A circadian rhythm or circadian cycle, refers to an internal process that modulates the sleep–wake cycle and recurs roughly every 24 hours. This piece delves into how consuming food in alignment with the circadian rhythm can enhance metabolic health.
What is the Circadian Rhythm Diet?
The circadian rhythm diet is essentially time-restricted eating in which you eat within a certain time frame, generally during daylight hours, and fast the rest of the time. Our circadian rhythms are influenced by light and dark, which tells our body when it’s time to wake and when it’s time to sleep. It is said that nearly every tissue and organ in our body is regulated by a biological clock, with a master clock located in our brain called suprachiasmatic nucleus, or SCN (explained in detail further), keeping all the other clocks in sync and on schedule. There is, however, one interesting thing to note about this system. Food actually overrides the clocks in all of our peripheral tissues. It doesn’t affect the brain tissue. The brain keeps going with the sun, but the clocks in the body are actually more controlled by the food that we eat. Food can un-synchronise the clock in your head from the clocks in your body and when they’re not talking well to each other, you don’t get optimal responses (from the body). (13)
Following the circadian rhythm diet entails timing your meals early in daylight hours and beginning your fasting before it gets dark or by 7pm. While following the diet, sleeping and waking up at the same time every day for optimal functioning of the body’s sleep-wake clock is encouraged.
Metabolism and circadian rhythm
Metabolism is a series of chemical processes in each cell transforming the calories we eat into fuel to keep us alive. These processes sustain life, everyday functioning and include breaking down food and drink to energy and building or repairing our bodies.
The circadian timing system or circadian clock plays a crucial role in many biological processes, such as the sleep-wake cycle, hormone secretion, cardiovascular health, glucose homeostasis, and body temperature regulation. Energy balance is also one of the most important cornerstones of metabolic processes, whereas energy imbalance is associated with many diseases (i.e., obesity, diabetes, cardiovascular disease). Circadian clock is the main regulator of metabolism, and this analysis provides an overview of the bidirectional effect of circadian rhythm on metabolic processes and energy balance. The circadian timing system or circadian clock plays a crucial role in many biological processes, but the increase in activities that operate 24/7 and the common usage of television, internet, and mobile phones almost 24 h a day leads to a gradual decrease in the adequate sleeping time. According to recent research, long-term circadian disruptions are associated with many pathological conditions such as premature mortality, obesity, impaired glucose tolerance, diabetes, psychiatric disorders, anxiety, depression, and cancer progression, whereas short-term disruptions are associated with impaired wellness, fatigue, and loss of concentration. In this review, the circadian rhythm in metabolic processes and their effect on energy balance were examined. (1)
The circadian clock can be divided into 2 parts: the central clock, residing in the suprachiasmatic nucleus (SCN) of the hypothalamus, which receives light cues, and the peripheral clocks residing in various tissues throughout the body. The peripheral clocks play an integral and unique role in each of their respective tissues, driving the circadian expression of specific genes involved in a variety of physiological functions. (2)
The main stimulus for the SCN is light . However, blind individuals have circadian cycles (e.g., sleep-wake cycles) and this cycle lasts longer than 24 h. This finding led to the idea that other stimuli, in addition to light, can act as a stimulus for the human biological clock . Scientists found temperature, hormones, nutrients, distribution of nutrients, some nutrients (alone; e.g., glucose, amino acids, ethanol, and retinoic acid), feeding/fasting state, sleep-wake state, physical activity to be effective stimuli for the circadian cycle in various peripheral pathways 
Metabolic homeostasis is an essential component that regulates energy metabolism, especially in adipose tissue. The adipose tissue is a central metabolic organ that regulates the whole-body energy homeostasis. The white adipose tissue functions as a key energy reservoir for other organs, whereas the brown adipose tissue accumulates lipids for cold-induced adaptive thermogenesis. Adipose tissues secrete various hormones, cytokines, and metabolites (termed as adipokines) that control systemic energy balance by regulating appetitive signals from the central nervous system as well as the metabolic activity in peripheral tissues . The adipocyte-derived hormone leptin, which has specific receptors on the hypothalamus, reduces body weight partially by activating the sympathetic nervous system, resulting in increased thermogenesis and energy expenditure by increasing thyroid hormones. In thermogenesis, UCP (uncoupling) protein inhibits ATP synthesis in mitochondria, allowing energy to be consumed as heat. . The release of leptin hormone occurs in a circadian cycle and serum leptin levels peak at night . Thus, disruption of circadian balance can affect leptin secretion, thermogenesis, and energy homeostasis, indirectly.
