The Ultimate Guide to Circadian Rhythm Fasting (and How to Make It Work for You)

Think of circadian rhythms as your body’s natural patterns over a 24-hour cycle. Throughout the day and night, your body pumps out chemicals to regulate cravings and behaviors. The interplay of these chemicals — like melatonin for sleep and cortisol for alertness — make up your circadian rhythm. This affects things like:

• waking
• hunger
• sleeping
• body temperature
• when you’re most alert or most relaxed

Humans’ natural circadian rhythms roughly match up with daylight hours. For example, science says that most teens feel the least alert between 3 a.m. and 7 a.m. and most energized between 10 a.m. and 1 p.m (which, honestly, same).

But environmental cues like light can interfere with your circadian rhythms. That’s why you’re more likely to nod off reading a book by candlelight than watching a movie on your phone with the brightness turned up.

Fasting means not consuming calories for a specific amount of time. Circadian rhythm is your body’s natural processes and behaviors over 24 hours.

Put that together and circadian rhythm fasting involves choosing your eating and fasting windows based on your circadian rhythm. Basically, you only consume calories during daylight hours and avoid eating and drinking when it’s dark.

Of course, daylight fluctuates throughout the year in most places. Most folks who do circadian rhythm fasting maintain the same 8- to 12-hour eating window all year long.

Intermittent fasting is an eating plan based on when you eat instead of what you eat. There are different intermittent fasting methods, but the gist is that you switch back and forth between eating and fasting windows. That could be flip-flopping between 8 hours of eating and 16 hours of fasting or even 2 days of normal eating followed by a full day of fasting.

Circadian rhythm fasting is a type of intermittent fasting with eating/fasting windows based on the sun. While all circadian rhythm fasting is intermittent fasting, not all intermittent fasting is circadian rhythm fasting.

Here are the key differences between these two fasting methods:

• Breakfast. Circadian rhythm fasting includes a morning meal, while most intermittent fasting stretches the calorie-free window until lunch.
• Fast length. Intermittent fasting can include longer fasts — even up to a full day.
• Environmental vs. individual. Circadian rhythm fasting is based on daylight hours, while intermittent fasting is up to an individual’s goals or preferences.

OK, let’s get down to the nitty-gritty. Here’s what drives people to eat and fast based on their circadian rhythms.

Healthy weight management

Research has linked high BMI and body fat percentages with consuming most of your daily calories at night (specifically after 8 p.m., which is past dusk at some times of the year).

While it’s tricky to determine whether this is a case of correlation or causation, it makes sense that limiting your eating window would result in less mindless snacking (even though we love some late-night nachos).

A 2020 research review also found that metabolism and circadian rhythm are deeply intertwined, though it’s still too soon to say exactly how.

Prevents blood sugar spikes

A super small 2019 study found that ending your eating window early leads to better blood sugar regulation.

That’s because when you fast, your body starts to rely on fat instead of carbs for energy. The result is stable blood sugar due to decreased insulin resistance. So if you’re a late-night snacker, switching for circadian rhythm fasting will give your body the break it needs to prevent blood sugar spikes and crashes.

Dials down inflammation

Research from 2015 found that reducing evening food intake and fasting through the night could lower systemic inflammation, which contributes to everything from heart disease to cancer.

A 2019 study also verified this. For 29 days, researchers asked 78 men to fast from sunset to sunrise. By the end of the study, the men experienced lower blood pressure and inflammation levels.

Might boost longevity

One 2020 study found that a fasting window of 14+ hours could actually change genes linked to longevity and overall health. In 2017, researchers also found a link between 12+ hours of daily fasting and improved lifespan.

So much of this research has been limited in time and human participants. More long-term research is needed to confirm that circadian rhythm fasting — or any type of fasting — can help you live longer.

Supports heart health

In a 2019 research review, experts found that intermittent fasting reduces risks for heart disease ❤️. While that sounds like a total #win, researchers warned that fasting isn’t healthy for everyone.

Following a circadian rhythm-based diet is pretty straightforward as long as you have a regular schedule. Meaning it’ll be an easier flex for folks with a 9-to-5 versus a night shift job. It’s a challenging intermittent fasting method if you live somewhere with limited winter daylight or limited time to make dinner before dark.

Here are a few ways to optimize circadian rhythm fasting for your life:

• Eat breakfast early. Aim to eat at the same time (7 a.m.) or shortly after dawn each day.
• Avoid artificial light in the evenings. This can disrupt your circadian rhythms.
• Follow a consistent daily schedule. Predictable mealtimes can help regulate your body’s natural rhythms.
• Expose yourself to sunlight. Unlike a paleo or keto diet, a circadian rhythm diet relies heavily on your body’s biological clock. Help yourself out by exposing yourself to light during the day and keeping the lights low at night.

Research on circadian rhythm fasting is still pretty limited, but limiting your calorie consumption to daylight hours could help you shed unhealthy weight, regulate your blood sugar, and improve your overall health.

Another benefit of circadian rhythm fasting is that the “rules” are straightforward and easy to remember.

Always talk with a healthcare pro before changing up your diet or meal schedule, especially if you take evening medications or have an underlying medical condition. Your doc can make sure that you choose the best fasting plan for you.