Our ability to measure things about the brain exceeds our understanding of how the brain makes the mind.
Maybe that's why that, at the Human Brain Mapping meeting in San Francisco this week, several groups are presenting work on sleep (example; example). Mostly, they're not studying sleep, per se. Rather, they're trying to exploit different states of mind, in order to figure out what some functional MRI brain signals might mean. The idea, roughly, is that because sleep is not consciousness, if something we measure looks the same in sleep and waking, then that something cannot reflect consciousness or mindfulness.
Of course, it's not quite that simple, because sleep is not just one thing; there are different kinds of sleep. And while slow-wave sleep is very different from the waking state, in REM sleep (also known as paradoxical sleep), brain activity is actually very much like that in the waking state. Or, as Nathanael West wrote in The Day of the Locust (1933):
His thoughts frightened him and he bolted into the house, hoping to leave them behind like a hat. He ran into his bedroom and threw himself down on the bed. He was simple enough to believe that people don't think while asleep.