RE: What happens in our brain when we sleep?

Well, the reality is that it all depends on the cause of death. If it is a Cerebrovascular Disease (hemorrhagic or ischemic), for example, the brain will be the main and first affected, and after it stops functioning due to the damage produced by the hemorrhage or ischemia, and then the other organs will stop functioning.

There is no single answer to this question, since everything will depend on the cause of death, and there may be hundreds or thousands.