strange sleep behavior on new iMac

iMac (21.5/3.06/2X2GB/5000GB/9400M, purchased 2/9/10) GOES to sleep just fine. The iMac and a Macbook (13.3/2.1/2X512/120/combo, purchased 8/26/08) are both wirelessly connected via an 118n Airport Extreme. Whenever the Macbook is opened and awakened from sleep, it also awakens the iMac from ITS sleep. The reverse does NOT awaken the Macbook! What's the story? Is there a setting I need to use?