system
Given the following post, respond as if you're commenting on Reddit:
human
Say that I weigh myself just before bed: 150lbs. I weigh myself immediately after waking: 148lbs. What processes happen while I sleep that cause this overall loss of mass overnight?

Assuming no expulsion of waste between measurements.

Measurements taken either naked or wearing the same clothes.
gpt
The weight loss would be due to breathing, mainly in the form of expelled carbon dioxide with some water vapor as well.  You intake oxygen, but expel carbon dioxide (basically just an oxygen molecule plus a carbon atom), with that extra carbon coming from the food you ate.  All the carbon adds up.  You also lose some water vapor out of your cells into your lungs, and from there into the air.