At the end of 2015, Geoffrey Woo was preparing to embark on his first experiment with radical intermittent fasting: a 60-hour stint, from Sunday night to Wednesday morning.

“At first I thought, is this even possible?” says Woo, co-founder of a San Francisco-based company called HVMN, which sells nootropic supplements the company says can “keep your brain sharp for the long term.”

“But I saw some of my co-workers doing it successfully, saying they were more clear and more productive.”

Woo didn’t begin fasting only for the reasons science tentatively recommends: to live longer or to lose weight. He was just as thin several years ago as he is today at age 30. What he was gunning for was a cognitive hack. “The plan was longevity and the cognitive benefit,” he says. Woo wanted to clear the “brain fog,” as it’s colloquially called, and optimize his mental agility.

“The first two times I tried to fast it was terrible. It was hard,” he says. “By the third time, it felt pretty refreshing. I felt pretty clear.”

Woo now cuts out all food for 24 hours once or twice a week. Every three months, he goes 36 to 72 hours without a meal. He justifies his cleanses based on several years of tracking his own blood sugar with a continuous glucose monitor he wears on his arm. While fasting, Woo says, his blood sugar remains steady throughout the day, instead of spiking and crashing after a big lunch.

As a Silicon Valley entrepreneur, Woo is perhaps the ideal guinea pig for this sort of diet experimentation. He’s part of an ever-growing movement of biohackers finagling with nutrition in service of the one part of our anatomy that no one ever sees: the brain.

Eating for brain energy and focus has become one of the central underpinnings of a series of diet trends growing in popularity. There’s intermittent fasting; the low-carb and high-fat ketogenic diet; the MIND diet, which combines the Mediterranean diet and the heart-healthy Dietary Approaches to Stop Hypertension (DASH) diet; and the Bulletproof Coffee diet, founded by biohacker Dave Asprey, which prescribes dunking a pat of butter into your morning brew for better energy and cognitive function. The diets have an especially notable following among technocrats like Woo. Twitter CEO Jack Dorsey says he does a daily 22-hour fast, and Kevin Rose, the co-founder of Digg, launched an app called Zero to help people track daily fasting.

“If somebody really feels like it works for them, honestly, you can’t argue with that.”

Data suggests that lifestyle behaviors, including diet, can play an important role in keeping the brain healthy and preventing cognitive decline, though dietitians and researchers caution against using small studies of unique eating trends to inform an entirely new food regimen. “More now than ever, people are looking for a quick fix,” says Marjorie Cohn, a registered dietician and spokesperson for the Academy of Nutrition and Dietetics. “I think a lot of what explains it is our access to information and social media. A lot of these diet hacks are being promoted and circulated not from professionals, but from nonprofessionals promoting nonprofessionals.”

Even so, eating patterns like occasional fasting or eating mostly fats and protein—while perhaps unnecessarily strict—may demonstrably improve a person’s reported energy and focus. Whether they actually change the brain is still being worked out. Meanwhile, the Woos of the world are eagerly adapting to new ways of eating before the results are clear. “Who did the randomized controlled trial on the FDA food pyramid?” says Woo.