I think I might have found the reason as why my
DS218 was waking up every night at 11:36pm. I have read on QNAP knowledge base that DHCP IP address lease renewal can also wake up their NAS units. My router assigns an IP address lease for 1440 minutes (1 day) so I thought its worth a shot to assign a static IP address to the NAS unit outside the range of IP address which are being assigned by router.
I have assigned a static address to NAS (outside the DHCP severs managed range) about 3 days ago and so far my NAS has not come out of hibernation. It may be incidence of some sort but I am cautiously hopeful at this point.
BTW I have also disabled all the log functions including hibernation log and disabled the USB copy feature. And I am running RAID 1 instead of Synology hybrid RAID system. .