Why I’m Fasting 15 Hours a Day


Last week I began fasting every day.

My daily target is to fast for 14-15 hours a day. This is a long term lifestyle change, not a fad diet. It means having breakfast around 9am, and eating my last food of the day by 6pm. Any caloric intake counts as food, so black tea and coffee is not allowed while fasting. Only water can be consumed during fasted hours.

After sharing my intentions to fast on Twitter, I had a few people asking me what the reasons behind it were. I plan to share these in this article.

To set things off, speaking in very broad terms, my ambition is that daily time restricted eating will improve my health, provide greater resistance to disease, increase my energy and crucially improve my endurance performance on the bike.

How did this all come about?

I’d been hearing lots of anecdotal evidence claiming some significant health and performance benefits from fasting. I’d even experimented with a 12 hour on / off eating cycle myself over the summer and have experienced some tangible benefits of the 12 hour restricted feed-cycle, which included better sleep and weight management.

The 12 hour approach was a great starting point for me but I suspected there were more benefits to fasting than I was aware of and that maybe I needed to restrict my eating window even further than 12 hours. Also, as an amateur athlete I would be lying if I wasn’t still seeking for a method of fasting that might give me a performance edge.

My search for a better understanding of fasting led me to the work of Dr Satchin Panda – a professor at the Salk Institute for Biological Studies. Satchin’s work deals specifically with the timing of food and its relationship with our internal biological clocks, or ‘circadian rhythm’.

It’s been Satchin’s research into this relationship of ‘when we eat’ with our body’s natural circadian rhythm that has compelled me to experiment with fasting for 14-15 hours every day.

Before sharing his research, I’d like to discuss what circadian rhythm is, how our lifestyles affect it, and the impact that the timing of our food intake has on our health and performance.

Circadian Rhythm

Our circadian rhythm is a 24 hour internal clock that regulates many processes in our bodies, such as the sleep / wake cycle, digestive system and even our ability to perform physical exercise. Every organ in our body has its own daily rhythms; in the morning our gut and digestive system begins to anticipate food; our brains are at their most alert around lunch time, and our muscles are programmed for peak performance in the mid afternoon. Even our saliva has its own circadian rhythm.

The most obvious circadian rhythm we all experience is our daily sleep / wake cycle, which is largely effected by exposure to light. To put it very simply – when it’s dark, our eyes send a signal to our brain that it’s time to sleep. Our brain then sends a signal to our body to release melatonin, which makes us tired.

Until very recently in human history, our bodies have been naturally in tune with our circadian clocks. Since the turn of the 20th century however, technology has allowed us to dramatically alter our lifestyles in unnatural ways. Many external factors in the modern world can wreak havoc on our natural circadian rhythms, which can lead to many metabolic and chronic diseases, as well as having a negative effect on our physical and mental performance.

Some of the major disrupters of our circadian rhythm include:

  • Exposure to artificial light after sunset
  • Lack of sleep
  • Irregular shift patterns (especially night shifts)
  • Eating after dark

How light and food can disrupt our circadian clocks

The advent of artificial light in the 20th century has been a major disruptor of our circadian rhythm. Exposure to artificial light (light bulbs, TVs, smart phones) after sunset confuses our internal circadian clocks and has been proven to disrupt the quality of our sleep. That’s why its a very good idea to not take your smart phone to bed, and try to limit your exposure to artificial ‘blue light’ at night. Trust me it really does make a difference.

In terms of food, we can count ourselves lucky that in modern western society we have access to food 24/7. While this is no doubt a privilege we should be grateful for, it has disrupted our natural circadian rhythm for eating. We’re able to eat from the moment we wake up and continue snacking right up until bed time. Combine this with the fact that food has become more than just a source of nutrition in modern society (we eat for social reasons and for comfort) and many of us can find ourselves taking in calories for most of our waking hours.

One recent study revealed that 50% of adults admitted to eating over 15 hours a day. Our digestive systems are not designed to process food for extended periods of the day like this.

It’s not difficult to understand how our circadian rhythms have become so disrupted in modern society. And these disruptions have been shown to lead to many of the common metabolic and chronic diseases we are afflicted by.

