Enable WiFi when in sleep mode

When I am at home, I do not have any ATT service. Therefore, to conserve battery power I enable airplane mode to keep the phone from searching for a signal. I do keep WiFi enabled for work email (exchange) and AIM.

I am noticing that when the phone is asleep, I do not get notification of new email messages. I assume that when the phone goes asleep it shuts off WiFi. Is there a way to keep WiFi turned on while the phone is in sleep mode?