An 85-year-old gentleman complaint of pain in leg while lying down, it lessens when he hangs the foot out of bed. What is the cause?

In peripheral aerial occlusive disease, as the disease progress, perfusion to the leg may be so severely compromised that anaerobic respiration occurs even at rest, typically affecting the foot and/or calf. The pain is exacerbated by lying down or elevation of the foot due to loss of the gravitational effects on perfusion pressure in the foot. The patient characteristically describes the pain that is worse at night and may be lessened by hanging the foot out of bed or by sleeping in a chair (effects of gravity restored)Ref: Bailey and love pg: 943