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A chronodiet takes into account your body’s natural rhythms to determine the optimal time to eat. It has been shown that erratic eating patterns can disrupt the coordination of the body’s systems leading to chronic disease while a  fasting and eating cycle that is matched to your body’s internal clock can prevent or reverse chronic disease (Manoogian, 2017)

I will explore this topic as well as the relationship between shift work, jet leg and health including body weight and cancer.  You can read this blog top to bottom or navigate using this Table of Contents

Chronodiet Glossary

Chronodiet – a pattern of eating that aims to align the master clock in the brain with the peripheral clocks of the digestive system by matching eating and fasting times with light and dark cycles. Also known as chrononutrition.

Chronotype – your preference for an early or late schedule but also takes into account your clock genes, cortisol, melatonin levels, social habits and season. Often classified as morning, intermediate or evening chronotype.

Chronodisruption – the synchronization between external environment cues and the body’s internal physiological processes is out of alignment. This desynchronization of the 24 hour rhythm results in adverse health effects.

Chronopharmacology – an area of research that determines the best time of day to take medications to correspond to the body’s natural rise and fall of hormones and other time dependent factors.

Circadian – circa = approximately and dian=day; around a day or approximately a day

Circadian clock – the body’s natural 24-hour rhythm, also called the “internal time keeper”.

Circadian misalignment –  describes a variety of circumstances including mismatch of sleep to day/night, a mismatch of sleep/wake with eating, or a mismatch of central and peripheral rhythm.

Circadian oscillators – an oscillator is a repeating circuit. A circadian oscillator is a repeating circuit that lasts about a day.

Circadian rhythms – biological rhythms that occur around a 24 hour time frame which help to coordinate and optimize biological, cellular, hormonal and other normal human physiology. Sleep, activity, light, darkness, eating and fasting are some of the key inputs that determine our circadian rhythms.

Clock Genes – also called circadian clock genes, are human genes that influence our bodies cycles. Examples include the CLOCK (circadian locomotor output cycles kaput) gene, cytochromes (CRY1 and CRY2), MTNRIB and PER (Per1, Per2 and Per3), Bmal1 (brain muscle ARNT-like1) and others. Clock genes are part of a two-way relationship; our genes affect our circadian rhythm, while our circadian rhythms affect the expression of these genes. Variations of these genes are associated with type 2 diabetes and inflammation (Jiang, 2017 and Xu, 2021).

Master clock – see Suprachiasmatic nucleus

Peripheral clocks – almost every cell in the body has a clock, these are the peripheral clocks (or peripheral oscillators). They can act independent of the master clock and take their cues from eating and fasting. They can be synchronized or desynchronized with the master clock.

Social Jet Leg – describes when a person’s circadian rhythm is out of synch with their eating, sleeping and waking not caused by travel or crossing time zones. Also used to describe more than 1-hour discrepancy in eating/waking during the workday versus weekend. It is calculated as the midpoint of sleep on days off minus the midpoint of sleep on workdays.

Suprachiasmatic Nucleus (SCN) – this is the ‘master clock’ of the body located in the hypothalamus and is responsible for sensing light levels and the cascade of endocrine and biological responses to light or dark. For example, melatonin will be released as the SCN detects lower light levels. For optimal health the SCN and the peripheral clocks should be in synch. Also called the master circadian pacemaker.

Time Restricted Feeding – a type of intermittent fasting that can be used to align the eating/fasting window with the light/dark levels in an effort to synchronize the peripheral and the master clocks. Read more about that here.

What is the Circadian Clock?

This is a cycle based on the 24 hour rotation of the earth and the change in light and dark levels. The circadian cycles or rhythms affect all living organisms including humans.

Some organisms are nocturnal – active at night and asleep during the day such as an owl and others are diurnal – active during the light and sleeping at night, such as humans and songbirds. Often times, a person’s chrono type is described as a “night owl or an early bird“.

Over the past decade, a large body of research has demonstrated that disruptions of the body’s own circadian clock, can lead to metabolic dysfunctions that are associated with obesity, diabetes, cancer and other metabolic illness (Jiang, 2017).

