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At first I thought it may be related to physics. You know, in one half of the day, the gravity changes, and the scale shows a different weight than the other. Then at night, gravity intensifies, and you become slightly heavier.

But I'm a bit skeptical about that. Is that true? That it's somehow related to gravitational fields?

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Veritasium created an excellent video just on this topic, which I would recommend watching, but will summarize here.

A common misconception is that we lose weight when we digest food or burn calories. Keep in mind that that broken down food and those calories* aren't "going anywhere", so technically you shouldn't loose any mass (unless you get rid of the molecules by going to the restroom).

However, in the "informal" tests done by Derek Muller, he lost about 250 grams each night. Since during the night you sweat (which evaporates or gets absorbed into the sheets and blankets), he estimated that about 150 grams was lost through perspiration and breathing out water vapor.

The more surprising loss of mass, was 0.012 grams of carbon lost with each breath when breathing in oxygen, and then breathing out carbon dioxide. Although this seems like a tiny loss, assuming you breathe in and out about 7,680 times each night, that tiny mass loss adds up to almost 100 grams of weight loss through breathing alone.

I guess health magazines should start running articles like "Top way to lose more weight? Researchers say breathe more!"

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  • $\begingroup$ * Technically calorie is a unit of energy and not an item that can be "burned". What we mean when we use the word is the process of molecules being broken apart, which releases energy that can be used by the body; however, the pieces of the "broken up" molecules still weigh the same as the molecule as a whole, so we don't loose weight just from "burning calories". $\endgroup$
    – IQAndreas
    Commented Sep 7, 2014 at 9:05
  • $\begingroup$ It's not really so surprising. Carbon is converted to Carbon dioxide which is exhaled. A quick back-of-the envelope calculation shows this makes sense: 1kg of fat has about 38000kJ, that's a bit over 3 times the energy used in a day, so if we don't eat, we convert about 300g fat into CO2 in a day, or 100g during sleep (1/3 day). $\endgroup$
    – uUnwY
    Commented May 25, 2020 at 19:37
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There is indeed a slight weight variation during the day. About 2 - 4 lbs (approx. 1 - 2 kg) [1, 2].

Some causes are:

  • water loss through respiration, perspiration or urination [1].
  • the relative long period without eating and drinking [2].
  • metabolic processing of food and drinks during sleep.

References:

  1. Cindy Banyai. http://www.livestrong.com/article/483853-how-much-can-weight-fluctuate-from-morning-to-night/
  2. http://www.weightlossforall.com/weighing-less-morning.htm
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    $\begingroup$ Not eating or drinking and Metabolic processing does not change your weight/mass, that violates conservation of mass. The only thing that affects body mass are excrements (from urine, sweat, and faeces), respiration, and very small amount of debris from your skin/hair. $\endgroup$
    – Lie Ryan
    Commented Sep 7, 2014 at 2:49
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    $\begingroup$ Mass can be converted into engergy. Setting something on fire does change the weight/mass of for instance your house, doesn't it? This is what your body does when you "burn" calories. You don't eat (add something), but your body stays 37 degrees -> the energy to keep it that temperature (even ignoring things like heartbeat and other processes like your brain) has to come from somewhere. Apart from that, the law of conseveration of mass deals with closed systems (also closed for energy!) which you body certainly isn't. $\endgroup$
    – Nanne
    Commented Sep 7, 2014 at 8:00
  • $\begingroup$ @Nanne Mass isn't "converted to energy", at least not in our human body (unless we go thermonuclear, which would be awesome, but just doesn't happen). What we do when we "burn" calories is break the chemical bonds between glucose(?) molecules; the breaking of these bonds releases the small amount of energy stored in them by the plants that spent that same amount of energy in order to produce those bonds. The combined mass of the "broken apart" molecules is still the same as the "complete" C6H12O6 molecule. $\endgroup$
    – IQAndreas
    Commented Sep 7, 2014 at 8:20
  • $\begingroup$ (If a biologist or chemist could double-check my comment, I wasn't sure if it was glucose or sucrose that gets broken apart. Moreover, as I only have a basic understanding in the field, I may have other minor flaws in my comment) $\endgroup$
    – IQAndreas
    Commented Sep 7, 2014 at 8:23
  • $\begingroup$ @Nanne Setting something on fire does change the weight/mass of for instance your house, doesn't it? > It doesn't; if you measure the increased weight of the air (the smoke particles in it) and the remaining ashes, there is no difference in weight. Technically there is a loss of mass, but it is unmeasurable by any equipment you could buy at a store. $\endgroup$
    – IQAndreas
    Commented Sep 7, 2014 at 8:32
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There is only one way to gain weight, consume things. Thus after any period of time not consuming anything (i.e. sleeping) we must necessarily weigh <= what we weighted at the start of that time period. Add in respiration, and that becomes a strict inequality!

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