Have you heard the term ‘circadian rhythm’ but you’re not quite sure what it means?
In this blog, Dr Lindsay Browning answers some frequently asked questions about the circadian rhythm, how it works and what factors influence it.
What is the circadian rhythm?
Our circadian rhythm is our internal clock, which helps to govern things like when we feel awake, when we feel sleepy and when we release hormones, as well as our temperature and our cognitive functions.
It runs on an approximately 24-hour cycle and is directly influenced by environmental cues (called zeitgebers) – bright light. In order to control when we feel sleepy, our circadian rhythm controls the release of melatonin. This is our sleep hormone.
During the daytime, we do not release much melatonin. As evening starts to draw near, we begin to release melatonin and this increases during the night, while we sleep. It then starts to reduce back to the lowest levels just before we wake up. This rise and fall of melatonin is the circadian rhythm in action.
People who have bedtimes and waketimes that vary by more than 90 minutes day by day tend to have worse health outcomes (Huang, Mariani, & Redline, 2020). In fact, one 2023 study found (Windred, et al., 2023) that how regular / consistent sleep was, was more important than how much sleep people got. Also, cortisol (related to stress) is another hormone that is linked to our circadian rhythm. Usually, cortisol levels peak in the early morning near to when we wake up and then fall to lower levels in the evening to allow for restful sleep.
If we are very stressed and our cortisol levels remain high as we approach bedtime, this can make it harder to fall asleep. Irregular sleep/wake schedules have also been linked to elevated evening cortisol levels (Scheer, Hilton, Mantzoros, & Shea, 2009) which, as well as affecting sleep, can promote visceral (belly) fat storage, as well as poorer glucose regulation and insulin response.
What affects or interrupts our circadian rhythm?
For most of human history, the sun has been our primary source of light. The sun increasing its brightness through the morning to its most intensive during the middle of the day has “anchored” our circadian rhythm. However, the ever-increasing number of other sources of intensive light and a ’24/7 world’ can cause major disruption to our melatonin production and circadian rhythm.
When your circadian rhythm is phase advanced, this means that your bedtime and wake time will move earlier in the day.
When your circadian rhythm is phase delayed, this means that your bedtime and wake-up time will move later in the day.
When people work shifts or experience jet-lag, it becomes necessary to try to actively phase advance or phase delay their circadian rhythm.
What influences our circadian rhythm?
Light is by far the most important factor in moving or disrupting our circadian rhythm. When our eyes see bright light, this sends signals from our eye to the suprachiasmatic nucleus (SCN) which is the brain’s clock to regulate our
circadian rhythm (or our circadian pacemaker).
In fact, manipulating when we are exposed to bright light is part of the therapy used to help treat people with a circadian rhythm disorder. However, the field of circadian research is blooming and new and ongoing studies suggest that more factors, such as when you exercise, socialising, ambient temperature and eating may also play a role in advancing or delaying our circadian rhythm. There is more research needed (especially to determine whether some of these factors are causes, correlations or just effects), but let’s explore each in turn:
1 – Light and light exposure
Light exposure is the most powerful regulator of our circadian rhythm. Bright light exposure suppresses melatonin secretion and helps our body know that it is daytime, whereas lack of bright light exposure helps to promote melatonin release and sleep. If we are exposed to bright lights at the wrong time, this can disrupt our circadian rhythm. Exposure to bright light in the evening, such as from an electronic device, can result in delays in melatonin release, increased evening alertness and a delay in the time it takes to fall asleep.
Many electronic devices now allow you to control the brightness of the screen and reduce the level of blue light emissions (a specific form of light, especially potent for cuing the circadian rhythm) and can even automatically adjust levels through the day and evening to minimise this impact. This is usually called a “night mode” function.
As well as the potential to cause unwanted disruptions to our sleep, specific timing of light exposure can be used to manipulate and shift our circadian rhythm, such as helping to beat jet-lag. As mentioned above, bright light very early in the morning results in a phase advance of the circadian rhythm (bedtime and wake time move earlier in the day), whereas bright light in the evening results in a phase delay of the circadian rhythm (bedtime and wake time will move later in the day).
