Wireless Sensor Networks (WSNs) can be used to monitor environments, and therefore have broad range of interesting applications. The applications which may use WSN can be of sensitive nature and therefore might require enhanced secured environment. As sensors are used to monitor sensitive areas therefore Security and energy efficiency is essential consideration when designing wireless sensor networks (WSNs). The Sensor nodes get their power from batteries. Since the sensor nodes are deployed in harsh environment they cannot be recharged. Due to unattended deployment and inability of recharging, the power consumption of the nodes should be optimal. To implement minimum power consumption Sensor networks periodically place nodes to sleep. This is achieved by using the media access control (MAC) protocols. These protocols are designed in such a way that they reduce the energy consumption of sensor nodes by keeping the antenna in sleep mode as much as possible. This leads to power saving. The MAC protocols change the sleep time based on the type of communication required. However, malicious nodes can be introduced in the network and these attackers use their information about the MAC protocol, by manipulating the sleep time of the node, so that life time of the node reduces. This is called as Denial of sleep attack .This paper, addresses the Denial of sleep attack in WSN while at the same time proposing a scheme for authenticating the new nodes which try to change the sleep schedule of the nodes. Only transmissions from valid nodes are accepted. Our scheme uses zero knowledge protocol (ZKP) for verifying the authenticity of the sensor nodes which pass the sleep synchronization messages. Also to enhance security further the interlock protocol is used during key exchange. The paper presents a detailed analysis for various scenarios and also analyzes the performance while implementing this secure authentication.