Our circadian rhythm, also known as our internal clock or biological clock, controls many of our everyday bodily functions. It synchronizes the sleep-wake rhythm, hormone secretion, metabolism and body temperature. At the heart of this is an approximately 24-hour cycle that is based on the alternation of day and night. Light and darkness have the greatest influence on the internal clock, but factors such as diet, exercise, stress and social interactions also play a role.
Scientific contributions describe the circadian rhythm as a complex sequence of biochemical and neuronal processes that run through the entire organism. Almost every tissue and organ has its own clock, which together with other rhythms - such as the shorter ultradian cycles of 90-120 minutes - form the overall picture of our biorhythm【11】. This integrated rhythm ensures that we feel full of energy in the morning and gradually become tired in the evening.
The master clock in the brain (SCN)
The center of these internal timers is located in the brain. The suprachiasmatic nucleus (SCN) in the hypothalamus acts as a master clock and coordinates the various biological clocks in the body. It consists of thousands of nerve cells that set the rhythm of the internal clock with the help of certain genes and proteins. Among other things, the SCN regulates the production of the sleep hormone melatonin via the pineal gland. When it gets dark in the evening, the master clock sends signals that increase melatonin production and thus trigger the feeling of tiredness; in bright daylight, production is inhibited so that we remain awake and alert. This adaptation to darkness and light helps the body to synchronize the energy balance and stabilize the rhythm. In humans, rhythms are optimized by bright daylight and dark nights - artificial light, on the other hand, can throw them out of sync.
Light, hormones and the hormone balance
In addition to the central clock in the brain, hormones and other messenger substances also influence our rhythm. Melatonin acts as a signal transmitter for sleep and rest at night. Its counterpart, cortisol - also known as the stress hormone - rises in the morning to give us energy for the day and falls in the evening so that we can rest. The sensitive interplay of these hormones is part of our hormone balance and is influenced by our lifestyle, our diet and our light environment. Researchers led by Czeisler have shown that bright light can significantly shift the human circadian pacemaker and thus push back bedtime【1】. Further studies suggest that not only the light intensity, but also the duration and timing of light exposure have a major influence on the internal clock【2】.
Chronobiology investigates these relationships in order to develop chronobiological strategies that can be used to positively influence the biological clock. Metabolic processes and the provision of energy in the body are also controlled by the internal clock【4】.
Chronotype: lark or owl?
The differences between people are reflected in different chronotypes. Some people are larks, i.e. active early in the day and rather tired in the evening, while others are owls, who only get into top form late in the evening and find it difficult to get going in the morning. These differences are partly genetic. Studies by Roenneberg and colleagues indicate that the internal clock in late types ticks slightly longer than 24 hours, while it is slightly shorter in early types【2】. Determining your own chronotype can help you optimize your daily routine. Modern tests and apps analyze sleep and activity data to find out when someone is at their most productive. If you know your chronotype, you can plan your work and leisure time better and thus reduce stress and use your energy in a more targeted way.
Many people suffer from so-called "social jet lag" - permanent tiredness because their natural rhythm does not match their social obligations. Late chronotyped people have to get up early for work or school, for example, even though their internal clock is still signaling "bedtime". Such a constant shift in rhythm acts like a minor jet lag and can impair health and performance in the long term.
Nutrition in harmony with the biorhythm
Research shows that not only what we eat, but also when we eat, influences our metabolism. A 2013 study showed that people who ate their main meal (lunch) before 3 p.m. lost more weight during a 20-week diet than those who ate later in the day【8】. Scientists have also found that insulin sensitivity is higher in the morning than in the evening, meaning that carbohydrates are processed more efficiently in the morning. Many experts therefore recommend eating a richer breakfast and a lighter meal in the evening to keep blood sugar levels stable.
Longer periods of fasting overnight - for example intermittent fasting with 16 hours without food - can trigger a circadian reset and stimulate processes such as autophagy, a cell recycling process that could benefit health. However, the links between fasting, autophagy and health benefits have not yet been confirmed by the EFSA; further studies are needed. Herbs containing bitter substances, such as artichoke, are traditionally used to stimulate digestion and bile flow - here too, the connection has not yet been confirmed by the EFSA, so further studies are needed. In general, regular meals at set times can prevent cravings and help to keep the metabolism in balance. When using herbs and dietary supplements, you should pay attention to approved health claims and seek medical advice if in doubt.
Circadian rhythm disorders: Causes and consequences
Circadian disorders are when the internal clock is permanently out of sync. Common causes are shift work, jet lag due to long-distance travel, irregular bedtimes or excessive screen use in the late evening. The National Sleep Foundation lists a number of sleep problems that can be attributed to rhythm disorders - including Delayed Sleep Phase Disorder and Advanced Sleep Phase Disorder. According to research, a chronic disruption of the rhythm can be associated with an increased risk of obesity, diabetes, depression and cardiovascular disease in the long term.
