Shocking simulation shows what happens to your body when you fast for 36 hours and the extreme impact

A viral 2025 clip that’s earned over 28 million views since publishing breaks down how a 36-hour fast changes the human body, but new guidance highlights an important warning.

Going without food for 36 hours might sound extreme, but fasting has been practised for centuries for religious and cultural reasons. Nowadays as well, though, intermittent fasting has become increasingly popular as a way of managing weight and better timing yours meals.

Restricting food to a six-to-eight-hour daily window to eating far less on selected days of the week, it can even be a straightforward way to ‘reverse aging’. As reported by outlets like those at Medical News Today, fasting can also be a fantastic go-to for controlling blood sugar levels and improving insulin sensitivity all in one.

This ever-increasing interest in modern times is what helped get so many eyes on the widely viewed clip from Wellness Wise, with the animated short breaking a 36-hour fast into stages — from the first few hours without food through to changes it says happen after more than a day, as the hours continue to climb.

However, the latest research stresses certain types of people for whom this practice isn’t suitable. Before we mention which, we’ll first go into the breakdown of 36-hour fasts specifically.

Your body goes through some mind-boggling changes if you don't eat food for 36 hours (YouTube/Wellness Wise)

What happens to your body during a 36-hour fast?

Four hours

In a 55-second video, posted to YouTube by Wellness Wise, it’s revealed that after four hours of fasting, your body 'stops digesting food, insulin drops, and it starts burning stored sugar for energy'.

According to popular practitioner Doctor Kiltz, this is the start of what is known as the ‘catabolic phase’.

On his website, Klitz Health, he says this period is ‘characterized by the breakdown of larger molecules of stored energy into smaller energy molecules which are mobilized to fuel your cells’.

Eight hours

When you haven't eaten for eight hours, this is the time when sugar starts to drop in your blood, and your body begins to use stored glycogen for energy instead of relying on new fuel coming in, as per the YouTube video.

12 hours

After 12 hours, your body is depleted of glucose, causing your liver to start breaking down fat into fatty acids called ketones — AKA, starting ketosis.

This process of turning ketones into food is known as ‘metabolic switching’ and, according to the BBC, is the reason why fasting can lead to weight loss.

Different things happen to your body the longer you abstain from food (Getty Stock Image)

16 hours

When you reach the 16th hour of a fast, a cellular process known as autophagy takes place. As per the Cleveland Clinic, this allows the body to disassemble its damaged cells and toxins — repurposing the salvageable bits and pieces into new, usable cells.

As the Wellness Wise video explains, this is essentially 'recycling'. Dr Hiltz explains that exercise and resistance training also help the autophagy process because if it gets disrupted, it can cause 'problems associated with abnormal cell growth'.

24 hours

A full day without food is a pretty long time for most, but this 24-hour mark is where major cellular repair takes place in the human body.

As also explained in the YouTube video, your body is fully in fat-burning mode, reducing inflammation and improving insulin sensitivity.

30 hours

The growth hormone spikes 30 hours after fasting, which reportedly helps to preserve muscle and promote fat loss.

Wellness Wise also states that this is the period where ‘deep healing’ begins.

36 hours

If you fast for 36 hours, then you’ve reached the stage of maximum autophagy, where your body regenerates tissues and boosts your metabolism after clearing out dead cells.

This full-body reset can be extended to upwards of 72 hours if you want to, as well.

What do researchers say about fasting?

Experts have long been divided over exactly how far the health benefits of fasting extend, particularly when findings from laboratory and animal studies are applied to humans.

James Betts, professor of metabolic physiology at the University of Bath, told The Guardian in 2024: “There [are] a lot of proposed benefits to [running on fats]. But a lot of the research hasn’t really [been borne out in] human beings. So we don’t see dramatic health benefits, certainly in the short term.”

More recent evidence, though, has given us a bit more clarity.

A major 2025 BMJ systematic review and network meta-analysis looked at 99 randomised clinical trials involving 6,582 adults. Researchers found that intermittent fasting strategies and continuous calorie restriction both reduced body weight compared with unrestricted eating.

Alternate-day fasting was the only intermittent fasting approach to show an additional weight-loss advantage over continuous calorie restriction, and the difference was relatively small. Interestingly, in studies running for 24 weeks or longer, intermittent fasting did not clearly outperform continuous calorie restriction for weight loss.

What’s more, studies have also found that people who regularly fast for more than 16 or 18 hours a day have a higher risk of gallstones, as per News In Health.

Meanwhile, Johns Hopkins neuroscientist Mark Mattson argues that there can be wider benefits to fasting.

He says: “Many things happen during intermittent fasting that can protect organs against chronic diseases like type 2 diabetes, heart disease, age-related neurodegenerative disorders, even inflammatory bowel disease and many cancers”

That said, Johns Hopkins' guidance, most recently updated in April 2026, nevertheless stresses that intermittent fasting is not suitable for everyone. Children and teenagers under 18, pregnant or breastfeeding women, people with type 1 diabetes who take insulin and those with a history of eating disorders are advised not to try it.

People with type 2 diabetes also need to consider their medication. Current NHS-backed guidance warns that fasting may increase the risk of low blood glucose for those using insulin or other medicines that can cause hypoglycaemia, and advises discussing the diet with a diabetes care team beforehand.

Whilst the simulation provides an eye-catching look at what may happen during a prolonged fast, its hour-by-hour timeline should not be taken as a guaranteed biological schedule.

Everyone reacts differently to fasting, so it’s worth checking with a medical professional before trying it out.