When it's hot out and summer time you can let the baby sleep in either a Onsie and Diaper or just let the baby sleep in just the diaper and maybe a shirt or just the diaper itself.
My kids have slept in just a diaper during hot weather although we did have air conditioning so if the air conditioner was on too low of a setting then I would at least put the baby in a Onsie and Diaper and they were just fine that way.