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Sleep & Circadian Health

Circadian Essentials

A person’s health is shaped by more than the hours they spend asleep. The body follows a daily 24-hour cycle, known as the circadian rhythm, that helps regulate when we feel alert, sleepy, or hungry. This internal rhythm responds to cues such as light and darkness and works alongside our need for sleep to support health across all 24 hours.

Understanding how our internal body clock works can help us recognize the connections between sleep, mood, and energy. It can also help us make positive changes to our daily routines that support better health and wellbeing.

Overview

  • Circadian rhythms are internally generated biological patterns.
  • Most people have an internal clock that runs slightly longer than 24 hours, and morning light resets it each day.
  • Circadian timing and sleep pressure work together, but they are separate coordinated biological systems that support health and wellbeing.
  • Light, activity, meals, hormones, and health conditions influence circadian patterns.
  • Poor sleep or an out-of-sync body clock can disturb the hormones and brain chemicals that carry timing signals, which can affect mood, appetite, and metabolism.
  • Persistent circadian misalignment plays an important role in many diseases and health conditions.

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What is Circadian Health, and Why Does It Matter?

Circadian health means that our internal body clock is working well and helping our body carry out important functions at the right times. These functions follow a daily pattern that helps us stay active during the day and rest at night.

Our circadian rhythms influence many aspects of our health, including:

  • Sleep and alertness: Helping us feel awake during the day and ready for sleep at night.
  • Hormones: Regulating when hormones such as melatonin and cortisol are released.
  • Digestion and metabolism: Helping the body process food and regulate blood sugar throughout the day.
  • Body temperature: Following daily changes that help support activity and sleep.
  • Mood and mental health: Influencing how we feel and function throughout the day.

When our internal body clock is out of sync with our daily schedule, these processes can be disrupted. This can affect how well we sleep and how we feel during the day. Over time, persistent circadian disruption has also been linked to an increased risk of health problems, including diabetes and cardiovascular disease.

Supporting healthy circadian rhythms is therefore about more than sleeping well. It is about helping our brain and body function well across all 24 hours.

Research increasingly associates persistent circadian disruption or misalignment with a range of health problems, including metabolic and cardiovascular conditions. It’s important to note that the relationships between circadian health and disease are complex and can run in both directions. Supporting sustainable alignment is one meaningful way to care for health across all 24 hours.

Circadian Rhythm and Schedule Conflicts

Everyone’s internal body clock is a little different. Some people naturally prefer to wake up early and go to bed early, while others feel more alert later in the day and prefer to stay up later. This natural tendency is called a chronotype. People are sometimes described as “morning larks” or “night owls,” although most fall somewhere in between.

Problems can arise when our natural sleep and wake times don’t match our daily schedules. For example, someone who naturally falls asleep late may struggle with an early work or school schedule. Shift workers may need to sleep during the day when their body clock is telling them to stay awake. Traveling across time zones can also temporarily disrupt our normal sleep patterns.

For some people, difficulties with sleep timing are persistent enough to be diagnosed as a circadian rhythm sleep-wake disorder. These conditions can make it difficult to fall asleep or wake up at the times required by daily life, even when someone is able to get enough sleep on their preferred schedule.

Not everyone has the flexibility to adjust their daily schedules to match their natural body clock. Work demands and caregiving responsibilities can make this especially challenging.

What Influences Our Internal Body Clock?

Our internal body clock responds to signals from our environment and daily routines. Some of the most important influences include:

Light: Natural daylight, especially in the morning, helps keep our body clock in sync with the day and night. Bright light in the evening can shift our internal timing later.

Meals: Eating at regular times helps support daily rhythms involved in digestion and metabolism.

Physical activity: Regular exercise supports healthy sleep and can also influence the timing of our body clock.

Age, health conditions, and medications can also affect circadian rhythms. While we cannot control every influence, understanding how our daily routines affect our body clock can help us make positive changes that support our health.

Circadian Effects Beyond Sleep

The timing of a person’s circadian rhythm matters beyond sleep. Our internal circadian clocks regulate the release of hormones, neurotransmitters, and neuroregulators. These messengers control not just sleep and wakefulness, but also essential biological processes. Our educational section Across The Body explores how sleep, circadian rhythms and different body systems and health conditions are connected in more detail.

Own All 24 Hours

The Own All 24 Hours approach encourages people to look beyond bedtime and recognize how health is connected across day and night. It means understanding personal rhythms, identifying what is within one’s control, and making informed choices. By protecting restorative activities and building sustainable habits that align circadian rhythms with the rhythms of daily life, people can take an active role in their wellbeing and take ownership of all 24 hours.

References

Burns, A. C., Windred, D. P., Rutter, M. K., et al. (2023). Day and night light exposure are associated with psychiatric disorders: An objective light study in >85,000 people. Nature Mental Health, 1, 853–862. https://doi.org/10.1038/s44220-023-00135-8

Facer-Childs, E. R., Middleton, B., Skene, D. J., et al. (2019). Resetting the late timing of ‘night owls’ has a positive impact on mental health and performance. Sleep Medicine, 60, 236–247. https://doi.org/10.1016/j.sleep.2019.05.001

Lloyd, M., Prakash, B. A., Zhao, L., et al. (2025). Circadian rhythms in metabolism and mental health: A reciprocal regulatory network with implications for metabolic and neuropsychiatric disorders. Current Opinion in Physiology, 45, Article 100836. https://doi.org/10.1016/j.cophys.2025.100836

Morris, C. J., Purvis, T. E., Mistretta, J., et al. (2016). Effects of the internal circadian system and circadian misalignment on glucose tolerance in chronic shift workers. The Journal of Clinical Endocrinology & Metabolism, 101(3), 1066–1074. https://doi.org/10.1210/jc.2015-3924

Phillips, A. J. K., Vidafar, P., Burns, A. C., et al. (2019). High sensitivity and interindividual variability in the response of the human circadian system to evening light. Proceedings of the National Academy of Sciences, 116(24), 12019–12024. https://doi.org/10.1073/pnas.1901824116

Wehrens, S. M. T., Christou, S., Isherwood, C., et al. (2017). Meal timing regulates the human circadian system. Current Biology, 27(12), 1768–1775.e3. https://doi.org/10.1016/j.cub.2017.04.059

Youngstedt, S. D., Elliott, J. A., & Kripke, D. F. (2019). Human circadian phase-response curves for exercise. The Journal of Physiology, 597(8), 2253–2268. https://doi.org/10.1113/JP276943

Medical disclaimer
This article is for educational purposes only and is not intended to replace professional medical advice, diagnosis, or treatment. Speak with a qualified healthcare professional about questions or concerns related to your sleep. Read full medical disclaimer

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