Millions of people around the world have tried fasting as a way to lose weight. Intermittent fasting and the 5:2 diet are both science-approved ways of keeping your waistline in check.
Now a Silicon Valley start-up has implemented a weekly 36-hour starvation period for all employees - not to help them lose weight, but to improve their workplace focus and concentration.
Employees at Nootrobox, a smart drug company which produces cognitive enhancers called nootropics, don't eat from Monday night until Tuesday.
"We're actually super productive on Tuesdays," co-founder and CEO Geoffrey Woo told San Francisco's Mercury News. "It's hard at first, but we literally adopted it as part of the company culture."
Some of Mr Woo's colleagues are way more hardcore. "Some people don't eat for even longer. They fast on Sunday night, then breakfast on Wednesday morning," he told Inverse.
Many Silicon Valley workers are trying to "hack" their brains as they strive to gain an edge in the city's ever-competitive market.
When a great idea can earn you millions of dollars - apps like Uber, Snapchat and Tinder once started as tiny ideas - there's an extra incentive to have your brain always working at full capacity.
"It kind of sucks, in the beginning, to not eat for 36 hours," Mr Woo said. "But it's fun to get breakfast together on Wednesday. We realised that a lot of people in our community want to do that as well, so we started a biohacker breakfast.
"We have 300 people in a Slack channel, nerding about fasting and different fasting protocols. Every Wednesday at 8am in San Francisco, we have a biohacker breakfast."
As well as fasting and taking nootropics, some employees are also consuming prescription drugs and small doses of LSD, The Mercury News reports.
"Entrepreneurs and executives and investors are not normal people," said Dr. Molly Maloof, who works with lots of biohacking clients from top-tier Silicon Valley tech companies.
"They are like high-performance race cars that are non-stop moving, and they need pit stops more often than normal people."
Nootrobox sells four different nootropics in tablet form, as well as chewable caffeine lollies.
Rise is designed to "enhance memory, stamina and resilience", Kado-3 is an Omega-3 tablet, Sprint is for "immediate clarity, energy and flow" over four to six hours and Yawn is to be taken before bed to help with a good night's sleep.
All are designed to increase brain performance and boost energy and can be purchased online without a prescription from a doctor.
Professor Anthony Hannah from the Florey Institute of Neuroscience and Mental Health says there is little scientific evidence to support fasting or taking nootropics to boost brain functionality.
"There are some companies who are conducting clinical trials on these drugs to improve cognitive function, but they're mainly to treat neurological conditions like alzheimer's, dementia and schizophrenia," Hannan told news.com.au.
"They're not for healthy people who think they are just going to perform better."
Every drug has side effects and risks which need to be taken into account, Prof Hannan said.
"If you can buy them online, it would be extremely unlikely that there is any proper scientific evidence of cognitive enhancement, because it's very hard for these companies to pay for those studies. You could argue they wouldn't want to know the results of that research," he said.
"While there is certainly scientific evidence that fasting is beneficial for weight maintenance and overall health, the idea that you're going to perform better cognitively isn't really there."
Hannah says the best way to improve brain functionality is to maintain a healthy diet, exercise regularly, keep your brain stimulated, get eight hours of sleep a night and reduce your stress levels.