Welcome to Tom's Forum!
First, huh?! Why are you allowing any Windows
Server to sleep??!! Next, you need to define via the BIOS AND in Window 'how' you want your "Server" to wake. Example, PME Event Wake Up, AC Back Function, Power-On by Alarm, ErP Support, Onboard LAN Function, Wake on LAN. Also, to enable WOL 'magic packets', etc in Windows. Next to decide on what Sleep modes S3, S2, S1. The worst sleep for a Server is Hybrid S3 + S4, and IMO stick to S2 and ..... maybe S3.
This is a problem when folks use a 'Desktop' MOBO for a 'Server' purpose. First, the Server BIOS defaults are all geared to 'always on' and minimal 'power savings', VT-d/Vt-x, etc in contrast to what you have now.