One of the primary mechanisms to mitigate idling energy consumption and reduce communication activities in Wireless Sensor Networks (WSNs) is to put nodes to low duty cycle operation as long as possible. Keeping a small time window (the listen period) is a challenging task, and thus requires appropriate tailoring of duty cycle so as not to compromise network performance. In this work, we explore an adaptive duty cycle mechanism for scheduling a node to sleep mode using node density. A forwarding candidate assesses its vicinity and adaptively adjusts its duty cycle. We show via simulation that the adaptive duty cycle mechanism can reduce the energy usage and prolong the network lifetime for the data delivery toward the sink. © 2011 IEEE.