In the area of wireless sensor networks, achieving minimum energy consumption is a very important research issue. A number of energy efficient protocols have been proposed, mostly based on a layered design approach, which means that they are focused on designing optimal strategies for "single" layer only. In this paper, we take an alternative approach i.e., a cross-layer design, and present a new MAC protocol named MAC-CROSS. In this new approach, the interactions between MAC and Routing layers are fully exploited to achieve energy efficiency for wireless sensor networks. More precisely, in the proposed MAC-CROSS algorithm, routing information at the network layer is utilized for the MAC layer such that it can maximize a sleep duration of each node. Through implementation on a Mica Mote platform and simulation study using the NS-2, we evaluate a performance of the presented MAC-CROSS and prove its substantial performance gains.