At 6:00pm on Saturday, February 14th, I stopped eating (clearly I didn’t have Valentine’s day plans).

I didn’t pick up a spoon, fork, knife, pair of chopsticks, beer bottle, or piece of finger-food for 120 hours until 6:00pm, Thursday February 19th.

Which raises the obvious question: “Why the hell would you do that?”

Let me explain.

Why Fast?

We eat three meals a day because… well because everyone else does. And because that’s how we were raised. And because we get hungry if we don’t eat for 4-6 hours.

But there’s no fixed biological rule saying that we need to eat every 4-6 hours. Homo Sapiens came onto the map 250,000 years ago but we only developed agriculture 12,000 years ago. There were 238,000 years where we were getting by as hunter gatherers, or more importantly, where we were getting by with unreliable access to food.

We might kill a gazelle one day, share it with our tribe, then go two days without access to meat and have little to subsist on. And this was perfectly fine. We certainly weren’t stopping in the middle of our 16-19 mile a day walks to have a protein bar to refuel.

Many of the foods in our modern diet (particularly grains and sugars), combined with eating constantly, has made our bodies lazy and largely turned off their ability to consume themselves. That might sound gross, but it’s a totally natural, healthy and important process.

But I still haven’t answered the question: why engage in this masochistic five day ordeal?

Reason to Fast #1: The Health Benefits

The best reason to fast is for the health benefits, and to start turning back on some of those processes I mentioned before.

Central to many of these benefits is a process called “autophagy.” Autophagy is the body’s natural process of killing off, eating up, or cleaning out bad cell matter that’s built up in your body. It’s an important system for staving off many diseases,including preventing cancer development.

Not only that, but reduced autophagy (the state that most of our diets leaves us in) leads to accelerated cell aging, which explains why in numerous studies on lab animals from single cells to mice to monkeys, restricting their caloric intake significantly increased their lifespan, even when that restriction was occasional (i.e. fasting every once in a while).

And it doesn’t stop there. Autophagy also helps with the development and retention of lean muscle which is probably why I’ve found intermittent fasting valuable for putting on muscle. And no, ladies, this doesn’t mean it will make you bulky, it means it will make you “toned.”

Oh wait? There’s more? Yes! It turns out that autophagy induced through caloric restriction also slows neurodegeneration and is one of the few things that can lead to production of new brain cells. This research suggests that fasting can protect you against neurodegenerative diseases like Alzheimers and Parkinsons. This is also likely why epileptic and autistic people respond very well to fasting.

And hey, while we’re at it, let’s throw in the evidence that periodic fasting helps with reducing chronic pains, rheumatic diseases, high blood pressure, and anything related to inflammation.

But we’re not done yet! Fasting also improves your insulin sensitivity, meaning your body can better process those Oreos instead of sending them to your waistline when you decide to cheat:

Then there’s the obvious fat loss benefit. After day two (maybe earlier) your body has nothing to run on but your own fat. The water weight you lose will come back quickly, but that fat loss is real. You can expect to lose ~1-2 pounds per day, but consider this a nice benefit, not the primary motivation. If you’re doing it for fat loss you might do it too long or ignore signs to stop.

Reason to Fast #2: The Challenge

I have a lot of fun doing new, difficult challenges, like not eating gluten for a month, so long as there’s a good reason to try it.

I assumed (rightly) that going five days without food would be a great way to challenge my willpower. I have fairly strong willpower (when sober at least) and this seemed like one of the biggest things I could do to put that to the test.

I’ve also just never done it. I’ve done 24-hour fasts before, usually after a cheat day, but nothing longer than that. I was curious if I could manage it.

Reason to Fast #3: The Meditative & Psychological Aspects

There were anecdotal reports that once you get through the painful first couple days of fasting, you enter into a totally new frame of mind that’s much clearer, and much more focused.

That’s what I’d heard about quitting coffee, quitting gluten, meditating, and from starting intermittent fasting, and all had been true. Naturally I had to try fasting then.

And, once again, it was true.

After the second day or so I entered into a state of total mental clarity like nothing I’ve ever experienced outside of meditation. I’ve never been so focused and hard to distract in my life.

You also develop a different relationship to hunger. Hunger becomes an option, not something you need to act on right now.

Finally, (and this one sounds super pretentious) you develop a sense of superiority to your feeble weak-willed hunger-driven peers. I was sitting in a 10:30am class and one of my classmates walked in complaining that she hadn’t been able to grab a snack before class. I hadn’t eaten in four and a half days at that point so I just quietly laughed at her in my head.

The Necessary Reason: You’re Interested

DO NOT, I repeat, DO NOT, fast as a weight loss method. That’s not the point, and it’s unsustainable.

