Dell XPS 15 W10 Shuts down instead of sleeping

Hi,
My Dell XPS 15 has developed the annoying habit of shutting down instead of sleeping.
It will go off after set time (Sleep after 45min). When I restart it, instead of just resuming, it begins from scratch with the DELL logo and I have to log-in with my password.
I have updated the BIOS and all other updates and windows updates are done.
Any clues as to what is causing it to ignore the setting (Sleep after 45 min) and shutdown instead?
Thanks
Bin

Well, just reading about this and one comment suggest that it is blue-screening, not shutting down. This fits as when I restart, it offers me any open Excel files etc. saved due improper shutdown.
So, slightly different issue, blue-screening not shutting down when all I want is for it to sleep!

Following another suggestion, I have set Sleep to Never / never and set Hibernation to 15 min / 45 min.....