TOG is short of the thermal overall grade. The higher the tog rating, the more warmth your baby can get. Kaiya Angel 3.5 tog sleep sacks are double-layer with 100g polyester padding. You can pick these sleep sacks when the room temperature is between 14-16℃/57-61℉. They are thicker than 2.5 tog sleep sacks. 3.5 tog is a suitable choice for cold days.