Your brain's visual centers remain active when your eyes are closed and even when you sleep. But in both situations, although electrical activity continues in your brain, the activity is represented by slow electrical fluctuations, rather than the bursts of activity that occur when you're awake with eyes wide open.
The slow fluctuation pattern can be compared to a computer screensaver, according to researchers. Though the function of this activity is unclear, there are a few possibilities. It is possible that neuron survival requires a constant state of activity, or perhaps the minimal level of activity enables a quick start when an outside stimulus is presented.
These new ideas differ starkly from older models of the brain, which posited that your senses are “turned on” by the switch of an outside stimulus, rather than being constantly active.
Live Science February 4, 2009