Human Circadian Rhythm

Our sleep, activity, eating, fasting, light and darkness perception all coordinate to create our unique circadian rhythm. This rhythm impacts many systems in the body including; digestive, metabolic, immune, reproductive, endocrine and cardiovascular as well as the function of the brain (Manoogian, 2017).

Our circadian rhythms evolved to be synchronized with our natural environment – when it was dark out, we fasted and went to sleep and when the sun came up, we ate and were active. Our circadian clocks are designed to help us survive by anticipating the demands for energy use and supply. Since indoor electrical lighting, this has changed and people now create their own individual daily routine not based so strictly on sunrise and sunset.

Circadian Rhythm and Metabolism, Digestion and Obesity

The relationship between circadian rhythm is bidirectional, meaning that your metabolism can affect your circadian rhythm and your circadian rhythm can affect your metabolism.

The organs involved in digestion have their own circadian rhythm. The pancreas, for example, has a circadian clock that tells it to produce a peak amount of insulin at about 5:00 pm (17:00h) but this can be overridden by eating (Manoogian, 2017). Same with the endocrine cells in the stomach that secrete the hunger hormone ghrelin before mealtimes (Laermans, 2015).

Peripheral clocks exist in the organs of digestion including the intestines, liver, lung, skin and fat cells (adipocytes) (Laermans, 2015). In addition to our organs, our microbiome is a collection of millions of live bacteria that have their own circadian rhythm. Animal research has shown that erratic eating patterns can disrupt the type and amount of these microbiome and create a dysbiosis (Frazier, 2020).

Shift work, regular travel across time zones and consuming calorie dense foods throughout the day and night can create metabolic alterations that contribute to obesity. In fact, this phenomenon has a name; chronobesity (Laermans, 2015).

Circadian Rhythm and Cancer

There is growing evidence that disruption of circadian rhythm may be directly linked to cancer (Laermans, 2015). Women working the night shift are at increased risk of developing breast cancer, and this risk increases with the number of hours and years spent working at night (Laermans, 2015).

The current working theory is that exposure to light at night and the resulting reduction in melatonin levels are thought to be the cause. This is why it is recommended to sleep in a dark bedroom, so that you can benefit from the rise in melatonin that comes from the brain sensing the darkness.

Circadian Rhythm and Cardiovascular Disease

While desynchronization of the body’s clocks can lead to obesity, which can lead to cardiovascular disease, there is also a direct effect of circadian misalignment on the cardiovascular system. The cardiovascular system has its own circadian rhythm with blood pressure, heart rate, and cardiac output all affected by a natural rise and fall.

The disruption of this rhythm can lead to direct effects on cardiovascular health . Circadian disruption is associated with high blood pressure and death due to ischemic heart disease (Laermans, 2015).

Circadian Rhythm and Inflammation

Circadian misalignment in non-shift workers has been shown to elevate several inflammatory blood markers including;

• hs-CRP (high sensitivity, C-reactive protein)
• TNF-alpha (tumor necrosis factor alpha)
• IL-6 (interleukin 6)
• IL-10 (interleukin 10)
• Resistin

In addition, circadian misalignment has been shown to increase blood pressure (Morris, 2017). This was also true in shift workers as well as non-shift workers (Morris, 2017).

A small study with 26 healthy young adults that changed their bedtimes, showed that the increase in inflammation and insulin resistance they experienced was not due to sleep deprivation or sleep quality but rather to the in circadian misalignment (Leproult, 2014). As you age, circadian rhythms deteriorates too, which may contribute to a wide variety of age-related disease (Manoogian, 2017).

Can Your Meal Timing Help with Weight Loss?

This was the first study of its type to test whether assigning individuals to a diet based on their chronotype would help them lose weight. This was a 3-month randomized trial, carried out in Spain of all places – a country known for its late dinners.

This study had an interesting way of blinding the study participants and researchers – they didn’t tell them the purpose of the study. For this experience, 209 overweight and obese individuals between the ages of 18 and 65 were recruited.

For 3 months. 104 subjects followed a typical low calorie diet divided into 5 meals and 105 followed the same calorie restricted but where advised how to distribute their calories based on their individual chronotype, which was determined by the Horne-Östberg questionnaire. Those that scored as night owls (16-51 points) were assigned to evening eating, and the early birds (52-86 points) were assigned to a morning eating window.

