A Wireless Body Area Network (WBAN) aims to provide real-time and ubiquitous body monitoring to perform autonomous early prediction, diagnostic, and tracking the response of the body to treatments. IEEE 802.15.6 is a communication standard for WBANs which pervasively and continuously monitor the human body. In this paper, we develop an analytical model for performance evaluation of the IEEE 802.15.6 standard under non-saturation regime. We validate the analytical results by a simulation model. The results of the paper indicate that adopting appropriate EAPs and RAPs lengths noticeably improves the performance of the network. Short EAPs and RAPs result in wasting the network resources due to growth of the time periods in which the medium is inaccessible. On the other side, long EAPs and RAPs increase the collision probability at the end of EAPs and may result in undesirable frame delays.