In this paper, we provide an extensive analysis of the MAC state operation for energy conservation and introduce both channel release and setup schemes regarding the uplink data traffic for wireless sensor networks. The introduction of a release timer has already been applied to some power saving schemes in the literature. In this paper, however, an additional release timer is used to let sink nodes go back to inactive state at time-out. Instead of using randomly distributed release timers as in the literature, we use Erlang-k distributed release timers here in order to improve the inaccuracy in the analysis. We find that a longer release time period can be a better design choice for arrival traffics with deterministic packet length.