The mesh nodes have to run either in always-on mode (which makes perfect sense if they are powered by, say, a wall socket, which they often are) or in a sampled listening mode. Sampled listening shaves some 98% off from the radio duty cycle compared to always-on mode, but still is not enough for the mesh nodes to run on coin cell batteries. Give them a larger pack of batteries and they'll run for months though.

To send data to a node in sampled listening mode, the sender sends a string of smaller wake-up packets that indicate when the sender intends to send the real data packet. When the receiver picks up the wake-up packet, it knows when it should wake up again to receive the data packet, so it can safely go back to sleep again for a while. And if the sender knows that the receiver is in always-on mode, because it is powered by a wall-socket, the sender can skip sending those wake-up packets, saving a bit of power.

This requires clocks to be synchronized, but only loosely - millisecond synchronization is enough. The trick is to strike the right balance between communication responsiveness and power consumption.