This article comes from one of my weekly Neurofeedback & Chill livestreams, where I ran a short neurofeedback session on myself and then walked through the science of fasting as a biohacking tool. I have stripped out the audience names and kept the substance. If you want the fully cited deep dive, I also wrote a companion piece: Strategic Fasting: Time-Restricted Eating for Metabolic and Cognitive Health.
I am going to talk about energy timing: when you eat, how much, and how you can use periods of not eating to shift your metabolism and your brain.
What Counts as Fasting, and What Doesn't
There is an alphabet soup of fasting terms floating around the biohacking world. Here is what they actually mean.
Intermittent fasting and time-restricted feeding (TRF) both describe a window where you eat and a window where you don't. The standard starting point is 16:8, sixteen hours fasting and an eight-hour eating window. Early time-restricted feeding (eTRF), where you stop eating earlier in the day, has better research support than late TRF. In a controlled crossover study, eTRF improved insulin sensitivity and blood pressure even without weight loss (Sutton et al., 2018). Eating your last meal late at night still produces some benefit over a no-fasting baseline, but you lose the metabolic edge of the early version.
Then you get the longer protocols. OMAD is one meal a day. Alternate-day fasting cycles a fasting day with an eating day. A "monk's fast" runs about 36 hours, dinner one night to breakfast a day and a half later. Extended or prolonged fasting goes past 24 hours into one or two full days. Water fasting and dry fasting sit at the aggressive end.
My line on what fasting is: if you are taking in energy, you are not fasting. Butter in your coffee is a meal with bad branding. Fat alone does trigger digestion and energy metabolism, even though it requires minimal insulin. Anything above about 30 calories will produce an insulin response. Black coffee, plain tea, water, spices, herbs: those are fine.
One more wrinkle worth knowing. Tasting something sweet, even a zero-calorie sweetener, can trigger a cephalic insulin response. The pancreas produces a small direct insulin signal in response to the sweet taste itself. That insulin rise and subsequent fall can generate the sensation of wanting to snack. This is consistent with basic metabolic physiology, and it is why a sugar-tasting zero-calorie drink can quietly erode a fasting protocol.
What Fasting Actually Does in the Body and Brain
Three levers run almost every fasting protocol: the volume of calories, the timing of when you eat, and the macronutrient composition. Different approaches emphasize different levers. Keto focuses on composition. OMAD focuses on timing. Caloric restriction focuses on volume.
When you fast, blood glucose drops and insulin falls. Around 10 to 14 hours in, the liver shifts toward producing ketones, and counterregulatory hormones (adrenaline and cortisol) rise modestly to defend blood sugar. That metabolic switch from glucose to fatty-acid-derived ketones is well described in the fasting literature (Anton et al., 2018). That combination of climbing ketones and a small adrenaline bump produces a focused, slightly elevated state. Many people who fast regularly describe sitting down at hour 12 to 14 and doing their best creative or strategic work. You are running on your own ketones, and the cognitive lift is real once your body has adapted.
Ketones do more than fuel the brain. High ketone levels are anti-inflammatory and they suppress seizures, which is why ketogenic diets have been used for decades to help children with epilepsy who can't tolerate medication (Neal et al., 2008). This matters beyond classic seizure disorders. Some research suggests subclinical epileptiform activity can appear across conditions you would not normally file under epilepsy. For a subset of people, fasting or deeper ketosis may quiet that activity. That is a stricter intervention than what you would use for general brain health, so treat it as a research consideration, not a casual recommendation.
On the metabolic side, fasting improves insulin sensitivity and gives the body a break from constant insulin signaling. Caloric restriction is among the most reliable longevity interventions across animal models in the research literature (Fontana et al., 2010). The brain's glucose handling matters here too. When neurons lose the ability to use glucose well despite normal blood sugar, you get the kind of metabolic brain fog and accelerated aging sometimes described informally as "type 3 diabetes" (de la Monte & Wands, 2008). Periodic fasting and ketone availability give those neurons an alternate fuel source.
How Do You Start Fasting Safely?
Start with a 12-hour fast. Stop eating after 8 pm, don't eat before 8 am. If that sounds hard, that is a sign you should probably do it.
Once 12 hours feels easy, move to 16:8 or 18:6 and track how it feels. Fasting gets dramatically easier with practice. The first few weeks are genuinely difficult; after that it becomes unremarkable. Run the simple version for several weeks before escalating.
After 16:8 feels unconstraining, try a 36-hour monk's fast once a week or once every couple of weeks. Then, every few months, you might try a 48- or 72-hour fast. I went through an alternate-day fasting phase around 2018 while teaching a course on the neuroscience of aging at UCLA. Paired with adequate protein and exercise, I got into the best shape of my life and preserved my muscle because protein intake stayed high. Later I cycled 22-hour, 44-hour, and 66-hour fasts with matching eating windows, and my body responded to that variation quickly.
A few cautions that matter.
Sex and age change the math. Men tolerate aggressive fasting better. Women, especially under roughly 40 to 45, can crash their metabolism with hard fasting. The 14:10 protocol that many women-focused practitioners recommend is mostly about getting sufficient calories without restriction.
Avoid staying in a caloric deficit week after week. Treat fasting as a tool, not as chronic calorie cutting. The body adapts to a sustained deficit by lowering energy expenditure and pushing you toward being sedentary, which is the rebound trap of old-school dieting. Look at your calories across a one- or two-week window, aim for break-even or a slight surplus, and build your fasts into that window. You get the timing and ketone benefits without forcing a long-term deficit.
