If you ever woke up with muscular back pain, and can’t recall how you got it, it could be a number of things.
1- Your mattress : cheap spring mattresses deform at the level of the body’s center of gravity when laying down, which is at the pelvis (buttocks/hip level). Remove your sheets and take a look at your mattress: is there a slight depression in the center? If so, when you lie on your side, your pelvis sinks and forces your lumbar spine to arc downwards, compressing the joints and discs on that side; if you sleep on your stomach it puts your lumbar spine into hyper-extension (arcs sharply) forcing the facet (rear) joints of your lumbar spine together; if you sleep on your back your pelvis sinks downwards and the mattress can’t support the lower back.
Solution: get a new mattress. I recommend the TempurPedic viscoelastic mattresses, which resist deformation and last a long time. Some people report good results on the Sleep Number air mattress as well.
2. You may have strained your fascia while exercising or engaging in other intense physical activity that required heavy use of the back muscles, such as rock climbing. The fascia is the sheath that encases a muscle, protecting it as it lengthens and shortens inside the body, rubbing against adjacent muscles. It may take longer for fascial strains to be noticeable as they don’t release as much inflammation as a strained muscle. So, if you wake up and you have a very focal, sore spot on one of your back muscles that gets progressively worse, chances are you have a myofascial strain. You probably incurred it the previous day doing a back exercise or other type of heavy exertion.
Solution: simply ice the area for 20 minutes using a frozen gel pack, and a kitchen towelette or two to protect from frostbite. Repeat until the pain goes away. An effleurage massage (long, light pressure strokes as opposed to short, heavy and focal pressure) would be helpful, too. Do the massage before the ice.