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You will lose a minimal amount of mass overnight. You won't do so because it is night—night has nothing to do with it—but because time is passing. You must burn calories to stay alive, and, even though you burn fewer while you are sleeping, you would still burn some just to keep your metabolism going. That's the basic loss of mass, and it is minimal: less than a pound.
I suppose if it were really hot, you could sweat off more, but that's the basic reason: even your resting metabolism burns energy.
When you sleep your metabolism is still working, so you burn calories which cause the loss of mass. Although it is not enough to be completely noticed.
I lose upto three pounds of mass overnight. Last night, I weighed about 181 in the evening and 178 this morning, for example. Its probably mostly water evaporating through breath and sweat, also heat etc, urine, but its kinda staggering nevertheless. I did a lot of physical work in the house yesterday and it was somewhat colder than usual inside.
Humans can lose upto 1.5 liters of water a day through respiratory water loss (breath). The amount depends on the temperature and volume of air breathed. The colder the air and the more intense the the activity - the more water evaporates in breath. 1.5 liters of water is about 3 pounds.
Three pounds of water is quite a bit over 8 hours or less. I need to drink more of it then because dehydration is very dangerous to the body - it can cause organ shutdown, deterioration and other nasty things over the long term.
That's about loss of weight overnight. If you are gaining weight overnight, you may be sleepwalking to the fridge - check you bed for crumbs and your face for leftover sauce, etc. :)
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