Neurological benefits of fasting
Many people report improved cognitive function while fasting but now we have some evidence of how that might happen. Autophagy, the ability of cells to renew themselves by by consuming then replacing damaged parts, is especially active in the brain during fasting.
The attached paper demonstrates in mice what we have observed for millennia in humans. Fasting makes you more alert, and improves the general health of your nervous system. I await evidence that it can help heal my neuropathy, an otherwise untreatable deadening of the peripheral nerves.
https://www.tandfonline.com/doi/full/10.4161/auto.6.6.12376
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I know that my own 18 hour fasting about 5 days per week gives me plenty of time an focus/energy. Eating does end all that with sleepiness and lethargy. I'd guess that at least part of that effect is diverting energy and bloodflow to digestive processes.
Could be, a significant percentage of our calories go into digesting food. You probably aren't eating sugar or high carbs, so it's not that.