I am so sorry that Cleo is poorly, we have fingers, paws, everything crossed here that she gets over this. Thinking of you, I know it is so hard.
Our vets have someone there 24 hours. The surgery has a flat above so even if there are no animals in overnight, there is always a nurse there in case an emergency comes in during the night. It did help when my last cocker, Sophie, was very poorly and had to stay there that there was someone with her all the time.
I remember looking at her chart which was attached to her cage and there were entries on it for each hour through the night and day.
Really hoping and praying that Cleo gets over this. I know what you are going through and my heart goes out to you