A wireless sensor networks is a wireless network organized with huge number of sensor nodes with specialized sensors that can monitor various physical attributes such as temperature, pressure, vibration, sound. Sensor nodes are powered up with batteries. Due to unattended nature of deployment, the sensor nodes cannot be recharged again. In this condition the nodes must optimally consume power. Various protocols are designed to reduce the energy consumption of sensor nodes by keeping the antenna in sleep mode 90% of time, so that power is saved. MAC protocols are designed to adaptively vary the sleep time based on the communication need. But attackers use their knowledge of their underlying MAC protocol, to reduce the sleep time of the node, so that life time of node reduces. Here we study the B-MAC protocol and the vulnerabilities in the B-MAC protocol and propose a solution to defend the B-MAC protocol against denial of sleep problem.