I found this paper by Benington and Heller that expands on the previously mentioned theory of sleep as a mechanism to renew metabolism. They hypothesise that sleep is necessary to replenish glycogen stores (mainly within astrocytes) in the brain. These stores are normally used to supplement blood glucose due to the high energy demands of the brain.
It is suggested that this may also result in the manifestation of feeling sleepy as a consequence of the exhaustion of glycogen supplies in specific small areas of the brain. Brief and localised depletion of glycogen stores mean that cells are operating with less energy than they normally have to work with. This causes an increase in synthesis of adenosine from the breakdown of AMP. The paper maintains that the increased levels of adenosine are detected by adenosine receptors which then triggers or increases (as seen on EEG scans) the feeling of the need for sleep:
In NREM sleep (which may lend some support to the theory as NREM sleep accounts for 80% of sleep and is most physiologically different to waking), this glycogen is most efficiently replaced. During NREM sleep the release of glycogenolysis inducing neurotransmitters is reduced, allowing glycogen-synthase to predominate and glycogen levels to be restored. However, these same neurotransmitters are key in the processing sensory stimuli (by tonically depolarising neurons in the sensory cortex). Therefore glycogen replenishment will always be associated with a (strongly) reduced response to stimuli.
This leads to finally answering the question as to why sleep has an evolutionary advantage, which I will quote verbatim to maintain the authors momentum:
Glycogen replenishment during waking would be maladaptive because it would impair the organism's ability to to process and respond to sensory stimuli. Sleep has therefore evolved as a state where animals retreat to a safe environment, behaviour is suppressed and glycogen stores are replenished.
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