Yes, most doctors will require you to wear the post-op knee brace (in locked position) for several weeks while sleeping after ACL knee surgery. While also keeping the knee/ leg elevated on a InteVision Foam Wedge Pillow (7.5" tall) while you sleep to help keep swelling down and also to keep your leg in a straight angle to help with extension.
Using the post-op knee brace at night will help protect your knee while you sleep, as well as keep it from losing full extension overnight. If you keep it in the post-op brace overnight, the knee will feel better in the morning. The longer you can wear the post-op brace while sleeping, the better your knee will be. At a minimum wear it for 4 weeks, optimally wear it for longer (7 to 9 weeks). At your second post-op doctor's appointment, the doctor will probably tell you that you no longer need to wear the brace at night.
I wore my post-op brace for about 8 weeks straight, then wore it a few more times over the course of the next two weeks. I've continued to use the Foam Wedge Pillow along with a knee pillow (for side sleeping) off and on, now going on 1 1/2+ years since knee surgery. I totally recommend getting both pillows for recovery after surgery.