In wireless sensor networks, power is a critical resource in battery powered sensor nodes. In this respect, as it is important to efficiently utilize the limited battery power, it would be desirable to make such nodes as energy efficient as possible. Many researchers who develop operating systems of wireless sensor networks have been trying to find a way to enhance energy efficiency of sensor nodes. In this paper, we present an overview of sensor node operating systems and some of its functionalities, and then present a performance analysis of task schedulers and task-related kernel routines of existing sensor node operating systems. The results of performance analysis show some advantages and disadvantages of the existing operating systems, and based on these information, we present some possible improvements for increasing the efficiency of sensor node operating systems.