There are other threads both here and if searched through Google that something is broken with sleepmode between Ubuntu and Intel NUC.
What is your power management settings in BIOS?
Im thinking if S4/S5 is enabled or not in your case?
No, this is not enabled.
In fact, I've turned off every power management feature to off as I have been having a secondary issue since early December where my wifi card (Intel 7260 dual band N) has been intermittently switching off to the point where it required shutting down and even then would sometimes not work. I am currently running a live version of Ubuntu 12.04.5 x64 to see if the wifi issue is resolved. If so, I will reinstall this version of Ubuntu and see if the sleep issue goes away.
I have read that thread and it is uniquely different from my problem (holding the power down in the other thread resolves the issue). Further, I am running Ubuntu rather than Windows.