Jessica, I used a 2.5 tog for mine during winter. When you buy a new bag it comes with a wall thermometer and it has a guide on the back for what to dress them in depending on what temp is. From memory on a cold night it is a long sleeve bodysuit with long sleeve PJ top, and a not so cold night is just a long sleeve bodysuit.

When mine were standing I switched them to snugtime sleepers and actually preferred them because they have legs in them (and long arms) the heaviest in them is a 1.5 tog, but I would dress them in pyjamas and a bodysuit underneath, and they were very snuggly - plus they looked like little teletubbies!! You can get them on ebay or there is also a website