Circadian fasting is a method of eating to align with your body’s internal clock. This eating pattern can overlap with intermittent fasting practices, but circadian fasting more strongly emphasizes an earlier eating window. In fact, circadian fasting practices are built on the notion that both modern meal timing habits of three daily meals (plus snacks) and the ability to stay up well after dark promote nighttime eating and therefore may disrupt the body’s natural rhythms and negatively influence metabolic health. Here’s what you need to know about circadian rhythm fasts.
What are circadian rhythms?
Circadian rhythms are 24-hour cycles with corresponding physiological, mental, and behavioral changes.  The cycle can generally be divided into two phases: time awake (light phase) and time asleep (dark phase).  From an evolutionary perspective, the light phase is designed for activity and eating, whereas the dark phase is intended for rest and recovery.
The hypothalamic suprachiasmatic nuclei (SCN) is your body’s master circadian clock. It’s the part of the brain responsible for receiving messages from light-sensing cells of the retina in the eye.  When it’s exposed to light, the SCN sends out signals to wake you up. When it’s dark, the SCN signals that it’s time to start winding down for bed. There are also peripheral clocks throughout the body in cells and organs (like in muscles or the pancreas). Gene studies show that more than 10% of expressed genes in any organ exhibit circadian fluctuations.  And while the circadian clock operates hierarchically—the master clock signaling to the peripheries—some organs may express their own rhythm independent of the SCN control.
Light is the main timer for these clocks, but they can also be entrained—or synced—with both biological rhythms like hormones and body temperature or external rhythms like feeding and fasting. 
Hormones related to circadian rhythms
The SCN is involved in the pathways of multiple hormones that exhibit daily fluctuations.
Melatonin: Melatonin is a hormone that promotes the sleep/wake cycle and its production is stimulated in response to darkness, peaking around midnight to 3 a.m. Light stops melatonin production. 
Cortisol: Cortisol, the stress hormone, is at its highest level in the morning, around 7 or 8 a.m. This provides the boost of stress needed to help you get out of bed. 
Insulin: Insulin is the hormone that helps lower blood sugar. Its peak secretion happens around 5 p.m. and dips to its lowest at 4 a.m. 
Testosterone: Testosterone peaks between 5:30 to 8 a.m., and the lowest circulating testosterone levels are seen about 12 hours later. 
Body temperature's relation to the body's internal clock
Body temperature can vary 0.45 to 0.9 degrees Fahrenheit daily. The lowest body temperature is reached at about 4 a.m. and the highest is around 6 p.m. This fluctuation is pretty consistent daily—even during a fever. 
The reciprocal impact of food and circadian rhythms
The relationship between eating and circadian rhythms is reciprocal. Circadian rhythms can influence energy intake and metabolism, but eating can also influence the expression of some of those peripheral clock genes.  For example, though insulin naturally peaks during the day and dips in the early morning, large meals or snacks at night still provoke the release of insulin—against the hormone’s natural cadence.
What disrupts circadian rhythms?
Circadian rhythms are capable of adapting to biological and environmental changes. But some of these changes may become disruptive and misalign circadian rhythms. Erratic eating and sleeping habits have been shown to increase the risk of chronic diseases and metabolic disorders as well as negatively impact the gut microbiome. [8, 9]
So, say a college student is regularly up until 2 a.m. studying. The bright library and screen lights can override and delay their body’s natural melatonin production. Late nights can also lead to late-night snacking, increasing insulin, and giving the body energy during a time when it's physiologically more primed for rest and recovery. Delayed melatonin secretion may also push back cortisol secretion in the morning since the two hormones are connected.  Together, these hormonal changes may offset your internal alarm clock and cause morning drowsiness.
One way to potentially prevent this disruption and synchronize your internal clocks is through circadian fasting.
What is circadian fasting, and how is it different than intermittent fasting?
Circadian fasting generally refers to narrowing the eating window to daytime hours (light phase) and fasting during the night (dark phase).  Since circadian rhythms are divided into two 12-hour windows, the minimum fast is a 12-hour fast overnight, but the fast can extend to 16 hours if desired. This allows for an eight to 12-hour eating window, usually commencing at a morning meal and ending in the evening. So, an eight-hour circadian eating window could be from 10 a.m to 6 p.m. and a 12-hour eating window could be 8 a.m to 8 p.m.
Compared to intermittent fasting, a circadian fast prioritizes an earlier eating window during the day. Intermittent fasting, specifically time-restricted feeding, involves longer standard fasts of 16 hours and an eight-hour eating window that can occur any time of day. Many people who practice time-restricted feeding choose to fast overnight and don’t break that fast until midday.