Shift work: a challenge for the internal clock
Shift work poses a particular challenge for the biorhythm. Shifting or frequently changing working hours can throw your internal clock out of sync. Studies show that regular night shifts increase the risk of metabolic disorders and can lead to chronic sleep deprivation. Those affected should try to keep to a consistent sleep-wake schedule, even on days off. A darkened bedroom during the day is helpful; wearing sunglasses on the way home after a night shift can help to reduce the effect of the morning light and make it easier to fall asleep. If necessary, brief light therapy with bright artificial light before the start of the night shift can be used to boost energy levels for the night.
It is also important to listen to your body's signals: If possible, shift workers should take short breaks or a power nap when tired. Caffeine can be helpful in moderation to stay awake during a night shift, but should be avoided in the hours before the planned sleep.
Jet lag: overcoming the time difference
Crossing several time zones quickly leads to so-called jet lag. Because your internal clock is no longer in sync with the local time, you often feel tired after long-haul flights, have digestive problems and find it difficult to concentrate. To avoid jet lag, it is advisable to gradually shift your sleeping times towards your destination time zone a few days before your trip. During the flight, you should avoid alcohol and excessive caffeine, but drink plenty of water. At your destination, daylight and moderate outdoor exercise will help to initiate the circadian reset, while short naps of less than 30 minutes will alleviate the worst of the tiredness. If you arrive very late, you should try to stay up until the local night to get into the new rhythm more quickly.
In some cases, doctors also recommend melatonin preparations to help you adjust your sleep-wake rhythm more quickly in the event of jet lag. It is important that such preparations are only used in the short term and in the approved dosage (max. 1 mg before going to bed). Statements that go beyond the approved health claim for melatonin should be avoided. If in doubt, it is always advisable to seek medical advice before resorting to sleeping pills.
Melatonin as a sleep aid
Melatonin can also be used specifically as a dietary supplement to make it easier to fall asleep. However, the legal situation in the EU must be strictly observed: There is an officially approved health claim for melatonin, which states that melatonin helps to shorten the time it takes to fall asleep - but only if at least 1 mg is taken shortly before bedtime. All other statements that suggest a therapeutic benefit beyond this are inadmissible. In the case of ingredients without approved health claims, it must always be pointed out that scientific proof is still pending and further studies are required.
Instead of pills, sleep hygiene should be optimized first: A dark, quiet bedroom, a comfortable mattress and avoiding screens directly before bedtime promote restful sleep. A consistent sleep rhythm - going to bed and getting up at the same time every day - also strengthens the effect of the internal clock.
Morning routine: start the day actively
A regular morning routine can help to set the body to 'wake-up mode' every day. Start the day with as much natural light as possible - for example, by taking a short walk outside immediately after getting up or by taking a light shower with a special lamp. Bright light in the morning activates the release of cortisol and has been proven to boost your mood. A healthy breakfast with complex carbohydrates and proteins is just as important: This first meal provides energy and signals the metabolism to start the day. However, if you skip breakfast, you run the risk of cravings later in the day and disrupting insulin regulation.
For many, a cup of coffee is also part of the morning routine, as the caffeine stimulates the circulation. However, pay attention to your personal sensitivity - a small amount is enough for some people, while others cannot tolerate caffeine at all. In addition, a few minutes of meditation or keeping a journal can help you clear your head and start the day with focus. Such rituals give structure to body and mind and support circadian health in the long term.
Evening routine: come to rest
In the evening, it is important to prepare the body for the upcoming rest phase. A fixed evening routine can work wonders here. You should gradually wind down about 1-2 hours before going to bed: Dim bright lights in your home and avoid screens or use blue light filters - blue screen light inhibits melatonin production and keeps us awake. A light snack or a small meal with protein-rich components is fine, whereas heavy, fatty foods just before sleep can put a strain on digestion and disrupt sleep. Alcohol and nicotine also have a negative effect on sleep quality and should be avoided in the evening. Instead, many people swear by a herbal tea (for example with valerian or lavender) or a warm bath to relax the body - however, the claimed calming effects of such home remedies have not been confirmed by the EFSA; further studies are required.
Relaxation techniques such as breathing exercises, light yoga or reading a book also help to relieve the stress of the day. Some rely on essential oils such as lavender oil to create a sleep-promoting atmosphere. Although some report positive effects from such aromatherapy, the links between fragrances and sleep quality have not yet been confirmed by the EFSA; further research is needed. As a general rule: put away electronic devices, dim the lights and choose activities that are calming. This makes it easier to fall asleep. If you have to get up early, you should also go to bed on time - ideally always at the same time - so that you get enough sleep.
Ultradian rhythms: Productive breaks
In addition to the 24-hour rhythm, humans also have ultradian rhythms - these are shorter cycles of around 90 to 120 minutes that determine phases of high concentration and phases of tiredness throughout the day, for example. By scheduling breaks in your working day to match these cycles, you can avoid dips in performance and exhaustion. One strategy that is often used is to take a short break of around 10 minutes after 90 minutes of concentrated work at the latest. During this time, you can get up, get some fresh air or do some exercise. Such regular breaks not only increase productivity, but also help you to stay mentally balanced.
