Why does pain seem to intensify during the early hours of the morning?

Not a new phenomena, I have noticed it for as far back as I remember, but recent events have brought the question to mind.

Why does it seem that an injury will hurt/ache more around, for me at least, 2 -3:00 am? It is not that I wake up worrying and focus in on the pain but that the intensity of the pain itself awakens me.
I go to bed with the injury, usually a dull throb at that point, wake up to sharp stabbing pain which gradually slackens off over the course of an hour and allows me to go back to sleep and then awaken with the same dull throbbing pain I had at bedtime.

Does it have something to do with REM cycles or normal sleep patterns? Would that explain why it occurs around the same time period?
I know I am not re-injuring the site while slumbering but am I perhaps stretching/stressing/pushing it past its limits it in my sleep?