In wireless sensor networks (WSNs) duty cycling has been an imperative choice to reduce idle listening but it introduces sleep delay. Thus, the conventional WSN MAC protocols are bound by the energy-latency tradeoff. To break through the tradeoff, we propose a radio wave sensor called RF watchdog that is dedicated to sense the presence of a RF signal. The distinctive feature of our design is that the RF watchdog can provide the same sensitivity but with two orders of magnitude less energy than the underlying RF module. With RF watchdog a sensor node no longer requires duty cycling. Instead, it can maintain a sleep state until its RF watchdog detects a communication signal. We also propose an on-demand MAC protocol called ZeroMAC that can effectively utilize the on-demand wakeup functionality of a RF watchdog by broadcasting a dedicated signal to wake up nodes before starting a communication. ZeroMAC wakes up only the nodes on the communication path by propagating wakeup signals in a hop-by-hop manner, avoiding unnecessary signal flooding. To further save energy, a node in ZeroMAC can turn off its RF module as soon as it detects the end of communication. According to our detailed packet level simulation results, ZeroMAC can deliver data packets at least 1.87 times faster by eliminating both idle listening and sleep delay while it consumes only 3% of the energy compared to X-MAC and A-MAC.