Numb thigh

For a few weeks now, whenever I'm in bed my left thigh from hip to knee becomes totally numb. Occasionally but rarely my left leg will also become numb and once it happened to my right thigh. Just now it happened and for the first time my knee also began to burn so badly. This happens every night. When do I begin to worry and what can I do to relieve the pain. This happens regardless of what side I sleep on.