Is there a best time to eat?

10th Jun 2022

We talk about what circadian rhythms are, why giving your gut a rest is important, the role of time-restricted eating or intermittent fasting and some practical tips for circadian health.

Key points

• Our biological timing system
• The history of circadian science
• Why circadian rhythms are so fascinating
• The role of circadian rhythms in glucose metabolism and diabetes
• Health lies in our evening activities
• Meal timing and gut health
• You probably eat outside a 12-hour window
• Breakfast is the most important meal of the day… but not for the reasons we typically talk about
• 6 healthy circadian habits
• Some resources to dive deeper
• Video
• Book
• Research papers

Listen to the full episode here or on your favourite podcast platform.

Our biological timing system

Circadian rhythms coordinate our entire genome and time daily physiology and behaviour. They are generated by circadian clocks found in every cell of the body that tell our genes the best time to be turned on or off. These clocks respond to light signals from the environment to align our internal biology with our surroundings.

Circadian rhythms are much more than sleep and wakefulness. They coordinate our behaviour including when we eat and fast and our physiology such as the release of hormones and the function of our heart. They optimise our entire physiology, contributing to many aspects of health including:

• Improving metabolism which balances our internal nutrient state
• Immune function
• Turning on the repair system that manages microscopic injuries
• Improving the brain’s neurotransmitter balance to optimise mood and intellectual performance

Healthy circadian clocks can promote health. So understanding what may nurture circadian rhythmicity and align our biological clocks to the environment is essential.

Professor Panda’s research looks at the effects of daily patterns of eating and fasting on circadian health and metabolism.

The history of circadian science

We have been living with circadian rhythms for our entire human history but we started understanding them in the last 50 years.

Interestingly, circadian rhythmicity is present in nearly all living forms including animals, plants, and microbes. Plants have circadian rhythms to anticipate the first ray of light in the morning and when the sun is going down. Animals also have circadian rhythms to anticipate and adapt to day-night cycles and temperature changes.

We used to think that these rhythms were driven by day and night until scientists started discovering that, even in the absence of light, sleep-wake cycles were regular. When living in caves for extended periods, people would still wake up and go to bed at regular times. These discoveries lead to our current understanding of circadian rhythms as an endogenous system that can sustain even in the absence of cues from the environment.

So scientists started investigating the genetic basis of circadian rhythms. Famous studies in fruit flies showed that mutations in specific genes influenced the period of circadian rhythms. It took many years for the scientific community to better understand and accept how genes govern circadian rhythmicity.

Why circadian rhythms are so fascinating

​​In the last century, most research has focused on what a gene or molecule does and its impact on diseases. There is rarely a component of timing. However, most diseases are a product of mistiming. In cancer, cell division becomes accelerated. In diabetes, blood sugar does not come down as quickly as it should after a meal. The biology of time is often neglected despite its major impact on health.

The role of circadian rhythms in glucose metabolism and diabetes

Glucose is an essential molecule for life: it gives our cells the energy they need to perform their functions. But too much glucose circulating in the blood is damaging to our health, which is why glucose regulation is essential.

An important organ for regulating blood glucose is the pancreas. Like all cells and tissues, it has a circadian clock that regulates the secretion of insulin. If we eat too early or late at night when the body is not expecting food, the pancreas is not ready and produces a suboptimal response. At night especially, the rising levels of melatonin blunt the release of insulin. So any food coming late at night can cause a much higher rise in glucose than it would during the day when the body is anticipating energy intake. Repeatedly missing this circadian-optimised window of insulin secretion can have a major impact on our blood glucose levels.

Health lies in our evening activities

The evening between 6 pm and midnight is the time when most people are out of work, school or daytime occupations. Habits we fall into during that time every day are essential for health.

We often tend to decompress by eating close to bedtime and having alcohol or other substances that affect sleep. As a result, our sleep-deprived brain the next day is more likely to fall back onto bad habits.

Meal timing and gut health

After your last bite of the night, the stomach takes around 5 hours to digest food and then pass it on to the intestine. So if you finish eating at 6 pm, your stomach will still be digesting until about 11 pm.

Why does it matter?

The gut needs time to heal and repair itself from the damage of digesting all the food we eat in a day. During digestion, cells lining the gut get damaged and need to be replaced or repaired. For efficient repair, we need 2 things:

1/ The gut needs to be relatively empty – not still digesting food from our last meal

2/ A growth hormone that is produced at night when we are asleep

Although these damages to the gut lining are normal, they need to be repaired to avoid gaps in the gut wall. If the gut lining is not repaired over an extended period, toxins from the gut can get into the blood and contribute to low-grade inflammation associated with many diseases.

So giving our gut a rest helps prevent hyperpermeability or ‘leaky gut’ that is involved in many common conditions.

You probably eat outside a 12-hour window

Your daily eating window is the number of hours between your first and last bite during the 24-hour day. Time-restricted eating emphasises a limited and consistent daily eating duration that does not involve calorie restriction. So eating all of our daily energy needs within a consistent 8 to 10-hour time window each day.

Most of us believe we are eating within a short time window. However, when our eating window changes from day to day, the circadian system gets confused – it doesn’t know when to expect food. So if you eat within a 10-hour window each day but the times are constantly changing, you are not truly eating within a short window. Your body is experiencing something similar to jet lag.

