SAN DIEGO -- Some San Diego residents are complaining that their mail doesn't arrive until well after dark, or worse: not at all.

San Diego residents on Canyon Mesa Lane in Mira Mesa complained Monday that their mail is consistently showing up after dark. One resident said his mail sometimes doesn't show up until the next morning.

A U.S. Postal Service spokesperson told NBC 7/39 that the local carriers are struggling to keep abreast of mountains of mail.

"There are a couple of factors at work here," said U.S. Postal Service spokesperson Mike Cannone. "First, we have the heaviest mailing season of the year, which started after Thanksgiving and will go until Christmas Eve. Second, we've been adjusting routes to try to balance out our carrier routes so they can deliver their mail in eight hours."

One local mail carrier told NBC 7/39 he worked a 12-hour shift Monday and was using his flashlight on a regular basis.