Fasting for long hours can clear the mind and enhance productivity. Is this a placebo, or is there actually something to it? In this article I share some of my findings and as it turns out fasting really does seem to boost brain power. Research in both animals and humans has identified a potential role for intermittent fasting for better cognition, and collectively, there are many reasons why people feel more productive, energetic, and clear-minded during a fast.

To start, humans are designed to thrive in a fasted state. Our hunter-gatherer ancestors needed to be sharp to track down food sources during periods of deprivation. Only after finding food could we truly afford to calm down and rest. Today, we spend most of our time in the latter “fed” state, since food availability isn’t exactly our issue. However, through voluntarily fasting, we can mimic these states of food shortage and take advantage of the cognitive benefits that ensue. The difference of course is that we can steer this mental energy towards our goals and ambitions, rather than finding our next meal. Whether it be short-term intermittent fasts, or prolonged multi-day fasts, being in the fasted state can be a way to fire up the brain.

By considering what happens when we eat, we can start to see why fasting keeps us mentally sharp, and why eating can slow us down. When food enters the system, blood flow is redistributed to the organs that participate in digestion, such as the stomach, intestines, liver, pancreas, and gallbladder. Food intake has further been associated with less blood flow to the brain. Since blood flow is responsible for delivering oxygen, fuel, and nutrients to the brain, when this is impaired, cognition is dulled and mental clarity may be lost.

Blood flow isn’t the only contributor, though. A meal also causes a number of hormonal changes that alter the central nervous system in a way that transitions the body into “rest and digest” mnode, and as a result make us feel less alert. So, while the body turns its attention to the incoming food, mental clarity and cognitive function may be temporarily blunted. This is one explanation for why we can often feel tired and sleepy after eating.

Now let’s consider what happens when we fast. Everything mentioned above in response to food works somewhat in reverse in a fasted state. Blood flow to the brain increases, delivering the materials it needs to function optimally, and the hormonal changes induced by fasting cause us to feel more alert.

Metabolically speaking, fasting causes profound changes in the brain. In the fed state (unless following a ketogenic diet), the brain’s primary fuel is glucose (sugar). Fasting depletes the body’s glucose stores (liver glycogen), which you would think İs bad for the brain since the brain requires a steady flow of fuel. However, we have a pretty cool back up system where the liver transforms fatty acids into molecules called ketones, which can largely fill the gap. This was our bodies way of prolonging survival when food was scarce for two main reasons. One: to maintain a constant flow of energy to the brain, and two: to spare muscle from becoming a major source of glucose.

The general scientific consensus is that the brain is very fond of ketones. In fact, fasting for ketosis was used in the early 1900’s to prevent seizures in epilepsy patients, and is part of the fascinating history of the ketogenic diet. Ketones have such a major impact on the brain that they are even being explored as a therapy for Alzheimer’s disease, traumatic brain injury, and brain cancer to name a few.

Compared to glucose, ketones are often touted as a more efficient fuel for the brain, with the potential to generate more cellular energy (ATP). Ketones can also upregulate the expression of particular genes and proteins that support our mitochondria and enhance energy production. If we want our brain to perform at its best, it needs the energy to do so. These effects might also explain why in animal models, fasting can reduce oxidative stress in the brain, which is a major culprit of brain damage and potentially learning and memory impairment as well.

Reaching ketosis through fasting unlocks a large reservoir of steady fuel - body fat and ketones - while stabilizing blood sugar levels. Brain function can fluctuate greatly in response to fluctuations in glucose caused by eating frequently, especially foods high in refined carbohydrates. Fasting and ketosis can prevent these fluctuations by normalizing glucose levels, and removing the brain’s dependence on sugar for energy. With that said, it’s possible that fasting beginners may experience a short transition of low energy and brain fog as your body becomes better adapted to running on fat and ketones.

Fasting has also been shown to increase brain-derived Neurotrophic factor (BDNF). possibly as a result of ketones. BDNF is like fertilizer for the brain, helping brain cells communicate, protecting neurons from dying, and promoting the growth of new ones. In other words, it helps build the brain and prevent its deterioration. It plays a vital role in how the brain performs, both short-term and long-term. So these effects can be long-lasting, and carry over into the fed state and throughout life.

It’s not all about ketones, though. Even practically speaking, fasting is a pretty effective productivity hack. Think about all the time and distraction that goes into preparing a meal, from hitting the shops to washing dishes, and everything in between. A lot of people find that fasting frees up new hours of the day where they can focus their attention on more pressing areas of their life. Plus without having to break up your day to eat, your workflow is spared from disruption. Whether you’re a morning lark or a night owl, you can position your fast to make use of your most productive hours without having to worry about food.

So, if you are building a fasting habit with the goal of more energy, clarity, and productivity, know that there’s plenty of science to back up your choice.