Why do I have a sort of restraint on my jaw in the morning?

Muscles tight. Only thing that restrains jaw movement (generally) is the muscles that surround of connect to jaw bones. These jaw muscles (used for chewing) may be subconsciously tight/clenched. Can occur in sleep or aroused state without even realizing it. If persistent and untreated, it can lead to chronic spasm of these muscles, and chronic TMJ syndrome--which is painful and difficult to rx. Check out now.
Correct. Dr kurzweil is correct. Problem probably from clenching/grinding. Sometimes can be from cartilagenous disc in the TMJ (jaw) joint being out of position. Have your "bite" and your joints and your muscle function examined by an occlusion expert, an orthodontist.