This bit of work got a flurry of attention recently, but I think is important enough to warrant repeating here. Xie et al. have used an elegant two-photon imaging technique to compare awake and sleeping mouse brains. They find that metabolic waste products of neural activity are cleared out of the sleeping brain at a faster rate than during the awake state:
...convective fluxes of interstitial fluid increased the rate of β-amyloid clearance during sleep. Thus, the restorative function of sleep may be a consequence of the enhanced removal of potentially neurotoxic waste products that accumulate in the awake central nervous system.The review of this work by Underwood has a nice graphic of the fluid-filled channels (pale blue) that expand to flush out waste:
The role of sleep in memory consolidation is well known, and further work of Tononi's group has suggested that in rats, sleep maintains an overall synaptic balance, by uniformly dialing down synapses that have expanded their activity during the day. (I have also previously pointed to work on fruit flies by Tononi's group coming to a similar conclusion.)
Work of this sort suggests that the 50-70 million Americans who have insufficient sleep or some kind of sleep disorder (like sleep apnea) are carrying around extra garbage in their brains during the day and have brain synaptic connection that haven't recovered from previous days' activities, both factors that would seem likely to compromise our mental function!