Do it for the health benefits, mental benefits, and because you’re interested in the experience. If you’re not really interested in it, and not committed to seeing what it’s like, you won’t make it.

Convinced that fasting is awesome and interested in doing it? Here’s how.

How to Fast Safely (or, all the advice I ignored)

Heads up! We’re getting into prescriptive territory where I talk about how you can try fasting. I’m not a doctor, veterinarian, spiritual healer, or anything else that would remotely qualify me to give you medical advice, so if you try this and you die it’s your own damn fault.

The fasting community is a little confusing.

Some people say “drink only distilled water” some people say “be sure you take a lot of electrolytes.” Some say “Make sure you take a vitamin supplement” some say “avoid vitamins.”

What I’ve learned from doing experiments like this is that when you get into new areas of research (like fasting), 99% of the info online is bullshit and you have to figure it out on your own.

So, do your research. You can die from fasting, usually from drinking too much water, flushing all the salt out of your body, and going into cardiac arrest. That’s why you only drink distilled water. Or is it why you don’t drink distilled water? No one agrees really, so I just drank a lot of Aquafina and Brita-filtered water.

Some people said you should only have 1-2L of water a day, some said just keep it under a gallon, some said to drink a ton to flush the toxins out. I just drank when I was thirsty.

Some said you need to stay as inactive as possible. They talked about staying in bed for most of days 2 and 3, not walking more than a half mile, and keeping their activity low even during the refeeding period.

I said screw that and went about life as usual. The only changes I made were not using my standing desk (until day five) and not working out, but I still averaged ~2 miles of walking and 10+ flights of stairs a day, most of it in sub-zero temperatures (Pittsburgh has been cold as the 9th circle lately).

They also said make sure you spend a week not eating carbs, reduce your diet slowly, etc. etc. I spent the day before eating an absurd amount of junk food (it was my cheat day, which, if you follow me on snapchat (nateliason) you’ll get an awesome story about each Saturday).

So… How Do You Fast Then?

These are the only things that I’m fairly confident about:

Don’t eat ANYTHING. Even eating a small amount keeps digestion going which will make the fast torturous.

Drink however much water you feel like you need. Don’t drink gallons, and don’t just drink a thimble, and you’ll probably be fine.

You don’t need the electrolytes / vitamins, but they probably don’t hurt.

Don’t drink “juice,” that stuff is terrible for you anyway.

Do as much activity as you’re comfortable with.

Sleep as much as you feel you need, but don’t lie in bed just because your body is more tired than usual. What I found was that once I started moving or working, my body provided me the energy I needed. If I stayed inert, I got tired.

A Caveat: My Competitive Advantage

Now, there’s something to be aware of here: I probably had a big leg up on most other people coming into water fasting.

First, like I mentioned, I’ve been doing intermittent fasting for 3 years. My body is used to at least short periods without food. It’s also used to running on its own fat instead of just glucose. This is most likely why I hardly had to decrease my activity.

I’m also used to cycling my carbohydrate intake, since I only eat carbs right after workouts and on Saturdays. I also only eat sugar on Saturdays, so my body already has great insulin sensitivity. This likely made it easier to switch off of running on glucose

And, like I mentioned, I have very strong willpower, so it wasn’t too hard to resist what cravings there were. Though, really, the cravings were minimal.

High Level: What Happened During the Fast?

If you don’t want to read the day by day, here are the Sparknotes.

I averaged 9-10 hours of sleep a night, that was the only change in my amount of rest though. No naps.

Days one and two were rough, day three was great except for when I was in classes. When I say “rough” though I mean I had to fight off the occasional pangs of hunger. It really didn’t affect my productivity or life that much. I’d compare it to having half a dozen itchy mosquito bites.

Days four and five I felt like a god. I got an incredible amount done, was completely undistractable, and just felt blissfully happy consistently.

All of the claims about mental clarity were true—I’ve never felt so “unclouded” in my mind. There was zero brain fog all hours of the day, especially on days four and five, and on day three when not in class. It was all the benefits of meditation, but constant throughout the day, and without actually needing to meditate. I stopped meditating for days 3-5 to verify that it was from the fasting, and the feeling stuck.

Another interesting thing: time moved very slowly, and I was never rushed. I have one theory: I always eat really, really fast. It’s a bad habit, I know. Without that rushed process twice a day, maybe I was less rushed in general. Either way, it was remarkable how calm I was.

The only real negative was that I was very, very sensitive to the cold. My theory is that my body was rationing its energy mostly towards brain functioning and movement, and not spending as much on thermogenesis. I was keeping my room 5-10 degrees warmer than normal, and still wearing a sweater.

Oh, one last positive. I’ve never had more muscular (especially abdominal) definition in my life. I would post pictures, but you would have to buy me a drink first. Hint: I like Malbec.

