There is some evidence to indicate that prolonged lack of sleep in mice may result in 25% permanent loss of certain brain cells.
Briggs, H. (2014). Lost sleep leads to loss of brain cells, study suggests. London: BBC Health News, March 19th 2014.
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Zhang, J. Zhu, Y. [and] Zhan, G. [et al] (2014). Extended wakefulness: compromised metabolics in and degeneration of locus ceruleus neurons. The Journal of Neuroscience: the official journal of the Society for Neuroscience, March 19th 2014, Vol.34(12), pp.4418-31. (Click here to view the PubMed abstract).