Use the forum search feature, magnifying glass icon upper right. There are a number of past discussions on the sleep issue with OS5 both in this subforum and in others. See if there is a solution to the issue in one of those past discussions.
It should be noted that the OS3 firmware likewise had sleep issues and in that case one typically had to disable services (Twonky/DLNA media server, iTunes, etc.) in the My Cloud Dashboard and or use SSH to disable various firmware modules. For example, many past discussions on the OS3 sleep issue in the single bay My Cloud subforum: https://community.wd.com/search?q=sleep%20category%3A105
I will take a look now, trying to stay away from using SSH never used it before scared if I mess something up and also looks very confusing to me. Twonky/DLNA media server, iTunes, etc are disabled within the My Cloud Dashboard.
User Fox_Exe has some directions that should work to roll back a single bay v2.x My Cloud back to OS3. One can try keeping their data and bypass the mkpart step(s) that involve creating and formatting the partitions.
Took the 4TB drive out of the My Cloud a couple of weeks ago now am having zero issues what so ever.
The main problem was the My Cloud Unit or My Cloud OS after taking the drive out and putting it a Linux NAS or HTPC I have faster speeds up to 120 MB/s, the drive goes to sleep as it should do, I can also shut down the server without un plugging it.
So yeah not buying another WD NAS device,only the drive in future.