A great deal of research suggests that there are better times to eat – for health, performance and body composition. The science suggests that much of this is related to circadian rhythmicity. Therefore, in this article I will cover a brief overview of circadian rhythmicity, the bidirectional relationship between your circadian rhythm and diet and the broader implications to health.
Keen to go straight to the practical recommendations for when to eat? Click for part 2 here.
What is a circadian rhythm?
Within biology many processes operate in a cyclical manner with specific rhythms – for example, wake/sleep, feed/fast, light/dark, activity/rest.
For example, a sleep cycles operate within a 90 minute duration (this is an ultradian rhythm since it lasts less than 24 hours) and the menstrual cycle operates within a 28 day period (this is an infradian rhythm since it lasts more than 24 hours).
A rhythm of about 24 hours is what we define as a circadian rhythm – examples of processes that have circadian rhythmicity include:
Circadian: about a day (Latin ‘circa diem’)
Chronobiology: specific rhythms that occur in biology due to internal biological clocks.
Chrononutrition: the interplay between nutrition and circadian biology.
The below image shows how different processes within the body are more or less effective during the the 24 hour period.
How are circadian rhythms regulated?
We have a master clock that regulates circadian rhythmicity throughout the body. It is in a part of the brain called the hypothalamus and is a bunch of neurons (brain cells) we call the suprachiasmatic nucleus (SCN).
There are also circadian clocks in cells throughout the rest of the body, such as the gut, liver, pancreas, adipose tissue and muscle – called peripheral clocks.
By definition, a circadian clock maintains this approximate 24-hour time period rhythmicity without any external influences. At a celluar level, this is due to the regulation of gene expression which results in particular physiological processes being more or less active within a 24 hours period, or known as having a diurnal pattern (1).
However, it becomes complex because there are certain factors that can influence and set circadian rhythms – in German the word for this is zeitgeber. The most powerful zeitgeber is light!
We have specialised cells with a photo-pigment called melanopsin, in the part of the eye called the retina, that detect light. The master clock, the SCN, is actually attached to the retina and receives light as a signal, enabling it to synchronise the body to the solar day or day/night time.
The SCN projects to other centres within the brain which can then communicate with the local peripheral circadian clocks throughout the body, coordinating them to be in sync with the day-night cycle
Although the peripheral clocks are regulated by the master clock (SCN), they can also regulate their rhythmicity independently and can be influenced by other zeitgebers that the master clock (SCN) cannot by influenced by, such as when we eat and temperature. These peripheral clocks are important for control the timing and rhythmicity of processes such as digestion, nutrient metabolism and appetite.
Why is your circadian rhythm important?
For physical and mental wellbeing, we require alignment between the master clock (SCN) and peripheral clocks. Disrupted circadian rhythms, which occur when there is misalignment, are associated with poor health and increased risk of chronic disease (2).
When someone is in circadian misalignment, it is likely to manifest as the following: poor blood sugar management, disrupted and elevated hunger, increased risk of insulin resistance and an unhealthy stress response. This is then associated with unhealthy lifestyle habits, such as being sedentary, eating too much and poor sleep, which is then associated with increased risk of being very overweight and developing chronic disease such as type 2 diabetes as well as poor mental health like depression (3).
Therefore we need to sync the habits that can influence the regulation of our circadian rhythm, such as viewing daylight, sleeping and eating food, so that there is alignment and our biological processes operate optimally. Then we can feel and function at our best.
What is the interaction between your circadian rhythm and your diet?
The master clock (SCN) directly and indirectly influences the feeding–fasting cycle and habits around food (e.g. hunger, appetite, fullness). When we eat, in turn influences the peripheral clocks that signal back to the brain through nutrients, hormones and how physically full our stomach is to regulate both food intake and/or circadian rhythmicity.
There is a bi-directional relationship between when we eat and circadian biology –
What this means:
When we eat can impact our circadian rhythm: and has the potential to influence our circadian alignment or misalignment – (although not to the same extent as light).
Our circadian rhythms impacts our response to eating a meal: and therefore there may be “better” or “worse” times to eat based on our circadian clocks.
Many processes that impact how food is broken down, digested and absorbed, have a circadian rhythm: – examples include:
Whilst the research is mixed, it indicates that the following habits may impact our circadian alignment:
But, the outcomes from the research are not conclusive and do vary depending upon who is studied, as well as other factors, such as meal composition and the calorie split of meals etc. More research is definitely needed on people who are active, fit and healthy – which will likely be different.