Can't wake my Mac from sleep

Guys,

I want to wake my Mac from sleep every 3 am so that my backup runs automatically. The backup is working and scheduled, but I can't get my Mac to wake up automatically.

One of the options in the "sleep" preferences (under "Energy saver") is "start up or wake", which I put at 2:55 am.

I would assume my Mac would wake up and my cron job would start the backup, but it really doesnt' wake. Am I missing something?

Tx,

Phil