Scientists believe they have discovered a new reason why we need to sleep - it replenishes a type of brain cell.
Sleep ramps up the production of cells that go on to make an insulating material known as myelin which protects our brain's circuitry.
The findings, so far in mice, could lead to insights about sleep's role in brain repair and growth as well as the disease MS, says the Wisconsin team.
The work is in the Journal of Neuroscience.
Dr Chiara Cirelli and colleagues from the University of Wisconsin found that the production rate of the myelin making cells, immature oligodendrocytes, doubled as mice slept.
The increase was most marked during the type of sleep that is associated with dreaming - REM or rapid eye movement sleep - and was driven by genes.
In contrast, the genes involved in cell death and stress responses were turned on when the mice were forced to stay awake.
Precisely why we need to sleep has baffled scientists for centuries. It's obvious that we need to sleep to feel rested and for our mind to function well - but the biological processes that go on as we slumber have only started to be uncovered relatively recently.