We use the Merlin to transition of baby out of his swaddle. He was about 4.5 months at the time and it worked great. We live in Arizona and of course we have the air on but it doesnt ever feel like he is overheating. (I frequently check his hands, feet, and stick my fingers in and feel his chest.) He can still access his hands, which he loves, but cant roll in it yet at 5 months.