The Day by Day Rundown

I was taking these notes as I was going, so it’s a little bit stream of consciousness / journal-like. For reference, the fast started on Saturday at 6pm. That was “Day Zero”

Day One: Sunday

8:00 am: Woke up, had a bit of water to start the day.

12:00 pm: Noticed some slight hunger around noon, have a bit more water.

1:00 pm: No headaches despite not drinking tea, good sign that I don’t have a caffeine addiction. I might miss the tea more than the food.

3:00 pm: hunger is starting to kick in. I normally live on a 16-8 intermittent fasting cycle, and this is around the point (18 hours in) when I normally HAVE to have food. But today… water.

4:00 pm: Water seems to help with the headaches. Maybe I was dehydrated?

7:00 pm: So tired, feels like it’s midnight or later. Hardly being productive at all, good thing I finished stuff earlier. I wonder if this is because I didn’t sleep quite enough last night or just from no food? Maybe both. Tomorrow will probably be rough if this fatigue is all from the fasting.

8:00 pm: Time feels like it’s moving slower.. maybe that’s just from the fatigue?

Day Two: Monday

9:00 am: Woke up at 9am after sleeping at 11, clearly need extra rest when I’m not eating.

9:30 am: Tried to play it conservative by sitting instead of standing, but my body wanted to stand for about an hour. Maybe the “rules” about inactivity vary person to person based on their prior activity levels? I’m probably a little different since I normally use a standing desk.

10:00 am: Despite not eating in over 36 hours now, I’m actually not hungry. No pain really, though my brain feels a little foggy, slight tunnel vision. Kinda like a mild hangover without the headache. I wonder if this is from the toxins coming out of my bad cell matter and getting dumped in my bloodstream (supposedly part of the autophagy process)… I should probably go out drinking less.

12:30 pm: Time definitely feels like it’s moving slower. There’s much less urgency.

3:00 pm: My business & culture professor talked about food for half an hour… it was terrible.

7:00 pm: Really hungry around dinner time.

8:00 pm: The fasting and hunger is not so noticeable when working, you get weirdly calm and focused, you just don’t think about it.

9:00 pm: Weird feeling, kinda like being tired, but still functional… sort of tunnel vision. I think my body is very tired but my mind is fine, maybe my body is conserving energy by leaving my body weak but still keeping my brain going? Everything is getting done though, it just doesn’t totally feel like I’m the one doing it. Kinda like I’m on autopilot.

Day Three: Tuesday

9:00 am: Woke up at 9 again, still 10 hours of rest. Though I woke up at 6:30 and 8 as well but I kind of forced myself to sleep, I was a little worried about getting that little sleep.

9:30 am: Sitting instead of using my standing desk to play it safe.

10:30 am: Barely any hunger, and much less pressure behind my eyes today.

12:00 pm: Time still feels really, really slow. Weird not having those interruptions of food. I’ll sit down to do an assignment or something then look up and go “wtf, that only took 10 minutes?”

3:00 pm: Climbing a lot of stairs at once is kind of tiring. I had to climb four flights and was very winded afterwards. I regain the energy quickly though, and it’s only a physical fatigue, not a mental one.

5:00 pm: Classes were horrible today, I had a 3-hour straight block that couldn’t have moved slower. I thought maybe the hunger was getting to me but it was something about the classes and the hunger, once I was out I was totally fine. It may also have been the boredom + time delay.

7:00 pm: Hyper focused on whatever I’m working on, had no idea this would be such a pronounced effect. I have no need for the Pomodoro method, the Self Control app, anything like that.

8:00 pm: Some small headaches throughout the day, mostly in classes though. I think I was dehydrated in the morning. They went away in the afternoon when I drank more water.

Day Four: Wednesday

9:00 am: Woke up feeling awesome, slept 10 hours again, no alarm clock.

9:30 am: Funny side effect, I keep putting my water pitcher in the fridge out of habit. It’s a problem because my body sucks so much at keeping me warm right now that I can’t drink cold water, only room temperature. Very European.

12:00 pm: Feeling awesome in general, super focused, no afternoon fatigue this time.

12:30 pm: Barely any hunger today, I started keeping a list of things that I want to eat when this is over and that helps a lot. As soon as I have a craving for something it goes on the list and the craving goes away. Saturday (faturday) is going to be nuts.

1:00 pm: Damn, my skin looks really good. So do my eyes. So do you, reader ?

12:00 am: Had a meeting that went until Midnight. I was worried about it since I’d been getting tired around 10, but I was 100% fine. This supports my theory that while fasting, your body provides energy on an as-you-need-it basis, and not by default. Since I had to do stand, walk around, and lead a meeting, I had the energy for it. When I’m sitting around reading, I don’t need much energy so my body provides less and I feel tired. As soon as I went home and relaxed after the meeting, I felt tired again.

