What Is Circadian Rhythm Fasting?

Circadian rhythms are natural, 24-hour rhythms that influence nearly every process in your body.(1)

Healthy circadian rhythms are associated with good sleep, lower risk of obesity, and other positive health outcomes, while unhealthy circadian rhythms are linked with insomnia, poor sleep, and health problems.(2)(3)

The best-known factors that influence your body clocks are light and darkness, but your activity levels and eating habits also affect your body’s circadian rhythms.(4)

In a nutshell, circadian rhythm fasting is all about timing your intermittent fasting according to your natural circadian rhythms for maximum benefit, no matter your goals.

Whether you’re interested in better sleep, healthy weight loss, or better overall wellness, circadian fasting is worth considering.

How to Do Circadian Fasting- 6 Easy Tips

What Is Circadian Rhythm Fasting?

Circadian rhythm fasting, sometimes called the circadian rhythm diet or sun cycle diet, usually involves an 8- to 12-hour eating window during daylight hours and avoidance of food for the rest of the day.

You can think of circadian rhythm fasting as nearly identical to any other form of intermittent fasting — except for the fact that the timing of the eating window must coincide with daylight hours.

And similar to other types of fasting, people use circadian rhythm fasting for a variety of goals: weight loss, general health, and disease prevention. 

But unlike most fasting methods that encourage users to fast whenever they wish, circadian rhythm fasting may be more beneficial for improving sleep. 

That’s because of the relationship between meal timing and circadian rhythms. Let’s learn how it works!

Does Food Affect Circadian Rhythm?

Yes!

According to a study published in 2017, the timing of meals can affect your body’s cell-based internal clocks.(5)

In the study, healthy participants received three meals per day on either an early or a late schedule. The late meal schedule shifted the time of day of some metabolic processes and also affected gene expression in adipose (fatty) tissue(5). 

While the study did not find sleep differences between the two groups, it may have been too short to detect those changes.

A separate study conducted in 2011 did find that eating a large meal before bed can negatively affect sleep quality and make it harder to get to sleep, especially for women.(6

The Science of Mealtime and Circadian Rhythm

For optimal health, your entire body needs to maintain proper timing. Every cell, organ, and process in your body is meant to run on a 24-hour schedule.(4)

Food intake is only one factor that influences your circadian clocks (the biggest ones are light and darkness in your environment), but researchers hypothesize that eating at the “right” or “wrong” times can still have a significant effect on your overall health.(5)

In nature, diurnal species (animals that are active during the daytime) mainly eat during daylight and become less active at night while they fast and rest. Because humans are naturally diurnal, nighttime eating is unnatural for us.(7)

That’s not to say that humans can never eat after dark, but rather that we appear to have evolved to eat most of our calories when it’s light out.(7)

Eating during daylight hours and fasting when it’s dark also allows more fat-burning, autophagy (cellular recycling), and repair of oxidative stress (cellular damage) to occur at night.(8)(9)(10

Other evidence suggests that eating patterns that shift the feeding time to earlier in the day may help reduce body weight, lower blood sugar, decrease insulin resistance, improve insulin sensitivity, and reduce the risk of heart disease and cardiovascular disease.(10)

And conversely, keeping an inconsistent sleep schedule, having poor circadian rhythms, and eating at night are all linked to higher rates of obesity and type 2 diabetes, elevated nighttime blood glucose, and other metabolic disorders.(11)(12)(13)

Does Fasting Reset Circadian Rhythm?

According to early scientific evidence, some types of fasting can probably help reset your circadian rhythms.

A recent study from 2020 found that in rats, limiting food intake to a 5-hour eating window helped shift the circadian rhythm of food intake.(14)

While humans aren’t identical to rats, it stands to reason that restricting your meals to a very limited window for a while could be helpful for changing your meal schedule.

For example, if you struggle with nighttime snacking, you could try a few weeks of eating only during a limited morning window of five hours or so. After this initial period, you may find it easier to only eat during daylight hours going forward.

