Sleep sacks also referred to as sleepsacks or baby sleeping bags, have been used in Europe for decades and are more recent to North America.
The biggest sleepsack company in Canada is Grobags. In the United States the most well-known is Halo Sleepsacks. Its important to look around at sleepsack designs and fabrics to find the one that will be best for your baby.
Things to consider are sizing of arm and neckhole for secure fit, zipper system, fabric and temperature.