Geoff Woo slurps egg yolks from his plate after breaking a 36-hour fast. Melia Robinson/Business Insider Geoff Woo says Tuesdays are his most productive days. He has the data to back it up, because he uses a software that tracks his time spent on email and coding.

His claim is somewhat surprising. Woo also doesn't eat on Tuesdays.

Woo, CEO and cofounder of a "cognitive-enhancement" supplement subscription service called Nootrobox, subscribes to an increasingly popular diet called intermittent fasting, which involves going without food for anywhere from 14 hours to several days. As part of his routine, Woo fasts for 36 hours straight at the start of each week.

"I thought it was impossible until I actually tried it," Woo tells Business Insider.

The science behind intermittent fasting is spotty. Most studies use rodents and fruit flies as test subjects, rather than primates and people, as Scientific American reports. Still, it's catching on among startup workers looking for ways to sharpen the mind and improve longevity.

When the body goes into fasting mode, it stops producing as many growth-related hormones and proteins, which are linked to cancer and diabetes. Instead, the body takes a little break to repair cells. This "maintenance state" may be the key to unlocking longer lives.

Research in animals from the Longevity Institute at the University of Southern California supports this idea. Mice that fasted for two to five days a month showed reduced biomarkers for diabetes, cancer, and heart disease, as well as a rejuvenated immune system.

Woo likens the body to a refrigerator that stores energy in the form of fat. When he fasts, something clicks that encourages his body to burn through the fat for fuel.

Monday night, Woo leaves the office and grabs a typical dinner from a Mediterranean restaurant or ramen shop. He says there's no need to binge. Then the fast commences.

Tuesday starts with a weight-lifting workout. Throughout the day, he fills up on water. Hydration is particularly important during a fast, since most water we consume comes from food, according to Woo.

He also takes dietary supplements manufactured by his startup, Nootrobox. The so-called "smart drugs" claim to make people feel better, smarter, and more alert.

He's not immune to hunger, however.

"Hunger pangs are normal and they come in waves," Woo says. "If I need to have a crutch, I'll have a black coffee" — a natural appetite suppressant — "or black tea."

As the hours whiz by, Woo says he feels a greater sense of focus, clarity, and "cognitive edge." He gave his diet a nickname for just this reason: "The monk's fast."

Wednesday morning, he breaks the fast in good company. Woo, his cofounder Michael Brandt, several of their employees, and a handful of fellow fasting enthusiasts meet for breakfast at an Italian restaurant near the office. Most attendees haven't eaten in 36 hours.

Members of the WeFast group break the fast with a meal together. Melia Robinson/Business Insider

Woo eases back into his diet with a low-glycemic meal that's high in healthy fats, like eggs with avocado and cheese. It helps preserve his fat-burning state for a few hours longer.

For the remainder of the week, Woo tries to keep his "feeding window" to eight hours a day. But he doesn't worry as much about what goes into his body. If pizza is on the menu, he's in.

Since he switched up his lifestyle about a year ago, Woo has become a fasting evangelist. He oversees a public chat room on Slack, called WeFast, where people can find the resources and support they need to be successful in their fasts. A majority of his employees follow his and Brandt's fasting routine, though the venture-backed company does not require it.

Woo says fasting is easier to maintain than a fad diet because you focus your discipline on a small, 36-hour window. There's no calorie counting or step-tracking involved.

"The idea here is that I intend to maintain this regime forever, until I die," Woo says.