Wireless sensor networks are increasingly used in many emerging applications. This type of network is composed of hundreds of low-cost sensor nodes, but with a limited budget batteries, low communication range, limited throughput, reduced computing power, low memory and low storage capacity. Communication protocols are proposed in the literature to deal with technical challenges coming from low intrinsic resources of sensor nodes. In most of these studies, simulations comparing a proposed protocol with other existing protocols are performed to show that the proposed protocol provides overall better performance. However, the environmental specification that made these comparisons is very often neglected or non-existent. In this study we show that it is essential to have the simulation environment very well defined before considering whether a protocol provides better performance than others. To do this we use two duty cycle MAC protocols, the standard IEEE 802.15.4 and SlackMAC (a protocol that we proposed). The aim of the paper is not to make an exhaustive comparison of protocols. We intuitively know that SlackMAC provides a better overall performance than the standard. What we are trying to show is the gap between performances according to simulation conditions. We will mainly focus on the topologies used and the capture effect. The results draw attention to the fact that it is essential to clearly define the simulation environment and also to reconcile the chosen conditions with the results when comparing the performances of two protocols.