There are other approaches that might work well for you, as we’ll cover below.

The Science of Mealtime and Circadian Rhythm

How to Do Circadian Fasting: 6 Easy Tips

Avoid Nighttime Eating 

At its core, circadian rhythm fasting is really about avoiding nighttime eating.

There are several different ways to define what “nighttime eating” means, but the basic idea is simply to get most of your calories during daytime (and preferably earlier in the day).

Your eating window should end when it’s dark outside (which will change with the seasons), or even earlier.

Skip Dinner, Not Breakfast

If you already have an intermittent fasting (IF) schedule in place, pay close attention to this tip.

Most people who use IF for health or weight loss skip breakfast. They begin their eating window sometime in the afternoon, then continue eating into the evening, or begin eating in the evening and finish up before bed.

You can easily transition from conventional IF to circadian intermittent fasting by skipping dinner instead of breakfast. 

And according to one study, eating a meal at 8 am rather than 8 pm has double the thermogenic (calorie-burning) effect in the context of intermittent fasting.(15

Don’t Eat Erratically

Regardless of which fasting schedule you choose, avoid eating erratically. 

According to some evidence, eating at random times that vary each day may “confuse” your body (and your circadian rhythms) and cause health problems, so it’s better to strive for consistency.(8

You can still change your fasting window when necessary, but try to limit how often you do this.

Longer Fasts and Daytime Fasts are OK, Too

Extended fasting windows (more than 24 hours) are excellent for your cellular health because they boost autophagy (cellular recycling), ketone production, and immune function more than shorter daily fasts.(16)

If you want to experiment with longer fast windows, they’re perfectly compatible with a circadian fasting approach. Just remember when you end any extended fast, the eating window should occur during the daytime.

Time Exercise and Physical Activity Carefully

Exercising in the fasted state can increase fat-burning, but may also reduce exercise performance.(17)

That means fasted exercise is good for fat loss, but not helpful when high performance is required (such as during intensive training or competition).

According to one study, performing cardio in the fasted state right before breakfast can increase fat oxidation for a full 24 hours, so it’s still a good idea to do morning cardio if you want to burn fat during circadian fasting.(18)

But overall, because circadian rhythm fasting means eating breakfast but skipping dinner, you’ve got to think even more carefully about when (and how) to be active for your goals. 

Skip Dinner, Not Breakfast

Manage Light in Your Environment

Exposure to light is the single most significant factor that determines your circadian rhythms, and you’ll also get better results from circadian fasting by accounting for light.

Starting your day with bright, natural light helps set your suprachiasmatic nucleus (SCN) or circadian pacemaker, an organ that helps control the release of cortisol (the primary hormone that determines your wake cycle).(19)

The SCN is also responsible for controlling melatonin release after dark, which affects the timing and quality of your sleep.(20)

A few simple ways to help your SCN, and feel energized:

  • Shortly after waking, spend a few minutes getting natural morning light on your eyes and skin.
  • Get exposure to natural light at regular intervals throughout the day.
  • Minimize exposure to light after the sun has set. Light no brighter than moonlight or candlelight is most ideal to set you up for a deep, restful sleep.

Final Thoughts

Circadian rhythm fasting may sound complicated, but it’s actually quite easy to tailor to your individual needs.

At its easiest, circadian fasting can mean eating only during a 12-hour daylight window or simply skipping dinner. 

However, if you’re new to circadian rhythm fasting, or new to fasting altogether, be sure to give your body time to adjust. 

You may need to stick with it for a few weeks or longer before you see significant changes, especially if you’re looking to reverse bad habits like nighttime eating.

Finally, to enhance the effects of intermittent fasting, remember to look at the big picture of circadian rhythms, which also includes light, darkness, and activity timing.

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Circadian Rhythm Fasting: The Science & How to Start
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Circadian Rhythm Fasting: The Science & How to Start
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Circadian rhythm fasting can enhance sleep, weight loss, and wellness. Learn the science of circadian fasting and how to get started today.
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