Accidentally fell asleep on couch overnight, now lower back really hurting!hurts to walk fast or go from sit to stand.what to do? how long pain last?

While it is hard to: Say with back pain without a physical exam, it sounds like you may have developed a muscle spasm in your low back, often lumbar paraspinals or iliopsoas after sleeping on a couch . Keep moving and walking, do not lay in bed with this. Should start getting better after two or three days with resolution within a week. If it doesn't, or pain radiates down leg, see your physician. Hope that helps!

Difficult to say: If there is no underlying proble, then it's a muscle strain that should go away within several days. Best wait a day or so and if it does not improve, then check with your doctor.