We are interested in the design of physical-layer aware medium access control (PHY-aware MAC) for self-organized, low power, low data-rate impulse-radio ultra-wideband (IR-UWB) networks. In such networks, energy consumption is much more of a concern than achieved data rates. So far, a number of different solutions have been proposed in the context of data rate efficiency for IR-UWB. However, the choices made for rate efficient designs are not necessarily optimal when considering energy efficiency. Hence, there is a need to understand the design tradeoffs in very low power networks, which is the aim of this paper. To this end, we first identify \emph{what} a PHY-aware MAC design has to achieve~: (1) interference management, (2) access to a destination and (3) sleep cycle management. Second, we analyze \emph{how} these functions can be implemented, and provide a list of the many possible building blocks that have been proposed in the literature. Third, we use this classification to analyze fundamental design choices. We propose a method for evaluating energy consumption already in the design phase of IR-UWB systems. Last, we apply this methodology and derive a set of guidelines; they can be used by system architects to orientate fundamental choices early in the design process.