Specifically, these were the calorie distributions:

Control group:

• 20% at breakfast
• 10% mid-morning
• 35% at lunch
• 10% mid-afternoon
• 25% at dinner

Morning ‘Early Bird’ Chronotype:

• 30% breakfast
• 10% mid-morning
• 35% lunch
• 5% mid-afternoon
• 20% dinner

Evening ‘Night Owl’ Chronotype:

• 20% at breakfast
• 5% mid-morning
• 35% at lunch
• 10% mid-afternoon
• 30% at dinner

All of the meals were designed by the study dietitian in accordance with the assigned calorie distribution and men received 1600-2000 calories per day and women 1000-1500 calories per day.

In addition, participants were asked to do moderate intensity exercise for 30 minutes, at least 5 days per week.

Results

While both groups improved their weight and anthropometrical parameters, the chronodiet group achieved a statistically significant greater reduction in percentage of total body weight loss, BMI (body mass index) and waist circumference compared to the control group, but…they also lost more fat free mass (Galindo-Muñoz, 2019).

When comparing blood results, the Chronodiet group had greater improvement in fasting blood sugar and Tryglyceride/HDL ratio, whereas the control group had better improvement in triglycerides. All participants improved their metabolic syndrome, which makes sense since all meals were based on a Mediterranean diet (Galindo-Muñoz, 2019).

One year later, the results were the same – those in the chronodiet group still had improved results compared to the control group although the long-term effectiveness was less than ideal.

Why Did Chronodiet Work?

One theory put forth by the researches of this study is that there was a higher energy expenditure in the chronodiet group as the calorie intake was synchronized with the circadian metabolism, especially in the first six weeks. After six weeks, the rate of weight loss was similar in both groups, which lead these researchers to suggest that the body then adjusted itself so that the metabolism aligned with the food intake.

Another important aspect of a chronodiet is the type and amount of microorganism in our intestinal tract. The microorganisms include thousands of species, which have millions of genes, which have their own diurnal variations and are therefore sensitive to what, we eat as well as when we eat (Jiang, 2017).

Bottom Line

I’m surprised that changing calorie distribution from 5-10% at a meal would make the difference that it did. To put this into perspective, these diets were 1000-1500 calories per day. A night owl would have 20% of calories at breakfast or about 200-3oo calories, while an early bird would have 30% or 300-450 calories. If this is all it takes to get better results with weight loss, then I think this is worth paying attention to.

Does Eating Late at Night Cause Weight Gain?

In an observational study of 872 middle aged and older adults AARP members (retired persons), researchers set out to discover if meal timing was linked to obesity (Xiao, 2019). Every two months for a year, participants completed a detailed 24 hour dietary report in which they had to record all foods, beverages and supplements from midnight to midnight. They also reported the time in 15 minute intervals and the amount of food consumed. They did one of these every two months.

Height and weight were measured three times throughout the year; months 1, 7 and 12. They also completed the Munich Chronotype Questionnaire and reported the time they went to bed each day as well as the time they got up.

In analyzing the data, the food consumption was divided into four time windows;

1. Morning – within 2 hours after getting out of bed
2. Early midday – before the midday of the waking period
3. Later midday – after the midday of the waking period
4. Night – within 2 hours before bedtime

Analysis by Time Window

The authors found that a higher percent of calories consumed in the morning, within 2 hours of waking was associated with a ~50% decrease in the odds of being overweight. If the person had a morning chronotype an ‘early bird then this association was stronger.

If there was a higher percentage of calories consumed in the night, within 2 hours of going to bed, this was associated with a ~80% increase in the odds of being overweight. For those with a late chronotype ‘night owl’ the odds were increased (Xiao, 2019). This surprised me, as I thought that having a later chronotype would allow someone to eat later at night will less impact on their weight.

Bottom Line

Because this is an observations trial it can only tell you associations e.g. nighttime eating is associated with overweight but not that it caused it. Having said that, I still think it’s important to consider not just what you eat, but when you eat it and to consider your chronotype.

Shift Work

Shift work is an example of dysregulation of the body’s normal circadian rhythm, in which workers are awake, active and eating during the evening or overnight and sleeping during the day. Shift work is associated with an elevated risk of obesity, type 2 diabetes, high triglycerides, low levels of HDL cholesterol, and cardiovascular disease (Laermans, 2015).

