Yes, but our brain is still working and processing information at those times as well, isn't it? Even if not in the same way as it does when we're conscious.
Anesthesia shouldn't take your brain offline. It just makes you unconscious, paralyzes you, and gives you amnesia. Your brain is still active under general anesthesia. What you were thinking or feeling for those 8 hours was just forgotten.
You might try https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8054915/ which states: "General anesthesia is characterized by loss of consciousness, amnesia, analgesia, and immobility." and further down shows brain activity recorded while under anesthesia via EEG. The paper looks at the differences and similarities of brain activity while under anesthesia and sleep. This is only possible because, however changed or slowed by it, the brain is still active while under anesthesia
