Wireless sensor networks are a class of wireless Ad Hoc networks for which the low power consumption is a major requirement. This paper presents an analysis of the performances of low power multiple access protocols designed for a network of wireless sensors. It considers the usage of Spatial TDMA for the transport of regular and frequent traffic and of a low power version of the non-persistent CSMA protocol for the transport of rare signaling traffic. The low power version of CSMA is obtained by combining it with the preamble sampling technique. The delay performances and associated power consumption and lifetime are computed analytically. The power consumption of TDMA is computed as well, taking into account the power cost of keeping synchronized two nodes having imprecise clocks. The lifetime that can be expected by a node with a single alkaline LR6 battery is given in function of the desired average time interval between collision free transmissions on the TDMA and on the contention channels.