Cells in the brain, probably the glial cells which keep nerve cells alive, shrink during sleep. This increases the size of the interstitial space, the gaps between brain tissue, allowing more fluid to be pumped in and wash the toxins away.
Dr Nedergaard said this was a "vital" function for staying alive, but did not appear to be possible while the mind was awake.
She told the BBC: "This is purely speculation, but it looks like the brain is losing a lot of energy when pumping water across the brain and that is probably incompatible with processing information."
She added that the true significance of the findings would be known only after human studies, but doing similar experiments in an MRI machine would be relatively easy.