The subtle neck injuries common to concussions often do not cause any stiffness. If you have stiffness, it would point directly to a neck issue. Without a stiff neck, it still can be a neck issue.

Your sleep struggles suggest neck inflammation that is disrupting blood flow to the brain and/or disrupting breathing so you get oxygen deprived. This can cause the waking up and the stressful dreams.

The delayed onset can be caused by slow onset of inflammation in the neck.

In my experience with vestibular therapy, there is a lot of head movement exercises. These can make any subtle neck problems more problematic.

Many have found that working with weights is a problem. If one watches weight workouts, a common observations is neck tensioning/stiffening. Many physical exercises cause muscle tension. A common symptom of a concussion is called postural rigidity when the upper body is held rigid. Paying attention to these posture issues and focusing on relaxing when exercising may help.

btw, If you use the Post Reply at the bottom left below the Post box rather than the Reply at the bottom right in the post box, the Reply post with not quote the previous post and be easier to scroll through.