Our bodies have a built-in clock known as the “suprachiasmatic nucleus (SCN).” The SCN is a structure within the hypothalamus that contains a collection of neurons that synchronize our internal biological processes with external light cues. This results in biological changes that run on 24-hour cycles known as circadian rhythms. Examples of circadian rhythms include body temperature regulation, sleep-wake cycles, hormone release, alertness, and metabolism. In addition to the SCN — the central clock— we also have peripheral clocks that regulate smaller changes within the cardiovascular, metabolic, endocrine, immune, and reproductive systems. The central clock synchronizes the peripheral clocks through a series of rhythmic signals including hormones, body temperature and eating times. This ensures that certain cellular and physiological processes occur when they are needed. You wouldn’t want to feel hungry in the early dawn because this would wake you up and disrupt your sleep cycle!
Typically, our light-dark cycles are synchronized with eating times, therefore, our central and peripheral clocks are synchronized. This means that our bodies optimally process food at specific times throughout the day, as this is when the biological processes needed to metabolize the nutrients are at their peak. Think of this as an engine. If you need to leave the house at 8:00 A.M. for work, you might turn on the engine ten minutes before to allow it to warm up. This ensures that the engine runs optimally and smoothly. Conversely, if you are called in to work for an emergency at 9:00 P.M., you will start driving without allowing the engine to warm up, making the ride a lot bumpier and potentially harming the motor.
The same applies to your digestive and metabolic systems! When you wake up, following a period of fasting during sleep, your SCN sends signals to the peripheral clock to produce enzymes that will aid in digestion and prepare other processes for nutrient intake. Therefore, once you eat, the food will be broken down and absorbed with ease. If our body is caught off guard with late-night snacking – a time when we normally would not eat – then the digestion and absorption of nutrients will be a lot harder. Furthermore, the energy intake signals to our peripheral clocks that it is time to be alert and awake. Therefore, even though our central clock is telling the peripheral clocks it is nighttime, energy signals are indicating that it is “daytime”. This desynchronization between the central and peripheral clocks can disrupt other circadian cycles, such as sleep.
Studies investigating the impacts of meal timing and metabolic processes found that healthy adults who were given a meal at night presented a higher rise in their blood glucose level as compared to individuals who were given the same meal in the morning. This spike in blood glucose levels results in an increased release of insulin that signals the central clock to increase alertness. Furthermore, desynchrony between the central and peripheral clocks has been linked to increased susceptibility to metabolic and cardiac disease. Desynchrony can also be caused by abnormal activity patterns such as jetlag or night work that result in circadian misalignment and have been linked to lower glucose tolerance and increased insulin resistance. Both factors can contribute to an increased risk for type II diabetes.
A person’s chronotype describes their circadian phase in comparison to the light-dark cycles. This leads to three general chronotypes which are morning, intermediate and evening. Studies that have characterized differences between these group found the evening chronotype – those with activities shifted later in the day – are more susceptible to a series of conditions such as metabolic dysfunction, diabetes, gastrointestinal issues, and cardiovascular issues when compared to morning chronotypes. This further supports the notion that there is a critical interaction between when we eat and how our body processes the food.
Current studies are trying to further understand how we might be able to use meal timing as a method to improve our health! Studies in animal models have found that time-restricted feeding, where mice ate at the same time every day in synchrony with light-dark cycles, attenuated the negative consequences of an unhealthy diet. This suggests that by timing our meals more consistently, we can optimize our metabolism! Further studies are needed to fully characterize the effects and implications of time-restricted eating in humans. One of the important considerations is that our social relationships heavily rely on the sharing of food and timing our days around meals is not always possible. Therefore, while time-restricted eating has the potential to improve our health, it should be approached with caution and consideration.
Daniela is a fourth-year (U3) Physiology student. She is very passionate about understanding the human body and how we can all individually adapt our daily lifestyles to improve its functioning.