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For several years during my early thirties, I was constantly tired. Starting around 2012, I was going for a five-mile run about three times a week, lifting weights just as often, and usually getting a good night’s sleep. I wasn’t training for the Olympics or running ultras, and I didn’t have a newborn keeping me up at night. I also wasn’t out of shape, overweight, or unhealthy. Yet every afternoon I felt the need to lie down for a nap—and if I didn’t, I was often irritable. I struggled to explain the problem to my wife and friends. I was fit and healthy. This was the prime of my life. Shouldn’t I feel great?

Over the course of three years, I spoke to several doctors, but none could pinpoint a major medical issue. One naturopath thought it might be mono, but the test turned out negative. An MD thought I might be depressed, but I didn’t feel depressed. Another naturopath thought it might be chronic fatigue syndrome, but that is, , a “debilitating and complex disorder characterized by profound fatigue that is not improved by bed rest and that may be worsened by physical or mental activity.” I wasn’t facing anything that extreme.

It’s only now, a few years later, that I’m confident I know what was going on: my blood sugar levels were out of whack. I had trained my body to crave simple carbohydrates, and I was caught in a cycle that left me crashing despite a constant sugar fix.

How did I realize this and end the sugar-up-then-crash cycle? I started by fasting for at least 12 hours every day.

Why I Started Intermittent Fasting

Last year, after dealing with chronic lethargy for three years, I read about the benefits of intermittent fasting, including and . Mark Mattson, chief of the laboratory of neurosciences at the National Institute on Aging, professor of neuroscience at Johns Hopkins University School of Medicine, and author of dozens of research papers concerning intermittent fasting, has argued in his work that humans evolved in situations where food was limited. “Evolution favored individuals with superior cognitive and physical abilities under conditions of limited food sources,” he wrote in a , “and brain function can therefore be optimized by intermittent dietary energy restriction and exercise.”

Three or more meals a day, Mattson pointed out, is not how most humans historically have eaten. Buddhist monks, Neolithic farmers, and traditional hunter-gatherers ate once or twice a day. Although breakfast, lunch, and dinner may seem timeless to us now, it’s only been in the past couple hundred years that became standardized in the West. This shocked me: How did the human race develop and thrive on just a one or two meals a day, yet I’m tired if I haven’t eaten a PB&J in the past couple hours?

Intermittent fasting—of which there are many different types, but all limit the time that one consumes food—most likely causes short-term stress on the body, somewhat similar to the beneficial stress of lifting weights. In a , Mattson noted that intermittent fasting can “protect neurons in the brain against a wide range of severe stressors,” like epileptic seizures, stroke, neurotoxins, oxidative stress, and amyloid, and can help cells cope with and resist injury and disease. However, some research indicates that stress-related , and others have argued that it .

Types of Fasting

I decided to give it a try. There are three main ways to do an intermittent fast: the 5:2 diet, in which you eat regularly for five days a week and reduce your intake to 600 calories during the next two; alternate-day fasting, where you rotate between standard and 600-calorie days; and time-restricted eating, in which you limit eating to a four-to-eight hour period each day.

The 5:2 diet, popularized first in the UK in 2012 and then in the United States in the past few years by , and are both geared toward weight loss, something I didn’t need. In contrast, with time-restricted eating, it’s easy to consume a regular caloric load each day while still getting some of the benefits of fasting. That sounded like a better fit for me, as I planned to remain active most days of the week.

I settled on the 16-hour fast, where I’d limit eating to an eight-hour window every day. As Mattson explains, the liver stores glucose (the body’s main energy source when you are regularly eating carbohydrates) for about ten to 12 hours after a meal. When you fast for more than 12 hours, your body switches from glucose metabolism to fat metabolism, which leads to a bunch of positive . So, if you fast for 16 hours each day, you’re receiving the benefits of those adaptive responses without limiting your total caloric intake.

Fasting 101: Cut Back On Snacks

Before I started fasting, I always ate a snack before bed, often a peanut butter and jelly sandwich or a bowl of granola. Just moving my bedtime snack back an hour made me slightly anxious at first; I was convinced I would wake up hungry in the middle of the night. But I did it for a couple days, then a couple weeks, and pretty soon I realized I didn’t need a snack right before bed—I just needed one a couple hours before bed.

Mattson compares starting a fasting routine to exercising: If you’re used to eating many meals throughout the day and jump right into fasting, it’s like trying to become a runner by going for a fast five miles without any training. It’s going to be unpleasant, and there’s a good chance you’ll give it up before you see the benefits. Gradual change and a slow buildup is key.

And so it went, with me moving my snack further from my bedtime and my breakfast further from waking up. Each time I moved the timing of a snack or a meal, I repeated the cycle of minor anxiety followed by acceptance, but it became easier. After about six months of intermittent fasting, I didn’t need snacks as long as I was eating nutritious, filling meals during the eight-hour window.

Does Fasting Make You Tired?

For the past few months, I’ve been eating breakfast around 10 a.m., lunch around noon, and dinner around 6 p.m. Technically, I’m intermittent fasting, but I don’t really think of it that way. It’s just my normal eating schedule. And if it felt difficult and restrictive at first, now, six months later, it feels liberating. I’m no longer constantly hungry or lethargic or irritable. Intermittent fasting alone didn’t solve all my blood sugar problems, and I’ve also noticed that regular exercise throughout the day and not eating processed carbs have helped bring my A1C—a measure of blood sugar—down as well. But fasting did help me understand how my body uses carbohydrates and fat for energy and why being too reliant on simple carbohydrates had contributed to my lethargy for years.

It also allowed me to cut processed carbs from my diet. Today, my typical meal consists of venison or fish, vegetables, beans or other legumes, and a good dose of healthy fat, like olive oil, nuts, or avocado. All these foods are broken down slowly, which limits the insulin spike and keeps me feeling full for hours. I still eat carbohydrates in the form of fruits and oatmeal, though only on occasion. If I’m going for a five-to-ten-mile run in the morning, I’ll do it fasted. If I’m going for a long run—say, 12 miles or more—I’ll take some  before the run and eat a bar during. And if a friend wants to get dinner after 6 p.m., I don’t sweat it. Most days I eat from 10 a.m. to 6 p.m., but intermittent fasting doesn’t require you to obsessively stick to a schedule.

These adaptations to how I eat haven’t changed the fact that I love eating. If anything, I enjoy the process of gathering ingredients, cooking, and eating now more than ever. I’ve realized that fasting, which is often considered ascetic, is compatible with a deep appreciation of food.

The Bottom Line to Successful Fasting

The key to fasting regularly is finding a program that suits your life and training goals. It could be one 24-hour period each month. Or the 5:2 diet. Or fasting on religious holidays that call for it. Fasting is like working out—different people will have different programs, but the goal is the same: to provide beneficial stress that helps your body become stronger, healthier, and more resilient when faced with larger, potentially damaging stressors.

I’ve been intermittent fasting for a year now, and I no longer need an afternoon nap. I’m not irritable if I haven’t eaten in the past couple hours, which used to happen to me all the time. I’ve also hiked the Colorado Trail and run a 50K, and I’m currently putting in 40 to 60 miles a week and lifting weights a couple times a week. Intermittent fasting hasn’t transformed me into a Super Athlete, but I have to say, I feel pretty great.