What to wear underneath

You buy a merino sack so you are not swapping the sack. You still dress for the room.

Warm night, around the top of 17–24°C: a cotton bodysuit, or just a nappy if they run hot. Cooler night, AC on, window cracked: long sleeves. The sack stays on either way.

Hands and feet run cold. They are a bad thermostat. Feel the tummy or the back of the neck. If that’s comfortable, leave them.

That’s the pitch, in practice: one sack on the crib rail, one decision about sleeves, and you both get a better shot at the night.