Dupes will sleep in unbreathable areas

If it is repeatable, watch the duplicants breath value through the night. Even if he had full breath before going to sleep, he would still drop to 32.5% (75 seconds) by morning, completely passing the "interrupt activity" of sleeping percentage, and actually getting into the suffocating range:

Interrupt activity at 73% (after 30 seconds)
Suffocation warning occurs at 45% (after 60 seconds)

I'm wondering if your duplicant has the -0.9% the entire time or not.