In recent years, an increasing number of researchers and members of the general public have become interested in fasting , a practice that involves intentionally not eating over specific periods of time. Studies suggest fasting can provide benefits like a longer lifespan, reduced risk of cardiovascular problems, and better cognitive ability.
Aligning when you eat with your internal body clock , a practice sometimes called circadian rhythm fasting, requires a specific type of eating schedule thought to provide multiple health benefits. We explore details about circadian rhythm fasting, associated benefits and risks, and tips for those wanting to try it.
What Is Circadian Rhythm Fasting?
Circadian rhythm fasting is a form of time-restricted feeding (TRF) in which a person eats all of their food across a span of hours in the earlier part of the day. There is no universal schedule used for circadian rhythm fasting, but different experts have suggested that it involves confining meals and snacks to a window of time ranging from as few as six hours to as many as 12 hours per day .
Ideal eating hours for a circadian rhythm fast might be between 8 a.m. and 6 p.m., but more research is necessary to confirm. This schedule suggests eating when it makes the most sense, given the daily waxing and waning of various hormones like cortisol, insulin, and leptin. In this hypothetical schedule, a person would be fasting, or abstaining from food, for 14 hours each day, from 6 p.m. to 8 a.m.
Circadian rhythm fasting falls under chrononutrition, or the study of how circadian rhythms and nutrition interrelate. Circadian rhythms are daily cycles the human body naturally carries out, which are driven by a master clock in the brain and informed by external factors like light and food. Sleep schedule, body temperature, hormone levels, and digestion are all greatly affected by circadian rhythms. Disrupted circadian clocks often lead to increased health risks.
Circadian Rhythm Fasting vs. Intermittent Fasting
Circadian rhythm fasting falls under a sub-type of intermittent fasting, but the two terms are not interchangeable.
Intermittent fasting (IF) involves abstaining from eating, either for certain hours each day or certain days each week. Multiple types of fasting can be categorized under the umbrella term “intermittent fasting.” Those engaging in the alternate-day fasting (ADF) sub-type might greatly decrease the number of calories they consume every other day or skip eating altogether for two nonconsecutive days a week. Others who engage in reduced meal frequency (RMF) fasting reduce the span of time in which they eat each day by intentionally skipping a meal.
Circadian rhythm fasting is a form of time-restricted eating (TRE), another sub-type of intermittent fasting. In this type of fasting, a person eats only during specific hours of the day, such as between 8 a.m. and 6 p.m. Unlike other types of intermittent fasting, time-restricted eating does not involve intentionally restricting the amount of calories a person consumes, only the hours during which they consume them. Time-restricted eating is consistent every day, so people practicing it do not go full days without eating.
Although circadian rhythm fasting overlaps with the time-restricted eating sub-type of intermittent fasting, it has the more specific purpose of aligning a person’s eating schedule with their natural circadian rhythm. As a result, circadian rhythm fasting is a form of time-restricted eating that begins earlier in the day. There are also time-restricting eating plans that involve eating later in the day. These two types of fasting are called early and late time-restricted eating, and only the early type aligns eating with the circadian rhythm.
Is Circadian Rhythm Fasting Effective for Weight Loss?
Research suggests that practicing circadian rhythm fasting may promote weight loss in multiple ways:
- Reduced calorie intake: Even though people engaging in circadian rhythm fasting are allowed to eat as much food as they would like, they often end up consuming fewer calories. In fact, one study found that people who stuck with a time-restricted eating plan unintentionally ate 20% fewer calories than they usually would have.
- Reduced appetite: Studies have found that people who are overweight or obese experience a reduction in appetite when practicing early time-restricted eating. This appetite reduction could be due to synchronizing eating with the release of certain hormones in the body.
- Improved metabolism: By aligning meal times with one’s circadian rhythm, a person can optimize their metabolism. Meals are consumed at times when hormones related to eating, like ghrelin and adiponectin, are at their peaks. These strategic meal times mean food is likely to be digested and metabolized more efficiently and less likely to be stored as fat tissue.
Benefits of Circadian Rhythm Fasting
Research suggests circadian rhythm fasting may provide many benefits:
- Weight loss
- Reduced body fat
- Lower blood pressure
- Lower LDL or “bad” cholesterol
- Higher HDL or “good” cholesterol
- Lower fasting insulin levels
- Lower fasting glucose levels
- Reduced insulin resistance
As a result, early time-restricted eating could be a strategy employed to prevent or help manage diabetes, high blood pressure, and high cholesterol. Weight loss might be a particularly valuable benefit of circadian rhythm fasting since many people find it difficult to lose weight or maintain weight loss through lifestyle changes alone, and surgical weight loss methods often come with multiple complications.
Some researchers hypothesize that an early time-restricted eating schedule could indirectly provide additional benefits by strengthening circadian rhythms. When circadian rhythms are undisturbed, a person faces a lower risk of disease and an increased chance of having a longer lifespan. Early studies also suggest that time-restricted eating can potentially help prevent cancer, reduce inflammation, and protect against neurodegenerative disease, but more research is needed.
Risks of Circadian Rhythm Fasting
Most people seem to easily tolerate circadian rhythm fasting or an early time-restricted eating schedule. However, this eating style might not be the best choice for everyone. Since early time-restricted eating tends to result in a reduction of calories consumed, those who are older or have a lower weight could be negatively affected if they do not eat enough while restricting their eating times. People who have experienced disordered eating should also take caution before restricting eating in any way, including by time of day.
Since time-restricted eating does not involve one particular eating schedule, different people might carry it out differently. Setting personal eating times too late in the day could actually cause misalignment with the circadian rhythm. Research suggests that an eating schedule that is misaligned with one’s circadian rhythm can increase a person’s risk of become obese or developing metabolic problems, like insulin resistance. Habits like eating bedtime snacks or skipping breakfast are considered misaligned eating.
What to Know Before Circadian Rhythm Fasting
Talk to your doctor before beginning circadian rhythm fasting or any new eating plan. Also, take into consideration your current eating habits to carefully plan how you can continue to receive the nutrients you need while on an early time-restricted eating schedule. Adhering to circadian rhythm fasting may be difficult for people who work a late shift or have a later family dinner each evening.
If your doctor says time-restricted eating is fine for you, keep these tips in mind as you proceed:
- Timing. Although more research is needed, experts currently think eating between 8 a.m. and 6 p.m. might be the best schedule for those wanting to align with their circadian rhythms.
- Nutrition. Even though restricted eating plans do not specify which foods you should eat, try to stick with the U.S. Department of Agriculture’s dietary guidelines , which recommend eating vegetables, fruits, and whole grains every day while limiting added sugar and salt.
- Drinks. Solid foods are not the only things you are avoiding during fasting hours. Sugary or alcoholic drinks count as consumption. Limit your liquids — except for water — to the eating schedule you set.
- Exercise. Following an eating plan alone is not enough to promote good health. If you are able, follow the Center for Disease Control’s physical activity guidelines , which call for regular cardio and strength exercises.
- Sleep. Optimal sleep is an integral part of health. Make sure to get adequate quantity and quality of sleep.
If you are looking to change your eating habits in order to improve sleep, know that diet and sleep are connected in ways beyond what time of day you eat. Researchers have also studied foods and drinks that promote sleep.