suspend – Ubuntu 18.04 restart my Lenovo Yoga 920

I am under Ubuntu 18.04.2 LTS on a Lenovo Yoga 920. Everything is fine except that when I go back to my laptop (quite regularly?), I discover that it has restarted. I guess this happens from time to time when the laptop goes into sleep mode, although by definition the problem has never occurred during the restart.

Does any one know what could be the reason? I know it's missing some details, but I do not really know what other information to include.

Here is the result of uname -a:

Linux cannon-yoga 4.15.0-45-generic # 48-Ubuntu SMP Tue, 29 Jan 16:28:13 UTC 2019 x86_64 x86_64 x86_64 GNU / Linux