Sorry, my phrasing was not clear. Your metabolic rate is generally lower during sleep than during the day; you definitely lose more weight overall during the day than while asleep. But, while you're sleeping, you don't eat anything. And quality sleep is necessary to maintain overall metabolic function. I'm not a scientist and didn't mean to make any strong claims!

Two short articles I was basing this off of:

- "When somebody loses weight, where does the fat go?" https://www.bmj.com/content/349/bmj.g7257

- "Sleep and Metabolism: An Overview" https://pmc.ncbi.nlm.nih.gov/articles/PMC2929498/

You exhale more CO2 during activity because metabolism increases. Burning glucose and fat produces CO2.

It doesn't make sense that you would lose more weight by sleeping only considering metabolism.

I can see an argument around sleeping always being a net loss, since you're never consuming food while sleeping. Sleeping more thus means you may eat less.

If I were to guess, the metabolism generates waste during activity that gets processed and exhaled as CO2 during sleep.