One　of　the　most　remarkable　applications　of　the　Internet　of　Things　is　health　monitoring.　Wireless　body　area　networks　(WBANs)　consist　of　several　body　sensors　implanted　in　or　place　on　a　human　body　and　a　coordinator.　One　of　the　essential　issues　of　WBANs　is　the　design　of　Medium　Access　Control　(MAC)　protocol　to　improve　system　efficiency.　In　this　paper,　we　modify　the　IEEE　802.15.4　standard　MAC　superframe　structure　and　propose　an　energy-efficient　MAC　protocol　for　WBANs　in　the　e-Health　Internet　of　Things　(IoT)　systems.　In　this　scheme,　the　time　slots　are　divided　into　mini-slots.　The　lengths　of　the　contention　access　period　and　the　contention　free　period　are　both　variable.　By　using　this　mechanism,　the　utility　of　time　slots　can　be　considerably　increased.