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Wireless platforms are becoming less expensive and more powerful, enabling the promise of widespread use for everything from health monitoring to military sensing. Like other networks, sensor networks are vulnerable to malicious attack. However, the hardware simplicity of these devices makes defense mechanisms designed for traditional networks infeasible. This paper explores the denial-of-sleep attack, in which a sensor node's power supply is targeted. Attacks of this type can reduce the sensor lifetime from years to days and have a devastating impact on a sensor network. This paper classifies sensor network denial-of-sleep attacks in terms of an attacker's knowledge of the medium access control (MAC) layer protocol and ability to bypass authentication and encryption protocols. Attacks from each classification are then modeled to show the impacts on four sensor network MAC protocols, i.e., Sensor MAC (S-MAC), Timeout MAC (T-MAC), Berkeley MAC (B-MAC), and Gateway MAC (G-MAC). Implementations of selected attacks on S-MAC, T-MAC, and B-MAC are described and analyzed in detail to validate their effectiveness and analyze their efficiency. Our analysis shows that the most efficient attack on S-MAC can keep a cluster of nodes awake 100% of the time by an attacker that sleeps 99% of the time. Attacks on T-MAC can keep victims awake 100% of the time while the attacker sleeps 92% of the time. A framework for preventing denial-of-sleep attacks in sensor networks is also introduced. With full protocol knowledge and an ability to penetrate link-layer encryption, all wireless sensor network MAC protocols are susceptible to a full domination attack, which reduces the network lifetime to the minimum possible by maximizing the power consumption of the nodes' radio subsystem. Even without the ability to penetrate encryption, subtle attacks can be launched, which reduce the network lifetime by orders of magnitude. If sensor networks are to meet current expectations, they mu- - st be robust in the face of network attacks to include denial-of-sleep.