|There is no "sleep mode"|
I'm going to guess you're actually talking about "Standby" mode. If this is correct then, you should know that standby mode is pretty much useless and has been ever since it was introduced.
Do yourself a big favor. Set it to "disabled" and then change the Power Options settings to "Turn off monitor" after say, 10 minutes, and the hard drive(s) after 1 hour.
This won't lock up or cause a reboot and is as effective as sleep mode for saving you electricity.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.