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In wireless sensor networks, energy is a scarce resource. Collision, as one of the major sources of the energy waste for medium access control (MAC) protocols, has been extensively investigated. However, it is mostly analyzed under the “saturated situation” in which the sampling rate is assumed to be equal to or larger than the transmission rate, which does not reflect the reality in many applications. This paper relaxes this assumption to consider the “unsaturated situation” in which nodes do not always contend to seize the media. It focuses on hybrid MAC protocols which enable nodes to define sleeping schedules. We take SMAC as a model protocol to analyze how the sampling rate affects the probability of collision. But the approach can easily be applied to other hybrid protocols such as BMAC and XMAC.