Luckily, there are many simple changes we can make to our daily routines that help us align to our circadian rhythms. These include:

  • Limiting exposure to artificial blue light (smart phones, laptops, TV etc) in the evening and especially before bed
  • Going to bed a bit earlier
  • Waking up at the same time every day
  • Avoiding eating or drinking anything other than water too close to bedtime

Why when we eat is so important

My own journey into fasting has been sparked by my curiosity into how the timing of when we eat and drink affects our overall health and performance.

Let’s return to the work of Dr Satchin Panda.

In one of his earlier studies, Panda found that mice who eat within a set amount of time (8-12 hours) were slimmer and healthier than those who ate the same number of calories in a larger window of time. He went on to discover that mice allowed to eat whenever they liked died younger and developed common diseases, whereas the time restricted feed group of mice did not show any signs of disease. Again, overall calories were the same in both sets of mice.

Wondering if time restricted eating could be used to reverse disease, Panda put a group of mice with existing diseases onto a time restricted eating pattern. The signs of disease in these mice were reversed in all cases!

Testing and analysis in humans is still in its infancy, but it is clear that there is huge potential for human health by restricting the time in which we take in our calories.

What about endurance performance?

This is where it get’s really interesting for athletes.

In one of Panda’s experiments, he compared the endurance performance a group of mice on a healthy non-time restricted diet, against a group of mice on an unhealthy time-restricted diet. The mice ran on a treadmill at low intensity until exhaustion.

The mice fed a time restricted unhealthy diet (high in refined sugar and fat) consistently outperformed the healthy diet group that could eat whenever they wanted. Again, overall calorie intake was the same in both cases.

Dr Satchin Panda noted that mice which were fasted for 12-13 hours benefited from improved health and disease prevention, while mice that were fasted for 14-15 hours a day, got all the health benefits, with the added endurance performance benefits on top.

I find these results so amazing it’s worth repeating.

With overall calorie intake constant for both groups, mice fed a time restricted unhealthy diet of refined sugar and fats could run for longer than a group of mice eating a non time restricted healthy diet.

In summary, mice fed a time restricted diet benefited from:

  • Longer life span
  • Disease prevention
  • Disease reversal
  • Better endurance performance

Panda has repeated the studies numerous times with consistent results every time. He states that ‘if mice eat all their food within 8 – 10 hours they are completely protected from all their diseases and benefit from improved physical performance and mental health.’

The research results have been so compelling that Dr Valter Longo and Dr Satchin Panda have concluded that:

“(Intermittent Fasting, Periodic Fasting and) Time Restricted Fasting have emerged as potential strategies for avoiding major dietary changes while achieving strong effects not just for one diseases risk factor but for an array of factors that constitutes the foundation for metabolic syndrome, cardiovascular disease, cancer, and possibly neurodegenerative diseases.”

These are very big claims, by very smart scientists who are dedicating their lives to understanding how fasting can improve health, longevity and performance in humans.

It was certainly enough for me to introduce a time restrictive fasting cycle into my own life.

Based on Panda’s recommendations, I am aiming to fast for 14-15 hours a day. Sometimes its a little less, sometimes a little more. Anything over 12 hours fasted is good, but 14-15 hours is the apparent sweetspot for performance gains.

Here’s what my first week of fasting has looked like.

my first 10 days fasting

  • I have averaged over 14 hours a day fasted for my first 10 days.
  • On 3 of these days I have cycled at a low intensity for 90 minutes in a fasted state (before breakfast).

While it’s too early to draw conclusions after only 10 days, I would like to share some of my early observations:

  • I am learning to control my hunger better
  • I go to bed feeling lighter and wake up with more energy
  • While cycling in a fasted state, I have noticed my heart rate is sometimes lower than usual
  • Restricting my access to food makes me appreciate the food I eat more
  • I eat more slowly and consciously
  • Not eating so late in the evening is naturally restricting my overall calorie intake, which will be good for my weight management

I’ll post any relevant updates on the blog as I continue with my fasting.