If I am overtired, I am less able to manage the stress of chronic pain. I will be FAR more irritable, needing more meds and/or more rest, with a pain level of 6/10, than I would be if I were dealing with a 6/10, which would be tolerable, if I were well rested. The added stress can also make it more likely that my pain will get worse.

If I am well rested, at least I have the shot at a decent day.

Getting decent sleep is a whole different issue and a huge challenge for many of us.