12:30am: Otherwise, felt amazing today. Super focused, generally happy, also very patient. Just a great day in general.

1:00am: Going to bed later today too, we’ll see how that affects things.

Day Five: Thursday

9:00 am: Woke up and had my usual glasses of water. Seem to be fine despite only sleeping eight hours and staying up till 1.

10:30 am: Class was much easier to go through today, feel so much better than Tuesday, and even better than Wednesday.

12:00 pm: This might be the best I’ve felt mentally in my entire life.

12:30 pm: Decided to push myself by sitting in a cafeteria for 1.5 hours around lunch time. Turned into one of the best willpower exercises I’ve ever done. I definitely wanted food badly, but I was able to keep focusing on work and other stuff and tune it out. It was a lot like meditation, every time my mind wandered to the enticing scents around me, I just reeled it back in.

2:00 pm: Had some turmeric and ginger tea to get the digestion going again. (Thanks Nicolas for the suggestion!) (Yeah yeah I know this isn’t water, but it’s part of ending the fast.)

6:00 pm: Started breaking the fast at 6:00pm, 120 hours after starting, with sauerkraut, athletic greens and almonds.

And then it was over! But, how do you end a fast?

Refeeding: How to End the Fast “Safely”

Now we come to the topic of refeeding.

Refeeding is the process of steadily introducing food to your body in such a way that you don’t totally crash your system.

The biggest risk is something called refeeding syndrome, where your reintroduction of food spikes your insulin so much that you go into shock and maybe die. It’s a concern on fasts five days or longer, and when you’ve lost a lot of body weight.

To prevent this, you steadily reintroduce foods to the body over one to two days, steadily moving up the chain of how hard to digest something is, ending with meat. Some say that you should re-feed for as many days as you fasted.

But here’s what doesn’t make sense: a lot of the sites that talk about fasting say that you should start with fruits and vegetables since they’re easily digestible. Vegetables make sense, but fruit? Fruit has a fairly high glycemic index, meaning it risks spiking your insulin. Again, don’t trust most of what you read on dietary science.

So I had a different theory. First, I repopulated my gut micro-biome, the healthy bacteria that live inside your gut and help with processing food. I did that with Athletic Greens and Sauerkraut, both awesome sources of probiotics.

Then, I ate nuts since they’re very low glycemic and they’re fat, which won’t spike my insulin and won’t be hard on my kidneys or gut. They’re also a good source of Magnesium, which is one of the mineral deficiencies that leads to refeeding syndrome. I threw in some Potassium and Phosphate supplements as well.

I did that at 6pm…

Then at 8pm I said screw it, ordered Taiwanese food, drank a beer, then had some wine while writing this article.

Why? Because I think people are way too careful with fasting. Numerous sources suggested waiting two or more days before reintroducing meat. If you were in the wild, and you hadn’t eaten in four days, and you managed to kill a deer, you wouldn’t go forage for berries before eating it to “work yourself up to it.” You’d eat the damn deer.

Also, occasional shocks are good for your body, that’s the whole point of fasting in the first place. If you treat your body like glass then it will stay like glass. This is the same logic behind body building, hormesis, vaccinations, cold therapy, and any other process where we inflict small damages to increase the strength of the system (what Nassim Taleb calls “Antifragility“).

And no, I didn’t go into shock, crash, get diarrhea, or anything like that. I just felt totally full, and that one beer got me way tipsier than normal.

Though, if I were to do a 7-day fast or longer, I might be a bit more careful. Again, not a doctor, and I have competitive advantages.

Your Own Fast

A LOT of people have emailed me since originally writing this article with questions about how they can do their own fast.

If you’re interested in trying your own water fast, I put together a day-by-day guide of what you need to do and be aware of as you’re doing it.

It’s based on my own experiences, as well as the experiences of friends who have done their own fasts of different lengths. It’s all you need to do your own water fast and see if you have the same experience I did.

What’s Next?

As I’ve mentioned a few times now, this was a very, very positive experience. There were hardly any downsides, and the few downsides were totally steamrolled by the positives.

On top of that, like I said, there are an incredible number of health benefits from fasting. I’ll definitely keep doing intermittent fasting. There’s no reason to stop that, it only does good things, and I don’t even know if I could now that I’ve gotten so used to it.

And I’ll most likely shoot for a fast like this maybe once a quarter. Maybe next time I’ll do a 7 day one.

Special thanks to Adil Majid and Zachary Rousselle for checking in on me to make sure I was alive during the fast.

This article also appears on nateliason.com