Alma 55:22

Now behold, this was done in the night time, so that when the Lamanites awoke in the morning, they beheld that they were surronded by the Nephites without, and that their prisoners were armed within.

This was done during the night. When the Lamanites woke up in the morning, they saw that they were surrounded by the Nephites on the outside. And their prisoners had weapons on the inside.

All of this happened during the night. When the Lamanites woke in the morning, they found themselves surrounded by Nephites on the outside—and facing armed prisoners on the inside.