26. March 2026
Fasting and the Circadian Clock
The Benefits of Time Restricted Eating (TRE)
by Michelle McKenzie (BSc, ANutr)
During our hunter gatherer days, we went without food for extended periods of time. We had no choice and our bodies adapted. During the summer months, we piled on the pounds eating lots of fruits, honey, and nuts. We became insulin resistant, which was protective in times of scarcity, and helped us to survive the winter months when food was scarce. When we fast, we trigger this pre-existing information that is encoded in our genes. Our bodies know what to do, and they do it well.
These days we can eat whatever food we want, whenever we want. The type of eating patterns we see today have set the stage for an explosion in cardiometabolic conditions, which are implicated with insulin resistance such as obesity, fatty liver disease, type-2 diabetes, high blood pressure and cardiovascular disease.
Fasting can be relatively easy to slot into your daily routine. Depending on the outcome you are striving for, there are several fasts to try. Time restricted eating, which is ideally timed with your circadian rhythm for maximum benefit, a 24-hour water fast, a 36-hour water fast, a 3-day water fast (this is best done under supervision) and the five-day fasting mimicking diet.
Time restricted eating is safe for most healthy adults, but it isn't for everyone. If you are pregnant or breastfeeding, have a history of disordered eating, are underweight, or are managing type-1 diabetes on insulin, please speak to your GP or a registered nutritionist before making any changes to your eating patterns. The same applies if you are on medication that requires food at specific times. Fasting is a powerful tool — like any powerful tool, it works best when used appropriately for the individual.
Research into fasting is ongoing and has a long list of reported health benefits. Most of the studies focus on time restricted eating (TRE), which is what we will focus on here. TRE is a type of intermittent fast that incorporates an overnight fast and usually an 8–10 or 10–12 hour eating window every day. There are differing opinions on how long the fasting duration should be. Some experts recommend leaving as long as is comfortable between dinner and breakfast. Some even recommend that you miss breakfast altogether and just have lunch and dinner.
There is clear data indicating that the time of day you eat, and the length of your fasting window, can help with weight loss, blood pressure, insulin resistance, glucose response, energy levels, improvements in the microbiome and your overall mood.
Autophagy, Stem Cells and Cancer Support
There are also clinical studies being carried out looking at autophagy, stem cell regeneration and cancer support, with initial results looking very promising.
Autophagy is one of the most exciting areas of fasting research right now. The word literally means "self-eating" — it is the body's built-in cellular housekeeping process, breaking down and recycling damaged or dysfunctional cells. Think of it as a deep clean that only gets triggered when the body isn't busy digesting food. Nobel Prize-winning research by Yoshinori Ohsumi confirmed autophagy as a fundamental biological process, and ongoing studies are exploring its role in reducing the risk of neurodegenerative conditions like Alzheimer's, supporting immune function, and slowing aspects of cellular ageing. Autophagy typically begins to ramp up after around 16–18 hours of fasting, which is one reason some people choose to occasionally extend their window beyond the standard 12–14 hours.
Your Circadian Rhythm — Why Timing Matters
Based on the research on time restricted eating, the data indicates that it is important to give yourself a minimum fasting window of 12 hours every day and ideally have your last meal or snack at 7pm. When you take into consideration our bodies' 24-hour circadian rhythm, this type of meal timing makes a lot more sense. In the evening the body prepares to go to sleep, melatonin is released a couple of hours before bed, digestion slows down — acid secretion, digestive juices, insulin secretion — and every organ in the body gets ready for some downtime.
Eating out of alignment with your circadian rhythm will affect your insulin secretion and when this is compromised it can spike your glucose levels, as our bodies cannot process the glucose properly. If this is a regular occurrence it could eventually lead to type-2 diabetes and fat storage. In the evening, how food moves through our intestines also slows down and the next day it can feel like the food hasn't been digested properly — it actually hasn't.
The simple maths from the stomach's point of view is:
- It takes approximately 5 hours to digest a good-sized meal, so if you have your dinner at 8pm the stomach will continue to work for the next 5 hours. This means that at around 1am our digestive system finally gets ready to have some downtime to repair and reset.
- The stomach needs approximately 7–8 hours of downtime every night to repair the stomach lining. Downtime is triggered after 7 hours without food being digested. So, eating at 8pm, followed by 5 hours to digest, followed by 7–8 hours of repair, means that it would be beneficial to abstain from eating until around 9am the next day.
