Still, check with the airline re: their maximum baggage connection time limit. It is airline-specific and each carrier has its own rules. If a through-check isn't possible---because your connection exceeds their limit---you'll know at check-in because their computer won't print you a tag with the second flight segment/city/flight on it. It's not simply a matter of whether it's "overnight", there is a time limit involved. I recall contacting AS about this on the above-mentioned SAN-SEA-SIT trip, and was told that AS's limit was eight hours. Our layover was 8.5 hours and I was told that some overnight layover people can check straight through as long as the total time at SEA is under eight hours. We were flying on award tickets, and there were no available SAN-SEA seats on the morning of our intended departure, but they did have free seats on the last SAN-SEA flight the night before. Given that RT Economy SAN-SIT was about $750, the moderately priced layover hotel at SEA was a bargain ($65), and we didn't mind being on the 7 am flight to SIT the next morning.