What Is Circadian Rhythm Fasting & Is It Better Than Intermittent Fasting?

Intermittent fasting, a type of dietary practice that focuses on when you're eating rather than what you're eating, has become a popular eating style as it is associated with a variety of potential health benefits.

Even further, you can incorporate intermittent fasting (IF) into any diet plan. 

But what if there was an eating plan that took it a step further by not only timing your meals but making sure those meals are in line with your body's internal clock?

Before diving into circadian rhythm fasting, let's talk about the circadian rhythm and why it matters.

Your circadian rhythm (also sometimes referred to as your sleep/wake cycle) is a 24-hour internal "clock" that regulates how awake and/or tired you feel during the course of a day.

Your circadian rhythm is controlled by an area in your brain called the hypothalamus1, which is super sensitive to light.

When your hypothalamus is exposed to light, it sends out signals to the rest of your body telling it that it's time to wake up. On the other hand, when it gets dark, your hypothalamus signals to your body that it's time to wind down.

In addition to light, the other major factor that regulates circadian rhythm is your eating schedule2.

All of this comes down to hormones—two especially: cortisol and melatonin. In a perfect world, cortisol rises in the morning, peaks again in the afternoon right before lunchtime, and then drops at night.

The circadian rhythm diet, also called the sun cycle diet, encourages timing your meals with the rise and fall of the sun, and the corresponding surges and dips in cortisol.

That's because cortisol has a significant effect on your thyroid hormones, which affect the metabolism of the food you eat. When cortisol rises in the earlier hours, your metabolism is also up and running3 and you effectively use the food you eat as energy.

When cortisol dips later in the day, your metabolism simultaneously slows down, which makes it more likely that your body will store the food you eat as fat.

Keep in mind that this is how it works with normal cortisol levels and cycles. If your cortisol levels get too high—like in times of severe stress—it can actually have a paradoxical effect where cortisol contributes to increased body fat4, especially in the belly area.

If you're under a great deal of stress, it's important to get that stress under control before doing anything else.

Circadian rhythm fasting also considers the role of insulin. When you eat, especially if you eat a meal that has a lot of carbohydrates, your body releases insulin in response to the rise in blood sugar.

According to researchers from a study that was published in Cell2 in May 2019, if insulin rises at odd times—like when you eat a meal late at night—it can actually disrupt your circadian rhythm and increase your risk for long-term health problems, like type 2 diabetes and heart disease.

Insulin also promotes the storage of body fat, especially if you eat too many carbohydrates or calories.

On the other hand, sticking to an eating schedule that aligns with your natural hormone cycles and avoiding late-night insulin spikes can help reduce your risk of developing chronic diseases, type 2 diabetes, and heart disease.

Timing your meals with your natural circadian rhythm has also been shown to help improve inflammatory diseases, like rheumatoid arthritis (or RA) and inflammatory bowel disease11 (or IBD), infections, metabolic disorders12, certain cancers12, and central nervous system disorders, like Parkinson's disease13.

While it shares similar characteristics with intermittent fasting, the circadian rhythm diet is a little stricter when it comes to timing.

Unlike intermittent fasting, which allows you to determine your own feeding schedule, as long as you stick to your 12- to 16-hour fasting window, the circadian rhythm diet requires that you stop eating by 7 p.m.

Another big difference is that the circadian rhythm diet encourages eating breakfast in the morning, when cortisol is the highest. Many people following regular intermittent fasting schedules skip breakfast entirely and eat their first meal around lunchtime.

The last real notable difference is that the circadian rhythm diet is typically built around two 12-hour windows. While you can extend your fasting window and stop eating earlier in the day if you want, it's encouraged that you eat between 7 a.m. and 7 p.m. and fast between 7 p.m. and 7 a.m.

While the overall timing of your meals is important, it's not the only thing to consider when following the circadian rhythm diet.

Felice Gersh, M.D., a board-certified obstetrician with expertise in hormonal management, recommends that, in addition to eating no later than 7 p.m., you also eat breakfast within two hours of waking up and eat no more than three meals a day.

Yes, that means no snacking. You should also focus on building your plate with lean proteins and healthy fats and avoiding processed foods and too many carbohydrates.

And since the circadian rhythm diet relies on a predictable rise and fall of cortisol and other hormones, it's important that your circadian rhythm is actually functioning as it should.

Artificial lights, extended screen time, and unpredictable sleep schedules can all negatively affect your natural circadian rhythm and throw your hormones out of whack.

Eating with your natural sleep/wake cycles can help start to balance your hormones, but if you're scrolling through Instagram on your phone until 2 a.m., it can negate your dietary efforts. 

To reap the full benefits, Shah recommends getting your sleep schedule in harmony with your eating schedule by going to bed between 9 p.m. and 11 p.m. and waking up between 5:30 a.m. and 8 a.m. each morning.

She also adds that getting two to five minutes of sunlight first thing in the morning can reset the hypothalamus and help with better hormone regulation.

Integrative medicine doctor and immune specialist Heather Moday, M.D., points out that it's also important to consider the artificial light and blue light you're exposed to before bed.

Previously, she explained to mbg readers, "If you use a tablet for reading, make sure that you always use the backlight dimming feature and/or wear blue-light-blocking glasses one to two hours before bed.