xbee sleep mode???

I am using a XBEE (1mW) to monitor the parallax motion detector.
I want the XBEE in sleep mode to save power and then wake up
when the motion detector sends a logic one. Then after receiving the logic one from the motion detector it will transmitt the logic one using Digital IO passing then go back to sleep. BUT, I do not know how to set the XBEE to do this, please help?