How the brain switches between sleep and wakefulness is an open question, but in the worm Caenorhabditis elegans it seems the process occurs passively.
Manuel Zimmer and his colleagues at the Research Institute of Molecular Pathology in Vienna used a new calcium-imaging technology to simultaneously view the activity of most of the worm’s brain cells during a particular stage of larval development. During this stage the creatures are prone to falling asleep — providing oxygen levels remain as low as those in their normal soil environment.
When the scientists lowered oxygen levels, around three-quarters of the neurons became silent. Those not silenced included neurons responsible for monitoring alarming environmental signals such as high oxygen levels, and these caused the animals to awaken when the scientists raised oxygen levels.
This indicates that sleep is an emergent property of neuronal circuits, rather than an activity strictly enforced by specific brain areas, as others have suggested.