M.K.
I've always had issues with this, since I didn't let my kids have blankets in the crib until they were 2yrs old (extreme, I know!)
We've had the air on lately, so I have closed my son's vent and have him in a cotton pj with long pants and short sleeves. If it's really cool, I simply put his pj bottoms on with a long-sleeved, comfy t-shirt. My son hates when his legs are cold!
Ideally, I put him in a lighter-weight cotton footie-pj. But these are difficult to find this time of year. In later summer, these will come out again (before the heavy fleece winter pjs). Buy a size ahead so you can be ready for next Spring and Summer.