What Causes Chronic Foot Pain At Night?

My mother has chronic foot pain in both feet at night. It gets bad when she is stationary and her feet are hot. Her doctor has her on strong pain killers but they only last about 3 to 4 hours before the pain wakes her again. She has had tests for vascular issues and told it is not a circulation problem. She is to have tests for arthritis. The condition improves if she exercises. She is 82 and not getting a proper nights sleep is really ðebilitating, never mind the pain she is experiencing. Any thoughts?