system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do people weigh less first thing in the morning (or, presumably, directly after sleeping) than they did immediately before sleeping?

This is without any excretions.
gpt
You constantly lose mass in two ways. One is that water evaporates from the damp insides of your lungs, and unless you are very cold, you will still sweat by a small amount.

The other is that living involves losing mass. You breathe in oxygen, and breathe out CO₂. For every two atoms of oxygen you take in from breathing, you breath them out with an attached atom of carbon. That carbon comes from stored sugars, fats or proteins in your body, and it leaves you - completely - as a gas.

It's like how the majority of the mass of a tree comes from thin air.