If your body carries a specific genetic or disease vulnerability, swinging hard in and out of fasting can trigger problems. I have a genetic condition that produces lipomas, and people in those communities have noticed proliferation when they go aggressively in and out of ketosis. Go slowly and listen.
Refeeding: The Part That Goes Wrong Fast
Breaking a fast carelessly is where people get hurt. After fasts longer than 24 hours, refeeding syndrome is a real risk. A large meal, too many carbs, too much electrolyte load, or any alcohol at the start can shift the osmotic volume of your tissues so abruptly that you become delirious or worse (Mehanna et al., 2008). This can become life-threatening quickly.
Ease back in. Start with broth and some fruit. Let yourself digest. Hold your first real carbohydrates until after your next bout of exercise, because the blood sugar will get pulled straight into muscle that is glycogen-depleted. Increase calories gradually over a couple of days back to normal eating. The "I fasted 48 hours so now it's pizza and ice cream" approach does strange things to your gut and metabolism.
Can You Combine Fasting With Neurofeedback and Other Biohacks?
Yes, and this is where it gets interesting for brain training specifically.
Fasting is a hormetic stressor, a controlled dose of stress that upregulates repair and adaptation. So are sauna, cold exposure, hyperbaric oxygen, and red light. When you add a hormetic stressor that supports metabolism and blood flow on top of neurofeedback, the research picture suggests you may raise plasticity beyond what neurofeedback produces alone. Meditation does the same thing. Stacking blood-flow and metabolic support, meditation, and neurofeedback together is a plausible way to drive large, building, plasticity-driven change.
I see this most clearly with people recovering from a metabolic insult: post-viral fatigue, concussion, hypoxic injury, residual effects from mold or chemotherapy-related brain fog. If the inflammatory event is mostly in the past and what remains is a metabolic drag, adding hormetic support appears to speed and deepen the response to brain training. I lean on HEG (hemoencephalography), a form of biofeedback that trains cerebral blood flow, as a primary tool here, but photobiomodulation, sauna, and fasting all fit. For the deepest plasticity, I would not fast a child or a cognitively impaired adult without real consideration of agency, but for someone who can manage their own nutrition, fasting is a legitimate addition.
If you want the broader plasticity picture, I cover it in Biohacking Plasticity: Unlock Your Brain's Adaptive Potential.
A Note From the Neurofeedback Session
Before I went through the fasting material, I ran a short single-channel session on myself, training at C4. I set an inhibit on theta (4 to 7 Hz), a reward on SMR/low beta (11.75 to 14.75 Hz), and a high inhibit on faster beta (22 to 34 Hz). The principle is straightforward: when all three bands sit in range for about half a second, the brain earns a reward, a beep and a visual cue. The brain figures out within minutes that something in the environment is reflecting it, but it takes a few sessions before that turns into change you feel.
Someone asked how often you need to train. Once a week generally does not produce reliable cumulative change. Joel Lubar's work on SMR training in the management of epilepsy and attention is part of the foundation here (Lubar & Shouse, 1976), and in practice three times a week tends to be substantially more impactful per unit of effort than twice a week, with four times a week only slightly better than three. We train three times a week in the office, and for home clients we push toward four times a week, often two days on, one day off, mixing protocols once benefits start diverging across sleep, attention, stress, and creativity. If you want the deeper mechanism on the SMR band specifically, see SMR Neurofeedback: Train Sleep, Focus, and Self-Control.
Reading Your Own Signals
As you adapt to fasting, the cravings and energy crashes fade and you get smooth energy. You start distinguishing real hunger from appetite. A simple test: if you would eat broccoli right now, it is hunger; if only the donut sounds good, it is a craving. Both hunger and cravings come and go in waves when you are fasting, and watching them pass is informative on its own. Sleep is often disrupted at first, then improves longer term.
The warning signs are the ones to respect. Energy that plummets, shakiness, an inability to recover from exercise, sleep that stays broken for several nights, severe irritability, dropping cognitive performance, or significant prolonged nausea all mean you have gone too far or your electrolytes are depleted. Keep salt, water, and magnesium on hand. Get real medical guidance before you run extended protocols, especially if you have an existing condition.
For tracking, I like Cronometer and the Zero fasting app. For measuring ketosis, breath acetone is the cleanest proxy I know of, because it sits late and downstream in metabolism and is not skewed by how well you burn ketones or how your kidneys retain them. The breath device I used most has unfortunately gone out of business, so I am hunting for a good acetone breath meter again.
Where Fasting Fits in a Bigger Plan
Match the protocol to the goal. General brain and metabolic health needs less than seizure suppression, which calls for deeper, stricter ketosis. Athletes building muscle want strategic carb refeeding, either eating carbs right after hard training or taking a 10-minute walk after meals so working muscle pulls blood sugar into tissue without a large insulin spike.
Protein comes first for me. Get enough protein, then roughly half as much fat, then half as much carbohydrate if you are trying to stay lean. Fasting is a powerful tool inside that frame.
If you want to keep building, fasting connects directly to several other pieces I have written about: Biohacking Your Morning for the circadian light and fasted-walk piece, Biohacking Sleep, and The Critical Aging Window for why metabolic health matters earlier than most people think.
Start with a 12-hour overnight fast tonight, track how it feels for a week, and add length only once the short version is easy.