Forget the sleep sack. It is HOT out! Babies who sleep on their backs are delayed in motor skills across the board. If they do not get enough tummy time/core stregth in the first 6 months then this will come back to haunt later on in school. It shows up as a handwriting problem, copying from the board problem, reading, decoding, auditory, vision and behavioral problems.....ALL that from not getting enough tummy time. Sleeping on tummy wakes up the central nervous system and playing on tummy increases all skills. A good rule of thumb for boys is 4 times a days for 30 minutes each time. If your child is below that then work on it. Get them out of chairs that sit them too early and develop the wrong groups of muscles. We all slept on our backs and we are so much stronger then this generation of children who are cognitively smart but cannot engage, communicate, show physical endurance and have lots of problems learning in a school environment.