Role of Cortisol & Melatonin
The master clock in the suprachiasmatic nucleus (SCN) receives light signals from melanopsin, a light-sensing protein in the retina. This light signal is transferred via sympathetic neurons in the SCN to the paraventricular nucleus (PVN), the intermediolateral column (IML) to the superior cervical ganglia then to the pineal gland. This light signal halts the production of melatonin by the pineal gland while stimulating epinephrine production by the adrenals via sympathetic signaling through the IML. The HPA axis along with the autonomic nervous system receive circadian signals and influence each other through neuronal signals and hormonal output. This network controls the circadian output of the adrenal glands in response to light and regulates sensitivity and consequent hormone production. Cortisol and epinephrine act as synchronizers of the circadian rhythm with the peripheral clocks.
Cortisol produced by the adrenal cortex can serve as a measure of a healthy circadian rhythm. According to Endocrine Reviews, since the discovery of the 24-hour variation in glucocorticoid levels in humans, an abundant literature has documented that the daily variation of circulating glucocorticoids is perhaps the largest and most robust circadian rhythm of all blood constituents in mammals. (14) Cortisol and the autonomic nervous system act as a bridge between the master clock and the peripheral clocks. The daily fluctuation in cortisol levels and glucocorticoid receptor (GC) activation play an integral role in modulating signals that direct biological activities confined to periods of light or darkness. (15)
As natural light dims in the evening, the signal to suppress melatonin decreases allowing melatonin to rise. The human body makes melatonin from the enzymatic conversion of serotonin to melatonin.  The circadian rhythm diet, also known as the sun cycle diet endorses timing our meals with the rise and fall of the sun, and the corresponding surges and dips in cortisol. As cortisol has a significant effect on our thyroid hormones, it affects the metabolism of the food we consume. Cortisol surges in the early hours revving up our metabolism. As cortisol declines later in the day, our body starts stepping into fat storage mode. Circadian rhythm eating ties into this predictable pattern of cortisol and other hormones.
Diet-induced thermogenesis (DIT) refers to the increase in metabolic rate that follows the ingestion of food, as well as changes associated with chronic alterations in the overall level of energy intake (i.e. the plane of nutrition). Studies suggest that the circadian system plays a major role in the morning/evening difference in early DIT and may lead to the effects of meal-timing on body weight regulation. Diet-induced thermogenesis (DIT) is lower in the evening and at night than in the morning.
Circadian Rhythm Meal Timing
Energy metabolism and appetite regulating hormones follow circadian rhythms which, when disrupted, could lead to adverse metabolic consequences. Such circadian misalignment, a mismatch between endogenous circadian rhythms and behavior, is most severely experienced by shift workers, due to nighttime wake, daytime sleep, and eating at night. However, circadian misalignment is not restricted to shift workers; milder shifts in sleep and mealtimes, termed social and eating jetlag, are highly prevalent in the general population. Social and eating jetlag result in later mealtimes, which may promote positive energy balance and weight gain. Earlier meal timing, specific to individual circadian patterns, could serve to reduce cardiometabolic disease and aid in weight loss. 
(Read our blog on meal timings and metabolic health here).
According to Dr. Lockley’s (a neuroscientist at the Division of Sleep and Circadian Disorders) statement to a digital publication, when we eat at night, our bodies can’t cope as well. Research affirms that shift workers, who are awake when it’s dark out and eat at unusual times of the day, are more susceptible to health problems that suggest metabolic dysfunction like obesity and cardiovascular disease. When you eat a large meal very late at night, your body is instructed to generate a lot of insulin during a time of day at which it’s used to resting. In the resting phase, your body doesn’t need to use glucose for energy (your body turns to burn stored fat at night) so you end up with excessive glucose in your bloodstream. This can impact your eating habits.
The timing of our meals and their effects on our body’s response dates back a long time. Ancient Chinese medics held the belief that energy flows around our bodies according to the sun’s movements. They believed that our meal times should be in rhythm with the trajectory of the sun. 7- 9 am was dedicated to the stomach, which meant this was the time when the largest meal of the day had to be consumed. 9 -11 am centred around the pancreas and the spleen. 11 am -1 pm was focused on the heart.5 pm-7 pm was considered dinner time, and a light meal was recommended. This time also revolved around the kidney. 
Circadian Rhythm Diet and Insulin Action
According to this study, the circadian timing system consists of a central brain clock in the hypothalamic suprachiasmatic nucleus and various peripheral tissue clocks. The circadian timing system is responsible for the coordination of many daily processes, including the daily rhythm in human glucose metabolism. The central clock regulates food intake, energy expenditure and whole-body insulin sensitivity, and these actions are further fine-tuned by local peripheral clocks. For instance, the peripheral clock in the gut regulates glucose absorption, peripheral clocks in muscle, adipose tissue and liver regulate local insulin sensitivity, and the peripheral clock in the pancreas regulates insulin secretion. Misalignment between different components of the circadian timing system and daily rhythms of sleep–wake behaviour or food intake as a result of genetic, environmental or behavioural factors might be an important contributor to the development of insulin resistance. Specifically, clock gene mutations, exposure to artificial light–dark cycles, disturbed sleep, shift work and social jet lag are factors that might contribute to circadian disruption.
