I cannot be 100% certain that sleep is not fatal. If I had some safe and reliable means of preventing sleep I would take it without hesitation. But it seems plausible that a person could survive sleep because it's a gradual process and one that everybody has a lot of practice doing. However, there are no such mitigating factors with general anesthetics. I will refuse general anesthetics if I am ever in a situation to do so. I believe a combination of muscle relaxants and opioids can serve the same medical purpose, which I do not believe would kill the person.