At higher temperatures, a bodysuit under the sleeping bag is usually enough. At lower temperatures, it is advisable to combine a bodysuit with pyjamas or cosy trousers and a top.
However, since every baby feels temperatures differently, you have to find out individually whether your baby is wearing too much or too little. You can find out by doing a quick test on the back of the neck: If the neck is moist and warm, the baby is too warm. If the skin is cold, your baby may be cold.