Yes — completely normal, and it has a clear physiological explanation. When you fast for an extended period, your brain switches to running on ketones, BDNF rises, norepinephrine surges, and inflammation drops. Together, these produce a distinct sense of mental clarity, calm energy, and sometimes genuine euphoria. Experienced fasters call it "the fasting high," and even Upton Sinclair described it in 1911 — "I read and wrote more than I had dared to do for years before." You are not imagining it.
When glycogen (stored glucose) runs low during a fast, your liver begins converting stored fat into ketones. Beta-hydroxybutyrate (BHB) — the main ketone — has been shown to increase GABA activity in the brain. GABA is the brain's primary calming neurotransmitter, the same one targeted by anti-anxiety medications. This explains why many fasters feel not just sharp but calm and untroubled — the brain is bathing in a natural GABA-boosting fuel.
Ketones also produce less oxidative waste during metabolism than glucose. The result is a cleaner form of neurological energy with fewer of the metabolic byproducts that contribute to brain fog.
BDNF — Brain-Derived Neurotrophic Factor — is sometimes described as fertiliser for the brain. It promotes the growth of new neurons, strengthens synaptic connections, and elevates mood. Fasting is one of the most reliable ways to raise BDNF: research measuring BDNF during structured fasting has shown significant increases that build progressively over days.
Higher BDNF is associated with better memory, faster thinking, stronger mood stability, and reduced risk of depression. The sense of ideas connecting easily, of being sharper than usual, of feeling almost electrically awake — these are the subjective experience of elevated BDNF at work.
Norepinephrine (noradrenaline) rises significantly during fasting as the body mobilises fat for energy. It is the same neurotransmitter elevated by caffeine — except the fasting-induced version is more sustained and doesn't come with a crash. Elevated norepinephrine sharpens attention, heightens awareness, and produces that characteristic sense of being "switched on" and energised without anxiety.
This norepinephrine surge also increases metabolic rate slightly — another reason experienced fasters feel they have more energy during fasting than when eating constantly.
Chronic low-grade inflammation — common in people eating a high-carbohydrate, high-sugar diet — impairs brain function in subtle but persistent ways. It contributes to brain fog, slowed processing, and low mood. As insulin drops during fasting and the body shifts away from glucose, inflammatory markers typically decline over the following hours.
The clearing of inflammation is what many fasters describe as "the fog lifting." Thoughts come more easily, the physical sense of heaviness disappears, and the world looks a little sharper. This is not imagination — it is measurable neurological improvement.
Some researchers have proposed that extended fasting triggers a mild endorphin response, similar to the well-documented "runner's high." Endorphins are the brain's natural painkilling, mood-elevating compounds. The subjective experience of fasters — the euphoria, the reduced sensitivity to discomfort, the sense of inner peace — is consistent with endorphin involvement, though this pathway is less studied than the ketone and norepinephrine ones.
Most people report the euphoric state appearing between hours 18 and 36 of a fast. The first 12–24 hours can feel uncomfortable — hunger, some irritability, possibly a headache as the body transitions. Once the body fully shifts to ketosis — typically by the end of day one, and faster if you are eating low-carb — the mood shift often happens suddenly and can feel surprising if you're not expecting it.
No — unless the euphoria is accompanied by confusion, disorientation, or rapid heart rate, which would suggest low blood sugar in someone with diabetes or another medical condition. For a healthy person, the fasting high is a sign the body is running cleanly and efficiently. It is a positive adaptation, not a warning.
The only practical note: the elevated mental state during deep fasting can make it tempting to overestimate your physical abilities. Exercise capacity may actually be slightly reduced even while mental performance is elevated, especially if electrolytes are low.
Want to learn more? Read our full article: How intermittent fasting promotes autophagy
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This is for informational purposes only and is not medical advice.