A Wireless Sensor Network (WSN) consists of spatially distributed autonomous sensors that monitor environmental data such as temperature, humidity, light, speed and sound. WSNs pose new security challenges because of their unattended nature and limited resources. Although prevention measures such as encryption and firewalls have been successfully applied, the attacker can physically access the node and modify it. Intrusion Detection Systems (IDSs) are a second line of defence that can be used to mitigate this problem. Building IDSs for WSNs is a new challenge because of the limited resources of the WSN nodes. IDS solutions for sensor networks should try to minimise the use of battery of the sensor nodes in order to prolong the network lifetime. In this paper we analyse different solutions that have been proposed for intrusion detection in wireless sensor networks. More specifically we analyse the impact of popular intrusion detection systems on the life time of the WSNs. Our study is quite general since we consider IDSs that are distributed on the sensor nodes and continuously monitor the networks for evidence of attacks. We also consider IDSs that are event triggered, which means that they require agreement between nodes when a suspicious activity is detected. The agreement is used to detect the attack and isolate the attacker. We analyse the effects of IDSs on battery life. The results show that, popular oral message algorithm of Byzantine generals problem should be considered for small scale WSNs because of the overhead introduced in terms of messages exchanged for decision. We conclude our paper with properties and recommendations for IDSs working for WSNs and some future works.