You were active during the day, that loosens up the issue in your back, but I'll bet that when you get up, you'll be having spasms again in your back.
The key to getting moving is to avoid the spasms, easier said than done. You have to keep your posture balanced, any lack of vertical alignment can put your back into another spasm which is re-injuring it. After awhile, it will loosen up and as long as you continue to keep moving, it will stay loosened up. But any extended period of not moving will allow it to tighten up.
Good luck with that, hopefully you didn't do something that will keep cropping up several times a year.