These 2 statements are both true, but I'm having trouble reconciling them, please help:
When you're sleeping in a sleeping bag, you want the sleeping bag to be form fit (hence the mummy shape). If it's not form fit, there's too much dead air for your body to effectively warm up, which may lead you to feeling cold
When you're sleeping in a sleeping bag, you want the sleeping bag to be an optimal thickness. If it's too thick, you'll trap too much body heat, which will lead you to overheat
I don't understand statement #2 in the context of statement #1. To me, it would seem that too much thickness means that there is too much dead air. So then why is it that in #2 the excessive dead air causes you to overheat, whereas in #1, the excessive dead air causes you to feel cold?
If I'm conflating multiple phenomena, please help me disaggregate