Energy efficient dense wireless sensor network design makes extensive use of spatial redundancy and appropriate MAC mechanism. CSMA is integrated with TDMA while filtering out the spatial correlation with a so-called HS-Sift MAC protocol. The entire sensing region is divided into three sub-areas for different channel access methods. The nodes near to the border sleep most of the time, nodes close to the event claim high channel utilisation and those lie in between compete under different priorities. A software simulation verifies effectiveness of the proposed scheme for energy consumption and access delays.