These are averages — digestion time varies depending on meal size and composition — but they give a useful framework for understanding why late eating has a knock-on effect well into the night.
Early Versus Late Eating Windows
Studies comparing time restricted eating windows — early versus later in the day — assessed whether the same benefits were reported regardless of timing. They also investigated how long was best to wait before having breakfast after waking.
The results revealed something we have all heard before: eat breakfast like a king, lunch like a prince and dinner like a pauper. Eating like this saw improvements in weight loss, blood sugar control, appetite suppression and fertility.
Earlier time restricted eating at 8am to 4pm compared to midday to 9pm was reported in 75% of studies to have the biggest improvement on glycaemic control and blood pressure. A Japanese study also reported greater insulin sensitivity, lower inflammation and greater diversity in the microbiome when adhering to the earlier window.
When we wake in the morning our melatonin levels are dropping and our cortisol is rising. Our pancreas isn't quite ready first thing, so insulin secretion will be affected by these hormonal changes. If possible, delay your breakfast by an hour or two and do not force yourself to eat. The pancreas is much better at responding 2 hours after waking and for the following 6–7 hours, with studies reporting a better glucose response in the morning and afternoon compared to the evening. A controlled feeding study looking at two different eating schedules showed that after a week of front-loading calories earlier in the day — breakfast and lunch — participants were able to reduce their mean glucose levels and increase insulin secretion.
Skipping breakfast was also reported to produce a higher glucose response with the afternoon meal. Plus, those who skipped breakfast tended to snack later in the evening. The conclusion was that it is best to push calories to the first half of the day, and that this lifestyle could be of benefit to anyone who has blood glucose issues or has been diagnosed with pre-diabetes or type-2 diabetes.
Based on the recent studies, it seems that the most benefits were seen when circadian misalignment was avoided, breakfast was delayed by 1–2 hours, and dinner was eaten earlier with eating stopped 3 hours before bed. Eat when you are on the move and not in the evening when you are sedentary — and if you do eat late, consider a lower carbohydrate option, as the timing of macronutrients also matters.
How to Start
If the idea of fasting feels daunting, start small. If you currently eat from 7am to 10pm, simply close the kitchen after 8pm for the first week. The following week, move that to 7pm. Gradually delay breakfast by 30 minutes at a time until you find a window that feels sustainable. Most people find that once they push through the first few days of adjustment — which can involve some hunger and mild irritability as blood sugar stabilises — the pattern becomes surprisingly easy to maintain. The key is not to white-knuckle it. Eat well within your window, prioritise protein and healthy fats to stay satiated, and let the fast take care of itself.
Fasting and Exercise
A common question is whether you should exercise while fasting. Gentle to moderate morning exercise — a walk, yoga, a steady run — is generally well tolerated in a fasted state and may enhance fat oxidation, as the body turns to stored fat for fuel in the absence of glucose. If you are doing high-intensity training or heavy resistance work however, a small amount of protein beforehand or immediately after can help protect muscle tissue and support recovery. Listen to your body. Feeling lightheaded or fatigued during fasted exercise is a signal to adjust, not to push through.
What Breaks Your Fast?
So what breaks your overnight fast? Anything that stimulates digestion or provokes an insulin response. For most people, plain black coffee and unsweetened herbal or green tea are considered acceptable during a fasting window and unlikely to significantly disrupt the metabolic benefits. However, if you are prone to acid reflux or find that coffee triggers hunger, it is better to save it until after your first meal and stick with water or herbal tea on an empty stomach. Milk, sweeteners, bulletproof-style fats, and anything with calories will break your fast. Keep it simple — if in doubt, water is always the right answer.
Should You Start TRE if You Already Feel Healthy?
Absolutely. Your goal is likely to stay healthy as you age, and daily time restricted eating will help you do exactly that. Invite it into your lifestyle and find an eating window that suits you. As an occasional intervention, you could try a 24-hour water fast, and a few times a year consider the five-day fasting mimicking diet.
When deciding how you personally fast, adherence is everything. If it's important to share a meal with loved ones at the end of the day, factor that in and count your window from there.
Let's face it, it's much easier to count hours than calories. And the beauty of time restricted eating is that it costs nothing, requires no special food, and works with the biology you already have.