This is just one of the strangest things I've come across barefooted, foot cramp. At first I was worried that I'd injured my feet (especially following my two summer marathons) as when I woke in the morning I found I was hobbling for the first few minutes of the day. But when this persisted long after my walks had been and gone I realised it was something else, something much more normal and everyday.
You must know the experience some days when you wake and find that your hands are all scrunched up and completely useless for a while, like it almost hurts to open them out. Well it's exactly the same thing, but as I'm actually using my feet to manipulate things and giving them a load of exercise in the day times (griping curb stones with my toes etc.) I have foot cramp each morning. It's really wonderful as it shows that I'm actually making use of my feel as something more than just stumps for walking on.