In fact, both nurses and flight attendants have been found to have higher rates of breast cancer and this finding has been attributed to shift work, with flight attendants being subjected to jet leg and radiation exposure as well as shift work.

Shift Work and Breast Cancer

In a study published in 2020, researchers from Spain analyzed 12 studies that all examined the relationship between shift work and breast cancer (Fagundo-Rivera, 2020). They examined the results of these studies and saw that most studies demonstrated a relationship between prolonged shift work; specifically, 3 or more night shifts per month for 15 years or more and breast cancer. The risk increased with permanent night shift and long term day-night shift rotations.

Why? There are several theories put forth to explain this relationship, namely;

1. Circadian changes in melatonin levels
2. Genetic changes
3. Changes in hormones such as estrogen and insulin
4. Poorer diet and exercise
5. Lower levels of vitamin D
6. Disturbed sleep
7. Compromised anti-tutor surveillance by the immune system

The risk was greater when the women began their night shifts at a younger age.Of course, other factors played a role in cancer progression too, including quality of sleep, obesity, diabetes, age of menopause, number of pregnancies and hormonal treatment.

Compared to women who had never worked night shift, those that worked 6 consecutive night shifts for 5 years had an 80% increased risk of breast cancer (odds ratio 1.8, 95% confidence interval). These were mostly estrogen sensitive cancers, and so the role of circadian dysrhythmia and changes in melatonin leading to changes in estrogen and progesterone hormones is significant (Fagundo-Rivera, 2020).

Shift Work and Prostate Cancer

This is another study from Spanish researchers (this topic really seems to be taking hold in Spain) that examined shift work and prostate cancer risk. In this study, 465 men between the ages of 40 and 80 who were recently diagnosed with prostate cancer were interviewed about their work history, as were a group of 410 men chosen from the public and matched for age that did not have prostate cancer, these were the ‘controls’.

All the men were asked whether they had worked between 10 pm and 6 am at least three times per month, if so, they were in the ‘night shift’ group. If they had not worked a night shift in the past year, this was the ‘never night shift’ group. All the men completed the Munich ChronoType Questionnaire.

When comparing the men with prostate cancer versus the controls the analysis revealed a risk for prostate cancer with night shift, particularly among those with a rotating night shift. Of the men with prostate cancer, 21% had worked night shift, compared to only 16% of controls. The risk of developing prostate cancer with night shift was 47% (odds ratio 1.47) and if that was rotating night shift, then the risk increased to 73% higher risk compared to non-shift workers (Lozano-Lorca, 2020).

Reducing Health Risks of Shift Work

Night shift work is very important work, but there are risks. The International Agency for Research on Cancer (IARC) has a very comprehensive document on the health risks of shift work and steps to mitigate the risks. Some of their recommendations are below, but if you or someone you love is a shift worker, you should read this entire document (IARC, 2010).

• Quickly rotating shifts are better than slowly rotating
• Clockwise rotation (morning/afternoons/night) is preferable
• Shifts should be regular
• Permanent night shift should include avoiding exposure to bright light after the shift
• No shift work during pregnancy and for 8 weeks after giving birth
• and more…(click on the hyperlink for IARC in the reference section below)

Jet Leg

Jet leg results from repeated rapid travel across time zone which upsets the circadian rhythm. Research in both mice and humans has shown that the irregular eating schedule of jet leg was associated with a failure to maintain the circadian rhythm of the microbiota which promoted glucose intolerance and obesity (Laermans, 2015).

Social Jet Leg

I must say, I love this term, and I can certainly relate to being off my schedule, staying up to late and eating at all hours. It’s basically, jet leg, without leaving home and results in a misalignment of your body’s internal biological clock with your external social time.

The official definition of social jet leg is; the difference between the midpoint of sleep time on your days off and your work days. It peaks in early adulthood and is associated with type 2 diabetes and elevated BMI (Kelly, 2020 and Roenneberg, 2014). Social jet leg is more common in those with a later chronotype (Kelly, 2020).

