My experience with the EVO is at night it shuts off EVDO, LTE, and WIFI and stops serving data unless you power on the phone.
Phone calls still come through just fine, as the 1x network is still active, but is not used for data.
It's an awesome power saving feature, I love it. Sometimes it's annoying when you are waiting for an email or a text and your phone is hibernating but that rarely happened to me.