The clothing your baby or toddler should wear beneath their sleep sack is determined by a combination of factors, including the room temperature, the TOG rating of the sleep sack, and your baby's personal comfort.
If your child's face appears flushed and their back or neck is perspiring, it could indicate that they are feeling overheated. To address this, you can either opt for a lighter sleep sack or take off a layer of clothing underneath the sleep sack.
If you notice that the chest, back, and belly of your baby feel cool or cold to the touch, it could be an indication that they are too cold. Instead of using a blanket, which is not recommended for infants under 12 months old, consider using a sleep sack that has a higher TOG rating or adding an extra layer of clothing underneath the sleep sack.