When you say sleep mode do you mean suspend or hibernation?
If you mean hibernation, then this can’t work without sufficient swap space configured, and you have none:
If you meant suspend, then this has no direct relevance, except that swap space should normally be configured even if it’s rarely used.
Indeed, that isn’t ideal. Here is a safer way:
Have you checked the Known Issues and Solutions section in recent Update announcements? Often there might be something related in those items.
Regards.