A severely hypothermic victim will become unconscious as a result of significant brain cooling; this is not going to sleep and is an inevitable progression towards death.

"All the photos in this article were taken in northern Sweden, well inside the Arctic Circle. I slept out in the shelter depicted in night-time temperatures that hovered between minus 20 and minus 30 degrees Celsius. This is colder than inside a domestic freezer. I did so without the aid of any sleeping equipment – no sleeping bag, bivvy bag, blanket or the like. Moreover, I was not at all concerned about freezing to death and I did get a reasonable amount of sleep...

...There is a common myth that you will not wake up if you get too cold. I can assure you that as soon as the fire dies down a little and you start to feel the chill of the night, you will be awake."