In a study from researchers in Ireland, they investigated if social jet leg was linked to higher HbA1c levels in adults with type 2 diabetes (Kelly, 2020). HbA1c (hemoglobin that binds glucose) is a blood test that reflects the average blood sugar level over the past three months, as opposed to a fasting blood sugar, which tells you your blood sugar level in a single moment.

Two hundred and fifty-two adults with type 2 diabetes, mean age 62 completed the Munich ChronoType Questionnaire. Shift workers were excluded from this study. This study then looked for associations between social jet leg and other health measures with the HbA1c levels.

There was no link to social jet leg and; BMI, average sleep duration, the number of calories consumed at breakfast or the quality of the sleep. Even when the researchers controlled for age and duration of diabetes, those with social jet leg still had significantly higher HbA1c – in other words, worse control of their blood sugar, with the younger people in this group being more likely to have social jet leg.

In conclusion these authors report that based on previously published research social jet leg is linked to higher levels of blood sugar and insulin resistance and their findings support that as well, showing that social jet leg was linked to higher HbA1c scores. This is more pronounced in young adults with more than 90 minutes of social jet leg.

How Do I Regulate My Circadian Rhythms?

Researchers from Belgium tackled this question and reminded readers of the old adage; ‘Eat breakfast like a king, lunch like a prince and dinner like a pauper’ (Laermans, 2015). While there is some truth to this, as people tend to have their calories disproportionally in the evening, time, your actual distribution could be customized to your chronotype and sleep schedule.

Here are some suggestions to help regular your circadian rhythms;

• Try to keep to a regular scheduled of sleeping, waking and eating
• Avoid eating late at night
• Try to limit diversions between workday and weekend schedules
• If you do shift work, then follow the guidelines listed above
• Sleep in a dark bedroom and limit light from devices while you sleep
• Dinner should be at least 3 hours before going to bed (Laermans, 2015)
• Avoid eating a high fat diet (Laermans, 2015)
• Use time restricted feeding to narrow your eating window
• Blocking blue light in the evening (computer, TV and phone screens)
• Dynamic lighting systems that boost blue light in the morning and red light at night to improve sleep
• Practice good sleep hygiene; cool, dark bedroom, warm bath, crisp sheets, no animals on the bed etc.
• If falling asleep at night is still a challenge you can speak to your doctor about chronobiotics;melatonin, circadian, Rameltoen, Tasimelteon, or Agomelatine.
• To help wake up in the morning speak to your doctor about Orexin

What’s My Chronotype?

Your chronotype is influenced by your age, with a late chronotype peaking in the early twenties (Kelly, 2020). You will need to score your honest answers to this questionnaire to determine your chronotype. The two chronotype questionnaires used in the research are;

Overall, a later chronotype is the unhealthy one. It is linked to depression, poorer health, cardiovascular disease and diabetes (Baron, 2015).

Chronotype and Covid-19

In a survey of 25,000 people in Argentina to assess the effect of covid shut down on chronotype researchers found that the length of sleep increased for those working from home, but no change for those working outside the home. Those working from home had more sleep, but developed a later chronotype (Leone, 2020).

Bottom Line for Chronodiet

The time you eat appears to matter. Since eating is one way to activate your peripheral clocks, eating during the day gives the best opportunity to keep your circadian rhythm in alignment.

In cases where you are not be able to change the time you eat breakfast, lunch or dinner based on work or family schedules; you could  change the proportion of food that you eat at each meal. In the Galindo Muñoz study, making a change of 5-10% of calorie intake at a meal made a difference.

Trying to limit social jet leg by keeping a sleep and wake routine similar from work days to rest days will help and if you work shifts, following the guidelines for safer shift work, like only rotating shifts in a clockwise direction can help to lesson some of the negative health effects of shift work.

Time restricted feeding is one way to help align your circadian clock. You may want to consider to this and you can read more about that in my blog post on;

Intermittent Fasting for Women Over 40

Are you wondering how fasting might affect cancer? If so, check out these two posts;

How To Starve Cancer

Fasting Mimicking Diets

References for Chronodiet

Biggs S. Activity- Are You a Night Owl or an Early Bird? Sleep Health Foundation, 2015. Accessed Jan 2, 2021.

