, Conservation of Mass would seem to imply that our weight always has to be exactly the same in the morning as the night before. Any difference can only be due to stuff we take in, or stuff we put out. It's easy to think of a few mechanisms by which we could lose weight overnight. One obvious way is to wake up and use the bathroom, but I'm assuming here for the sake of argument that we don't do that.

Another way is via perspiration. Perspiration is obvious during heavy exercise, or in extremely hot weather. But in fact, we are all perspiring to some degree all the time, and might particularly perspire under heavy bedcovers. I don't think we lose a huge amount of water this way, but even a little bit of water has a measureable weight.

There's another less obvious way we lose water: by breathing. Our lungs are damp, and have a huge internal surface area (a Wikipedia article estimates that a pair of adult lungs has a total surface area of about 70 square meters (750 sq ft)). With that large warm surface evaporating water, the air we exhale is extremely humid. You can plainly see that this is the case if you step outside on a cold day. Then, when you exhale, you can watch the water in your breath condense into droplets in the air. Particularly in dry weather, one is always losing water in one's breath.

So it appears that we can only lose weight while we're sleeping. Short of waking up to drink a glass of water, or sleepwalking to the refrigerator for a snack, we don't take in anything during the night.

Or do we? It occurred to me that there's a purpose, after all, to our breathing: we are bringing in oxygen. We use this oxygen to power our metabolism. Does some of it end up in tissues our body is building? Could we gain some weight that way?

We derive energy from oxygen, in essence, by using it to "burn" carbon. I put the word "burn" in quotation marks because this is a chemical process. Obviously, there's no flame. In terms of input and output, breathing takes in oxygen, and puts out carbon dioxide. In the air, oxygen does not exist as single atoms, but rather as the molecule 0₂, two atoms of oxygen bound together. In chemical terms, we inhale 0₂, and exhale CO₂.

Thus the primary overall effect of respiration is ejecting carbon. So we don't gain weight by breathing, we lose weight by breathing, above and beyond the aforementioned weight we lose by exhaling water vapor.

Oxygen is metabolized in our cells, and it's carried there by hemoglobin in our red blood cells. In the lungs, inhaled oxygen molecules becomes bound to hemoglobin molecules, which transport them throughout the body and drop them off at the cells. Carbon dioxide molecules from the cells attach to different sites on the hemoglobin molecule, to be carried back to the lungs to be exhaled. ^Note 1 

Does all the oxygen we inhale get "burned" into CO₂, and subsequently exhaled? Or could some of it be incorporated instead into muscle, fat, or bone? I'm pretty sure the number of CO₂ molecules exhaled closely matches, and in fact may be exactly equal to, the number of O₂ molecules inhaled. I'm not a biochemist, so if I'm mistaken, I'd love to hear about it. Let me know, and I'll add a note here.

But I think a valid retort to the title of this entry, "Gaining weight overnight" is, "You can't". As far as I can tell, there is no way (without eating or drinking) to gain weight overnight. One can only lose. Far too much stuff is thrown out (perspiration, water vapor, carbon), and nothing, or at least very little, is taken in and retained.

Let me end with an odd story. I once dieted by joining a group called "Weight Watchers at Work". This was a regular Weight Watchers^® group, but instead of my going to one of their locations, Weight Watchers sent a group leader to our company. The meetings were often scheduled just before lunch. They started with a "weigh-in", after which we would eat lunch during the meeting. At one particular session, I had picked up my lunch, but I couldn't immediately weigh myself because there was some problem setting up the scale. I waited for them to get it working.

The Weight Watchers group leader asked me why I didn't just eat my lunch, and weigh myself afterward. I noted that in all our previous meetings, I had been weighing myself before eating lunch, so I wanted to weigh myself before eating for consistency. At that point, the leader said something I found rather astounding: "But if you eat and then weigh yourself, you won't have digested the food yet, so it won't add to your weight."

I was more than a little stunned by the thought that an intelligent person could possibly imagine that by eating food, I wouldn't immediately add the weight of that food to my weight as measured by a scale. Nothing seemed more obvious. If I weighed myself before eating, I would get some number. If then, still standing on the scale, I picked up my sandwich, obviously my weight would go up by the weight of the sandwich I'd be holding in my hand. If I then ate the sandwich, clearly, in my stomach, it would add to my weight just as if it were in my hand.

The logic is so trivial that I had difficulty imagining that anyone could possibly think I could eat a sandwich, and its weight would only be gradually added to my weight as I digested it. But that's precisely what the weight watchers group leader seemed to think. Perhaps she had simply never thought it out. After all, I've previously discussed, in my entry My brain on Ovaltine, a few very obvious things that I had to learn myself. ^Note 2 
 

Note 1:   Carbon monoxide is poisonous because it binds to the sites on the hemoglobin molecule (four of them) that normally carry oxygen, and won't let go. This effectively takes that hemoglobin molecule out of use. If enough hemoglobin molecules are thus compromised, the body is essentially suffocated.   [return to text]

Note 2:   Does the picture at the top of this entry look familiar? I used it in my entry Eleven.   [return to text]