Generally speaking, getting bright sunshine earlier in the morning will help to move your bedtime and wake time earlier. This helps you to wake up more easily in the morning and then fall asleep earlier at night.
2 – Exercise and movement
Some studies have found that the timing of exercise can either advance or delay someone’s circadian rhythm. For example, exercising in the evening was shown to result in a phase delay (waking up later and going to bed later). Although this may be potentially helpful for someone wanting to stay awake if they are falling asleep too early or adapting to night shifts (Easter, Gupta, Vinent & Ferguson, 2024), vigorous exercise within 90 minutes of bedtime may make it harder to fall asleep, so should be genealogy avoided..
Whereas, some studies suggest exercise earlier in the morning day may was shown to phase advance (cause bedtime and wake time to move earlier in the day), although this is more unclear as it depends on exactly when and how intense the exercise.
It has not been well researched yet, but there is some indication that exercising in the morning may help to wake you up and phase advance your circadian rhythm a little. In which case, it is likely to be a beneficial way to start your day, especially if you struggle to wake up in the mornings.
3 – Eating
The circadian rhythm controls our ability to properly digest food, primarily through controlling the release of hormones, such as insulin, which increases as melatonin release decreases and vice-versa. This means we are not designed to digest food when our circadian rhythm thinks we should be asleep and we are less able to absorb blood sugar from food during the night. In fact, it has been found that shift workers who often have to eat during the night have a higher incidence of digestive issues compared to people who work a typical 9-5 day.
Studies also suggest that the regularity of mealtimes may serve as a signal back to the circadian rhythm. As well as light exposure, eating at the right times is a recommended step to help with jet-lag.
Keeping consistent meal times (such as having a regular time for breakfast, lunch and dinner) also help the circadian rhythm (Wehrens, et al., 2017). Further, generally eating meals earlier in the day rather than having a large meal close to bedtime support better weight control and insulin sensitivity (Reytor-González, et al., 2025).
4 – Temperature
It is long established that our circadian rhythm controls our core body temperature, and that we have a warmer body temperature during the daytime (when we are awake). Then, as bedtime nears, our body temperature starts to drop. Our core body temperature is lowest and melatonin levels are highest during sleep itself. However, recent research published in Nature has looked at ‘circadian clock neurons’ and found that they can be changed through environmental temperature, as well as light exposure, suggesting the temperature could be a cause, rather than an effect, of our circadian rhythms. That would make sense when considering that the sun (providing light and warmth) has been the primary influence of the circadian rhythm throughout human history. More research is needed to fully understand how we might be able to change our circadian rhythm by changing our ambient temperature.
Anything that maintains an elevated body temperature will interrupt your sleep. This is most obvious to people when the weather is particularly hot in the summer and many people struggle to sleep because their body temperature wants to reduce through the evening and during the night.
Some people experience a similar effect when sleeping in a different house or a hotel where the heating or bed covers may cause you to be hotter than you are used to or want to be.
Knowing this information, it may seem counter-intuitive that a warm (but perhaps not hot) bath can actually helps you get to sleep. However, by having a bath, it temporarily and artificially elevates your body temperature and as you cool your body is mimicking what happens naturally to your core body temperature as you feel increasingly sleepy and drift off.
5 – Social connection
This last one is not quite proven as a stand alone factor. Instead, the influence of socialising on our circadian rhythm may actually be due to a number of factors.
Studies have shown that when we increase our social connection (such as by going out to meet people or socialising) we can modify our circadian rhythm to stay awake longer. However, it may not be the social connection itself that is affecting our circadian rhythm, but rather the fact that as we meet people and socialise we are likely to experience increased light exposure in these social settings, compared to staying at home, for example.
How can you work with and strengthen your circadian rhythm?
To strengthen your circadian rhythm it is important to eat meals at the same (and traditional) times each day. If you exercise, avoid vigorous exercise within 90 minutes of bedtime and ensure you get outside each day for natural bright light exposure (especially in the morning to help with falling asleep earlier at night). A brief walk, especially on a sunny day, in late morning is ideal.
In the evening try to avoid bright light close to bedtime, and enable “night mode’ on your devices to dim the screens and help shift the lighting away from the higher blue frequencies. Also, ensure you have natural fibre bed linens that allow your body to naturally cool.
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Full references available on request