Either a sleeping bag or co-sleeping
Co-sleeping regulates baby's temp and if you are both hot, you just sleep disentangled and baby knows you are just there. Generally, Oscar gets warmer quicker than we do and will hang his feet over the edge of the bed to cool off, whilst keeping a hand on the boob. He sleep on my side, so he's definitely not getting too hot from body heat. And because he knows I'm right there, he doesn't wake even half way for a feed in the middle of the night, he just stirs, attaches and falls back asleep so that I get a restful sleep, too