Scientists at Washington State University have come up with a new theory suggesting that people feel sleepy when parts of their brains are actually asleep.
The researchers also contradict the suggestion that there is a control centre in the brain the dictates when it is time for one to drift off to dreamland.
Instead, they suggest that sleepiness results when independent groups of brain cells become fatigued and switch into a sleep state, even when an individual is awake.
Consequently, the researchers add, a threshold number of groups switch and people doze off.
Lead author James Krueger says that the view of sleep as an "emergent property" explains familiar experiences that the top-down model doesn't, such as sleepwalking, in which a person is able to navigate around objects while being unconscious, and sleep inertia, the sluggishness we feel upon waking up in the morning.
"If you explain it in terms of bits and pieces of the brain, instead of a top-down phenomenon, all of a sudden you can make sense of these things. The old paradigm doesn't even address these things," Nature magazine quoted Krueger as saying.
Krueger insists if sleep were being directed by a control centre, the whole brain would respond at the same time.
However, he adds, the brain behaves like a self-directing orchestra in which most sections are more-or-less in sync, but a few race ahead or lag behind at any given time.
According to him, when a person is sleepwalking, the neuronal groups needed for balance are in a wake state while those needed for consciousness are in a sleep state.
On the other hand, in sleep inertia, enough neuronal groups are in a wake state for one to be awake in a general sense, but some groups are still in a sleep state-enough to hamper one's ability to perform tasks.
"Everybody has sleep inertia every morning. It takes 30 minutes to an hour to recuperate from being asleep" and get all your neuronal groups up and running," said Krueger.
The report on the theory proposed by the Washington State University researchers has been published in Nature Reviews and Neuroscience.