To understand the nature of consciousness, it is necessary to forego the notion that we are either conscious or unconscious. Some consciousness "stuff" does not suddenly get added to our other brain functions, and we are conscious. Rather, consciousness emerges gradually from the "mindlessness of a slug". In humans, neurologists recognize levels of consciousness from minimally conscious states to full consciousness. Let me illustrate this phenomenon with a couple of examples.
A famous case from the philosophical literature is that of the long distance truck driver. Think of yourself driving a truck at night for hundreds of miles along an interstate highway. Suddenly you realize that you have been driving all that distance without being aware of what you have been doing. You have no memory of the past hours. Yet during all those miles, you did not hit the median barrier, drive off the road at some unknown exit, nor did you fall asleep. But were you conscious? From a neurologist's point of view, you were conscious enough to manage successfully all the curves in the road but your level of consciousness was not high enough for full awareness of what you were doing.
Another example of levels of consciousness is a neurological condition called transient global amnesia. It is characterized by a memory disorder that can last for hours. Personal identification is intact and the patient's behavior looks normal except for incessant questions like, "How did I get here?" These patients are always capable of high-level intellectual and motor activity like driving safely, but like a long distance truck driver, they have no awareness or memory of what they have been doing.
The importance of these cases is that they lead us away from the idea that consciousness is like a switch, either on or off. They guide us toward the possibility that consciousness is nothing more than the brain's ability to gain information plus the amount of content that information contains. In the two examples above, the content was sufficient to drive long distances successfully but not sufficient for complete awareness of what was going on. Consciousness, therefore, could be considered at wakefulness and content, with the amount of content defining the level of consciousness.