Difference Between Intermittent Fasting and Circadian Rhythm Diet
Intermittent fasting (IF), when followed in the right pattern, not only helps you with weight loss, but also helps in detoxifying your body, regulating your sleep cycle and even reversing type 2 diabetes. However, the same lifestyle pattern can affect your health in a negative way if you constantly change the fasting period. Intermittent fasting that follows the rise and setting of the sun can be termed as circadian-based. It’s natural for diurnal (active during the day) animals such as humans to eat during daylight when we evolved to digest food. When you eat breakfast, you’re breaking an overnight fast with a meal that stimulates your pancreas to pump out more insulin. This hormone tells your cells to store glucose—which controls blood sugar levels and provides your cells with energy—as well as to make more of the PERIOD protein that enables all of your individual cells to “keep time” according to the circadian rhythm and stay in sync with each other. This is supposed to happen every day to ensure your cells are all working on the same schedule. (19)
Adapting IF to your circadian rhythm can have myriad benefits.
Dr. Locke (quoted above) suggests that it may be more beneficial to focus on timing your meals to your circadian clock over how many hours you go without eating. It’s not about restricting. It’s about getting back to a more natural cycle where we don’t eat at night.
Benefits of Circadian Rhythm Diet
In experiments with mice, leading researcher and The Circadian Code author Prof Satchin Panda found those whose calorie intake was not restricted, but whose eating timeframe was restricted to eight to 10 hours, did not become obese or diabetic and had normal liver function and cholesterol levels. Weight loss as a result of eating in line with circadian rhythms is something Prof Heilbronn also found in research she conducted into time-restricted eating, but other notable health benefits included a reduction in blood pressure and cholesterol levels. Eating for a limited amount of time per day gives all the cells in the body time to rest. So, your insulin levels come down and your fasting or nutrient signalling pathways come up. Also, regulating eating patterns allows your body to anticipate and get to know that it can expect a glucose load at certain times of the day, so it’s prepped and can make the most appropriate responses. (20) Moreover, Research shows that your insulin response is better in the beginning of the day compared to later in the day. If you eat identical meals at breakfast, lunch, and dinner, your glucose levels will be the lowest after breakfast and the highest after dinner. Chronically high blood sugar levels are associated with many markers of metabolic disease and pre-diabetes such as insulin resistance, fatty liver and fatty-pancreas disease, and obesity. Excess blood sugar can also damage the blood vessels in the heart, leading to heart disease.
Practicing a circadian rhythm diet can speed up your metabolism by helping restore your sleep-wake cycle. This enables you to achieve REM sleep. REM sleep raises your basal metabolic rate to its highest level during sleep by increasing your body temperature and energy expenditure in the brain. (21)
A circadian diet can also jumpstart a sluggish digestive system. Research shows that the rate of intestinal motility and gastric emptying is higher in the beginning of the day than at night time. Following a circadian rhythm diet may also improve digestive symptoms associated with circadian disturbances such as abdominal pain, bloating, diarrhea, and constipation. 
A circadian rhythm diet may also have beneficial effects for your gut health. Research suggests that stabilizing your circadian rhythm may prevent inflammation and increased intestinal permeability. Restoring your circadian rhythm to its natural state can also reduce your risk of intestinal dysbiosis. This condition occurs when the bad bacteria in your gut outnumber the good bacteria. 
Early meal timing may also enhance your immune system and reduce your risk of health problems. One study found that fasting later in the day when your melatonin levels are high, improves immune cell recovery, specifically B cell recovery. B cells are a type of white blood cell that produces antibodies that signal your immune system to destroy pathogens like bacteria and viruses.
The circadian rhythm affects health by influencing the body temperature, food intake patterns, metabolism and the hormone balance. Circadian rhythm diet is also termed as the body clock diet, where you eat during the daylight within a window of maximum 12 hours and fast for the rest of the 12 or more hours. Diet Induced Thermogenesis (DIT) or Thermic Effect of Food (TEF) is the amount of energy the body utilizes to metabolise food. The DIT is higher in the morning than in the evening and night, when it is in sync with the normal sleep/wake cycle. Adapting intermittent fasting to your circadian rhythm can have myriad benefits. The circadian clock controls food intake, energy expenditure and whole-body insulin sensitivity.
Disclaimer: The contents of this article are for general information and educational purposes only. It neither provides any medical advice nor intends to substitute professional medical opinion on the treatment, diagnosis, prevention or alleviation of any disease, disorder or disability. Always consult with your doctor or qualified healthcare professional about your health condition and/or concerns and before undertaking a new health care regimen including making any dietary or lifestyle changes.