system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we prefer cold pillows to warm pillows?

Even though warm pillows are more comforting, why do we still prefer cold ones?
gpt
When you're awake, the normal functioning of your body is generally producing more heat than your body needs to maintain its preferred internal temperature. So your body is often trying to dump that heat out into the world, or else you'd overheat and that'd be bad for you. 

So when you lay down and put your head on a warm pillow, that's basically like wrapping a blanket around part of your head. You're insulating that part of your body, so it can no longer be effectively used to dump excess heat into the atmosphere. It's not comfortable because it's holding heat in that part of your head. 

While your sleeping, your body is generally much less active, and as a result creates less heat, and therefore needs to dump less heat into the outside environment or even conserve heat in order to maintain your body temperature. That's why in the morning a warm pillow might feel nice, even though 8 hours earlier all you wanted was a nice cool pillow.