system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do flannel sheets feel warmer than regular sheets when you first get into bed?
gpt
The way the human body senses temperatures is weird and relative. As your skin comes into contact with something that is a different temperature the two set about reaching an equilibrium. 

Plastic toilet seats feel "warmer" than porcelain ones. But they aren't. Porcelain is a more efficient thermal conductor, so it drags heat out of your arse much faster than plastic. The more rapid theft of heat with porcelain is what we perceive as cold. 

Same principle applies with flannel sheets. The effect is more pronounced because flannel has more of a texture, so less of it is in contact with your body (stealing warmth) at any one time.