Why do we weigh less in the morning?
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!"