During REM specifically, very high, many times even higher than typical for that person actually.<quoted text>
I think God is always at work and don't think we are semi-dead or lifeless when asleep. Many REM studies reflect high levels of brain activity.
During this time there are several events taking place that require the full potential of the brain itself. The first is pretty mundane but vital, the brain is organizing recuperation, and the body does the vast majority of it's healing at this time as well. Ever notice how you never actually see any wound heal, like as in see the last moments when the skin mends?
The other part is where the dreams themselves come from. The brain is "compiling" the memories it contains, compressing bits and discarding other bits, since the access to the sensory neurons is random at this state, the brain mixes up the sensory input with the memories. It's much the same effect as a hallucination but a lot lighter and looser. It's the most fascinating part of the life cycle of brains that it has captured the attention of anyone who delves into it. The brain's memory storage is limited, very limited, and since we are not well adapted to living more than 30 years it has to compile even more than it once did. This is why we often get de ja vu or forget why we walked into a room, if the event or thought is similar to one that's already stored, the brain forgets the new one and replays the old one to help conserve memory space. You can't grow new neurons, so it's vital. It's also why if we don't stimulate the brain enough with new things the older we get, the more we forget. Those who suffer insomnia will often forget things very easily if they do not remain highly active mentally, because until you enter REM the real compression cannot take place, so the brain just starts discarding what it thinks is not important.