Set Your Body Clock: Eating in Time to Optimise Function
When it comes to healthy eating, we naturally tend to focus on what’s on our plate, but we should also be thinking about when we eat.
Research has shown that as many as 50% of us spread our meals and snacks out across 15 or more hours of the day. However, evidence suggests that 10 hours or less may well be an optimal eating window for many of us.
Having periods of time in every 24 hours where we are not eating is essential for repairing, resetting and rejuvenating all of our organs and tissues, and strengthening our immune system.
There are numerous benefits associated with compressing your eating window, or Intermittent Fasting as it is more commonly known, including:
· Better sleep
· Improved digestion
· More energy
· Weight loss
· Improved gut health
· Better kidney function
What happens in our body when we have a period of time when we are not eating?
Chrononutrition studies the relationship between food, metabolism, meal timing, and the circadian system. It is a rapidly developing field of science which is beginning to show that our ancient biology is at odds with our modern lifestyle.
We have evolved with 24 hour light-dark and eating-fasting cycles so we are designed to go through at least 12-14 hours of fasting every day. This fasting has to be consistent from one day to another and occur at the same time so that our body’s internal programme recognises that fasting is going to happen.
After your last bite of food, although you have finished eating, remember that your food still needs to be digested. For the first 5 hours of an overnight fast, the body is busy digesting food and then nutrient absorption occurs in the small intestine for the next few hours. During this time, the body is using most of the glucose and stored glycogen for fuelling.
When it runs low on glycogen, which happens between 7 – 10 hours after our last meal, it slowly begins to burn stored fat to produce ketones. And this is no bad thing because the evidence suggests that ketones are anti-inflammatory and positively influence the immune system.
Timing of food intake may be almost as important as our food choices.
New research on Intermittent Fasting in mice has found that by imposing a strict feeding-fasting cycle, 80% of their genes changed what time they were turned on or off, and how much their expression went up or down. It also found that the effects of fasting are wide reaching and impact nearly every organ in the body.
Controlling meal timings can have a positive impact on sleep, and when sleep improves, repair processes improve. When your fasting period aligns with your sleep, then your gut lining, along with other tissues which get a lot of damage during the day, can start to repair. But this process can only happen when you are in deep sleep and in a fasting state.
Once you are sleeping well, cravings for energy dense foods tend to decrease. So that means, that by controlling meal timings we can inadvertently change our nutrition quality and quality. This has been evidenced in many clinical trials on Intermittent Fasting. By simply eating within an 8 -10 hour window, the caloric intake of participants was shown to modestly reduce without them even trying (in some cases by up to 20%). People also tended to reduce their caffeine and alcohol intake.
The Ideal Body Clock
In first hour after waking our sleep hormones, particularly melatonin, are still very high in most people. During this time the body is not ready to digest food and assimilate nutrients well because melatonin inhibits insulin release from the pancreas. This means that any food or coffee and tea with milk and/or sugar consumed within 1 hour of waking may not be processed properly and will cause blood sugar levels to shoot up.
Breakfast is the first meal that breaks your fast and it is the most important factor for resynchronising your body clock. Although its recommended not to eat for at least one hour after waking, it is far more important that you have your breakfast at a consistent time each day.
Caffeine also resets the body clock so it can be good to wake you up in the mornings however it is best consumed with breakfast, to reduce the acid reflux effects of coffee or tea.
Various studies have shown that our internal circadian body clock is a strong regulator of metabolism. In the first half of the day our blood sugar control is much better. We lose sensitivity to insulin as the day goes on, which means that we are less tolerant of sugar later in the afternoon and in the evening, compared to in the morning.
Essentially we process our food more efficiently in the morning than in the evening. However, the most common pattern of eating in the UK is to consume most of our calories in the evening (roughly 40% of our daily energy intake), and fewer in the morning.
If you habitually consume a lot of energy dense foods later in the day, you’re going to have a longer elevation of sugar and fat in the blood which can contribute to weight gain and put you at risk for various metabolic diseases.
Aligning meal timing with the body’s circadian cycle can help to optimal glucose and insulin responsiveness. A 2023 meta-analysis reported significantly greater weight loss in groups with higher energy intakes earlier in the day compared to groups with high energy intake later in the day, as well as significantly greater reductions in LDL cholesterol, fasting glucose, and insulin resistance.
The recommendation is to have your last meal of the day as early as possible (to fit with your lifestyle) but at least 2 - 3 hours before bed to make sure digestion does not impair sleep quality.
If you behave out of sync with your clock it can take you a few days to recover. As food resets the body clock, if you decide to eat in the middle of the night then your body will think you have moved to a different time zone. So will reset the clock, anticipating that the next day you will also eat in the middle of the night.
Eating late at night when your body is ill prepared, will impair digestion and absorption, turning even healthy food into junk. It will also take you several days for your body clock to readjust back to your regular schedule. During this time, you can feel more hungry, have more gut issues and feel more fatigued.
Eating Windows
Fasting for 12 hours and eating over 12 hours (12:12) has shown some benefits in clinical trials on healthy individuals including better sleep, more energy, reduced acid reflux, better gut health and improved athletic performance. This is often where I ask my clients to start, whilst they condition their bodies to fasting.
In clinical studies, individuals with metabolic syndrome who followed a 14:10 eating window (fasting for 14 hours, eating for 10) saw improvements in weight, blood sugar control, blood pressure and blood triglyceride levels.
I often recommend that clients progress from a 12:12 to a 14:10 eating window, but I don’t often ask them to go beyond this. Although Intermittent Fasting eating windows are down to personal preference, the research shows that there are limited benefits to fasting for longer than 14 hours.
Food, whilst very important, is not the be all and end all when it comes to optimising our fasting windows.
Light exposure is very important because whereas our body clock is set by food, our brain clock is set by sunlight. Our body clock should be in sync with our brain clock but most of us don’t get enough light in the morning to synchronise our brain’s circadian clock and to reduce sleepiness and depression.
Light is a strong anti-depressant so try to get outside for a 30 minute - 1 hour walk first thing, or if you can’t fit that in, take your morning tea outside to drink in the sun. Even on a cloudy day this is an extremely beneficial practice.
Intermittent Fasting may not be suitable for people with eating disorders or existing health conditions. If you are taking regular medication, such as blood pressure or blood sugar lowering medication, please speak to your health provider before following the any of the advice contained in this article.