Fagundo-Rivera J, Gómez-Salgado J, García-Iglesias JJ, Gómez-Salgado C, Camacho-Martín S, Ruiz-Frutos C. Relationship between Night Shifts and Risk of Breast Cancer among Nurses: A Systematic Review. Medicina (Kaunas). 2020 Dec 10;56(12):680. doi: 10.3390/medicina56120680. PMID: 33321692; PMCID: PMC7764664.

Frazier K, Frith M, Harris D, Leone VA. Mediators of Host-Microbe Circadian Rhythms in Immunity and Metabolism. Biology (Basel). 2020 Nov 25;9(12):417. doi: 10.3390/biology9120417. PMID: 33255707; PMCID: PMC7761326.

Galindo Muñoz JS, Gómez Gallego M, Díaz Soler I, Barberá Ortega MC, Martínez Cáceres CM, Hernández Morante JJ. Effect of a chronotype-adjusted diet on weight loss effectiveness: A randomized clinical trial. Clin Nutr. 2020 Apr;39(4):1041-1048. doi: 10.1016/j.clnu.2019.05.012. Epub 2019 May 21. PMID: 31153674.

Baron KG, Reid KJ. Circadian misalignment and health. Int Rev Psychiatry. 2014;26(2):139-154.

IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. Painting, Firefighting, and Shiftwork. Lyon (FR): International Agency for Research on Cancer; 2010. (IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, No. 98.) 1, Definition and Occurrence of Exposure. Available from:

Jiang P, Turek FW. Timing of meals: when is as critical as what and how much. Am J Physiol Endocrinol Metab. 2017 May 1;312(5):E369-E380. doi: 10.1152/ajpendo.00295.2016. Epub 2017 Jan 31. PMID: 28143856; PMCID: PMC6105931.

Kelly RM, Finn J, Healy U, Gallen D, Sreenan S, McDermott JH, Coogan AN. Greater social jetlag associates with higher HbA1c in adults with type 2 diabetes: a cross sectional study. Sleep Med. 2020 Feb;66:1-9. doi: 10.1016/j.sleep.2019.07.023. Epub 2019 Aug 5. PMID: 31770613.

Leone MJ, Sigman M, Golombek DA. Effects of lockdown on human sleep and chronotype during the COVID-19 pandemic. Curr Biol. 2020 Aug 17;30(16):R930-R931. doi: 10.1016/j.cub.2020.07.015. Epub 2020 Jul 8. PMID: 32810450; PMCID: PMC7342078.

Leproult R, Holmbäck U, Van Cauter E. Circadian misalignment augments markers of insulin resistance and inflammation, independently of sleep loss. Diabetes. 2014 Jun;63(6):1860-9. doi: 10.2337/db13-1546. Epub 2014 Jan 23. PMID: 24458353; PMCID: PMC4030107.

Lozano-Lorca M, Olmedo-Requena R, Vega-Galindo MV, Vázquez-Alonso F, Jiménez-Pacheco A, Salcedo-Bellido I, Sánchez MJ, Jiménez-Moleón JJ. Night Shift Work, Chronotype, Sleep Duration, and Prostate Cancer Risk: CAPLIFE Study. Int J Environ Res Public Health. 2020 Aug 29;17(17):6300. doi: 10.3390/ijerph17176300. PMID: 32872503; PMCID: PMC7503878.

Manoogian ENC, Panda S. Circadian rhythms, time-restricted feeding, and healthy aging. Ageing Res Rev. 2017 Oct;39:59-67. doi: 10.1016/j.arr.2016.12.006. Epub 2016 Dec 23. PMID: 28017879; PMCID: PMC5814245.

Morris CJ, Purvis TE, Mistretta J, Hu K, Scheer FAJL. Circadian Misalignment Increases C-Reactive Protein and Blood Pressure in Chronic Shift Workers. J Biol Rhythms. 2017 Apr;32(2):154-164. doi: 10.1177/0748730417697537. Epub 2017 Mar 27. PMID: 28347188; PMCID: PMC5858578.

Roenneberg T, Allebrandt KV, Merrow M, Vetter C. Social jetlag and obesity. Curr Biol. 2012 May 22;22(10):939-43. doi: 10.1016/j.cub.2012.03.038. Epub 2012 May 10. Erratum in: Curr Biol. 2013 Apr 22;23(8):737. PMID: 22578422.