We are interested in the design of physical layer (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 the achieved data rates. So far, a number of different architectures have been proposed for IR-UWB, but in the context of rate efficiency. 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 low rate operation. Our aim is to present the different design alternatives, to evaluate their suitability and assess their costs and benefits for networks with very low energy consumption. We identify the four main functions a PHY-aware MAC design has to provide: (1) interference management, (2) access to a destination, (3) sleep cycle management, and (4) signal acquisition. Then we present a non-exhaustive list of the many possible building blocks and use it to analyze the design choices that existing proposals make. Finally, we review the performance implications of these design choices with respect to a very low power ultra-wideband networking architecture.