What are the potential benefits of circadian rhythm fasting?
Human studies investigating the interaction of food, nutrition, circadian rhythms, and health are still in the early stages but focus on metabolic health and inflammation.
It may improve measures of metabolic health
Meal timing has been most studied for its role in energy regulation, weight, and body mass index (BMI).  For example, research shows that an early peak energy intake (before 3 p.m.) is associated with lower overall calorie intake, a higher-quality diet, and more structured meal patterns. Conversely, a higher percentage of daily calories consumed at night and food intake after 8 p.m. are associated with higher BMI and body fat. 
And that may be due to favorable metabolic changes that occur after eating a meal in the morning compared to the evening—although why these changes occur is not well understood. These changes include: 
- Improved blood sugar control
- Increased thermic effect of food (the body spends more energy breaking down food)
- Enhanced absorption of nutrients in the intestines
A small, 2019 crossover trial found that a 12-hour eating window (from 8 a.m. to 8 p.m) for four days significantly decreased average daily glucose levels, altered lipid metabolism, and positively impacted circadian clock gene expression of those periphery organs.  The combination of poor sleep and circadian disruption may also play a role in blood sugar response to food. A study published in Frontiers in Physiology in October 2021 found that, when paired with recurring circadian disruption, chronic sleep restriction significantly increased blood sugar levels after a morning meal. 
While the exact mechanism of how these rhythmic biologic changes impact metabolic health isn’t fully understood yet, the current evidence indicates that frontloading food intake to earlier in the day improves blood sugar control and energy regulation, and circadian rhythm disruption (such as through night-shift work) negatively impacts metabolic health. 
It may lower inflammation
The frequency and timing of meals can impact markers of inflammation like hsCRP, which is a protein found in your blood as a marker of inflammation. One study collected data from 2,650 participants' calorie consumption between 5 p.m. and midnight, the frequency at which they ate, and the length of their overnight fasts. Analysis of the data showed that each 10% increase in the proportion of nightly calories consumed was linked to a significantly higher concentration of hsCRP regardless of other lifestyle factors.  The researchers concluded that eating more frequently, reducing night-time energy intake, and fasting for longer may help lower inflammation.
While this study illustrates the potential connection between timing and frequency of food intake and inflammation, more research is needed to understand the full effects.
What to know before trying a circadian fast
While fasting is not a new practice, the research of synchronizing your feeding and fasting window with circadian rhythms is still evolving. Most human studies are correlational in design or have a small number of participants and short durations. 
Compared to intermittent fasting when you can set your eating and fasting window whenever you want, circadian fasting requires that you break your fast earlier in the day and begin your fast in the evening to coincide with the sun rising and falling. However, in modern society, it can be more challenging to get in the bulk of calories earlier in the day, as late dinners after work or after picking up the kids from practice are common. To keep to a circadian eating window, you may have to be more observant of when you’re eating. And as daylight hours shorten during the winter months, the fasting window doesn’t have to shorten with it, although you may choose to. The body is capable of handling these types of seasonal variations (even if it does get dark at 4 p.m.) and keeping the majority of your eating window earlier in the day can still support your circadian rhythm. 
If you’re unsure of whether a circadian fast or fasting, in general, is right for you, talk with a healthcare professional.
- Circadian rhythms are 24-hour cycles with corresponding physiological, mental, and behavioral changes
- These rhythms can be synced with hormones, body temperature, eating, and fasting.
- Circadian fasting describes a pattern of eating and fasting that may help sync those rhythms that focus on eating primarily during the daylight hours and fasting overnight.
- Research is still evolving, but getting the bulk of your food and energy earlier in the day is associated with better blood sugar control, weight status, and markers of inflammation.
InsideTracker currently recommends intermittent fasting for specific health goals like healthy aging. Although it’s not specifically a circadian fast, the eating window could be adapted to earlier in the day to align with daylight hours and resemble a circadian fasting schedule.
Quality sleep also plays an integral role in circadian fasting and overall health. InsideTracker analyzes biomarkers like cortisol, glucose, and vitamin D combined with sleep data from your fitness trackers, to deliver personalized recommendations on how to improve sleep quality. Wondering how to get started? Purchase any InsideTracker blood plan and sync your fitness tracker to your InsideTracker app. If your goal is to improve sleep, opt for Ultimate Plan for the most accurate recommendations.
Molly Knudsen, MS, RDNMolly is a Content Writer and Team Nutritionist at InsideTracker. As a Registered Dietitian, Molly enjoys connecting people to the food they eat and how it influences their biomarkers. When she’s not writing about the latest nutrition science, she’s likely in the middle of a yoga flow or at the beach with a good book.