Check in once or twice?

We are going round trip from SMF to BOS. The first leg out to SEA arrives in SEA late in the afternoon and the flight SEA to BOS is the next morning. (There are other flights later than the arrival time in SEA, but they are red-eyes which we choose not to do) Both flights are on the same ticket. We will be staying overnight in a hotel in SEA. We will keep overnight items in our carry-ons.

Will we need to pickup our checked bags in SEA or will AS hold them overnight and put them on the plane to BOS? (Or is it safer to pick up the bags and recheck them the next morning, just be sure they will arrive the same time we do?)

Related to this, when we check in at SMF does this also check us in for the flight the next morning or do we do the app check in for the flight the next morning?

TIA