Eating within a consistent short period every day is important to allow body cells to anticipate food intake versus rest. So it can time physiology appropriately. Early evidence suggests that time-restricted eating may optimise circadian clock function and improve health. The idea is to maintain a consistent window of eating with small variations.

Breakfast is the most important meal of the day… but not for the reasons we typically talk about

Breakfast is important because:

1/ It’s the meal most of us have control over being typically eaten at home

2/ Eating a filling breakfast reduces cravings for snacks and improves energy levels throughout the day

3/ Eating breakfast at a consistent time each day may support circadian rhythmicity

6 healthy circadian habits

1/ Have a consistent sleep schedule

Go to bed and wake up at consistent times. Try to spend around 8 hours in bed so you can get 7 to 9 hours of restorative sleep.

Why? During sleep, our circadian clocks produce hormones and chemicals to repair the brain and body. Sleep has numerous benefits including synaptic plasticity, learning and memory formation.

2/ Wait for a least 1 h before your first meal after waking up

Avoid eating at least 1h after waking up

Why? So our organs have the time to fully wake up and adjust to daytime: night hormones go down and day hormones go up. Giving our body time after waking up is important for better digestion and regulation of blood glucose.

3/ Time-restricted eating

Concentrating food intake within a consistent 10-hour window each day is the core of circadian optimisation. It allows for a daily period of fasting where the body repairs and regenerates. This revolutionary idea was a breakthrough from Professor Panda’s lab at the Salk Institute almost 10 years ago and is now popularised as intermittent fasting.

Starting from breakfast, eat all your meals within the next 8 to 10 hours, leaving the remaining 12 to 16 hours for fasting.

Why? Eating within a consistent time window each day helps maintain robust circadian rhythms. It helps the body anticipate when you eat so it can optimise nutrient metabolism. The period of fasting each day also allows the body to rest, restore and repair itself. Overall, time-restricted eating may help regulate many areas of health like blood glucose, blood pressure and the immune system.

4/ Get at least 30 minutes of daylight each day

The importance of daylight is another major breakthrough from Professor Panda’s lab.

Step outdoors each day to get at least 30 minutes of daylight, even on cloudy days.

Why? Even on a cloudy day, being outdoors is the best way to re-synchronise our brain clocks. It is especially important for brain health. It reduces depression and elevates mood.

5/ Daily exercise

Exercise every day for at least 30 min, especially in the late afternoon or early evening when muscles are most efficient and the body is more flexible.

Why? Exercise can help synchronise and maintain circadian rhythmicity by entraining the circadian clock to follow a regular 24-hour day/night cycle.

6/ Winding down 2 to 3 hours before going to bed

Create a consistent bedtime routine that you enjoy: dimmed light, no food, calming activities.

Why? Avoiding food close to bedtime promotes gut health by improving digestion and reducing acid reflux. Dimmed lights can promote the production of night hormones like melatonin to help you get into restorative sleep.

For more, including eating strategies for shift workers and the importance of animal research to understand circadian rhythms, listen to the full episode here or on your favourite podcast platform. Simply search ‘The Doctor’s Kitchen Podcast’ - #153 Time Restricted Eating (TRE) with Professor Satchin Panda PhD.

Share your thoughts 💬

How many of these habits do you already integrate into your daily life?

How many habits do you want to add?

What are some tips that help make circadian habits easier?

What are some ideas to improve our societal structures for circadian health?

For example, designing buildings for optimal natural lighting, improving work schedules, shift work, etc.

Some resources to dive deeper

Video

How optimizing circadian rhythms can increase healthy yearsSatchin PandaTEDxBoston

Book

The Circadian Diabetes Code

Research papers

When to Eat: The Importance of Eating Patterns in Health and Disease

Manoogian EN, Chaix A, Panda S. When to eat: the importance of eating patterns in health and disease. Journal of biological rhythms. 2019 Dec;34(6):579-81.

Time-restricted eating and glucose control

Zhao L, Hutchison AT, Liu B, Yates CL, Teong XT, Wittert GA, Thompson CH, Nguyen L, Au J, Manoogian EN, Le HD. Time restricted eating improves glycaemic control and dampens energy-consuming pathways in human adipose tissue. Nutrition. 2022 Jan 4:111583.

Workplace randomised controlled trial to introduce time-restricting eating among shift workers

Manoogian EN, Zadourian A, Lo HC, Gutierrez NR, Shoghi A, Rosander A, Pazargadi A, Wang X, Fleischer JG, Golshan S, Taub PR. Protocol for a randomised controlled trial on the feasibility and effects of 10-hour time-restricted eating on cardiometabolic disease risk among career firefighters doing 24-hour shift work: the Healthy Heroes Study. BMJ open. 2021 Jun 1;11(6):e045537.

Most people eat outside a 12-hour window

Manoogian EN, Wei-Shatzel J, Panda S. Assessing temporal eating pattern in free living humans through the myCircadianClock app. International Journal of Obesity. 2022 Jan 8:1-1.

The importance of natural light to re-synchronise our brain clocks

Wright Jr KP, McHill AW, Birks BR, Griffin BR, Rusterholz T, Chinoy ED. Entrainment of the human circadian clock to the natural light-dark cycle. Current Biology. 2013 Aug 19;23(16):1554-8.

Exercise can help synchronise and maintain circadian rhythmicity by entraining the circadian clock to follow a regular 24-hour day/night cycle

Hower IM, Harper SA, Buford TW. Circadian rhythms, exercise, and cardiovascular health. Journal of circadian rhythms. 2018;16.