There's no mechanism built into the AP radio to do this. A couple of recurring WLSE config jobs could do the trick--one in the evening to turn off the radios and one in the morning to turn them back on. You could also write a simple script to do this too if you don't have the WLSE.
Another option, and I don't know if it's supported or not in the AP IOS is a time-based ACL that blocks all traffic from 6pm to whenever in the morning you want to start allowing traffic again. The down-side to this approach though is that the AP will still beacon and respond to probes and wireless clients might still be able to associate. They just wouldn't be able to do anything on the network.