Chronobiology: genes and chronotherapy
Chronobiology also investigates how our genes influence our internal clock. Mutations in certain clock genes such as PER and CRY can lead to extreme sleep-wake cycles. The Nobel Prize winners Hall, Rosbash and Young identified the period protein (product of the PER gene) and its feedback loop back in the 1980s - a discovery that laid the foundation for today's understanding of molecular clocks. More recent studies are investigating gene variants that lead to rare circadian disorders such as Non-24-Hour Sleep-Wake Disorder, in which the sleep-wake rhythm shifts by hours every day. This disorder occurs particularly in blind people, as they lack the crucial light stimulus for synchronization.
Genetic tests can help to detect such deviations. Based on this, chronotherapy - i.e. therapy through targeted adjustment of sleep times, light and activity phases - can be individually tailored. For example, someone with a delayed sleep phase can be gradually guided to earlier bedtimes. It is important to be closely supervised by specialists so that the body adapts gently and no additional stress is caused.
Stress and the biorhythm
The role of stress for the internal clock is often underestimated. Prolonged stress - for example due to high work pressure or personal stress - increases the release of cortisol and adrenaline in the body. These stress hormones put us on constant alert and can throw our circadian rhythm out of balance. Relaxation techniques such as meditation, yoga or progressive muscle relaxation are simple methods to get the body out of alarm mode and into rest mode. Just a few minutes of slow, deep breathing before going to bed can help to reduce stress levels and make it easier to fall asleep. If you get enough exercise during the day and consciously relax in the evening, you help your internal clock to send regular signals and prevent sleep problems.
Sleep rhythm and mental health
Mental health is also linked to the circadian rhythm. Changes in the sleep-wake cycle can contribute to mood swings and increase the risk of depression. People with a disturbed sleep rhythm often feel listless and irritable. Conversely, regular bedtimes and daily sunlight in the morning have a stabilizing effect on mood. A study by Duffy and Czeisler shows that light therapy can be helpful for seasonal low moods - the so-called winter blues【3】. This involves exposing yourself to a bright light source (approx. 10,000 lux) for a certain amount of time in the morning. However, this therapy should not be seen as the sole cure; it can only alleviate symptoms. Professional medical care is essential for severe depression.
A possible link between disrupted circadian rhythms and neurodegenerative diseases such as Alzheimer's and Parkinson's is currently being discussed【9】. However, it is still unclear whether a disrupted biorhythm is the cause or the consequence of the disease. Research in this area is still in its infancy.
Sleep hygiene: sleep better
For healthy sleep, it is important to take a holistic approach to sleep optimization. In addition to fixed routines, external factors such as room temperature, light, noise levels and bed quality also play a major role. The bedroom should be cool, well ventilated, dark and quiet. If ambient noise is disturbing, earplugs or white noise can help. You should avoid caffeinated drinks in the afternoon, as caffeine has a long half-life and can keep you awake in the evening. Heavy meals shortly before going to bed are also unfavorable - it is better to eat a light meal 2-3 hours before going to sleep so that digestion is not disturbed.
A relaxing evening ritual signals to the body that bedtime is approaching. For example, a warm shower or foot bath before going to bed has proven to be effective. Some people also like to drink a detoxifying herbal tea as part of their routine. However, many so-called detox teas lack scientific evidence of their effect - it is important to remain realistic and, if in doubt, rely on tried and tested measures. Although numerous herbs have a calming effect, there are (as yet) no approved health claims for many of them. Therefore, when in doubt, it should always be pointed out that the suspected connections have not yet been fully confirmed scientifically. If you have persistent sleep problems, it is worth seeking professional advice.
Time change: mini-jet lag due to summer time
The change from winter to summer time is stressful for the body. When the clocks go forward one hour in spring(Daylight Saving Time), many people experience a little "jet lag". According to the National Sleep Foundation, this change can reduce the amount of sleep, impair decision-making and productivity and is associated with a short-term increased risk of heart attacks and accidents【13】. These problems arise because the altered exposure to daylight temporarily throws the biorhythm out of sync.
Experts recommend gradually adapting in the week before the time change. This means going to bed 10-15 minutes earlier every evening a few days beforehand and getting up earlier accordingly. Daily activities such as meals and exercise can also be brought forward a little in the days before to make the changeover easier on the body. In the first week after the changeover, you should get as much daylight as possible in the morning hours, as bright light in the morning synchronizes your internal clock most effectively. In the evening, it helps to darken the bedroom well so that you get tired on time despite the longer daylight hours. This can speed up the circadian reset after the time change and reduce the symptoms of mini-jet lag.
Seasons and light
The time of year also has an influence on our biorhythm. In winter, the days are shorter and there is less light. Many people experience a slight drop in energy or even the winter blues (seasonal mood swings with increased tiredness) during this dark time of year. In addition to plenty of exercise in the fresh air, morning light therapy with a special lamp (10,000 lux) can help to stabilize the hormone balance and start the day alert. In summer, on the other hand, the days are very long, which can make it harder to switch off in the evening. Here it makes sense to darken the bedroom well to create a clear difference between day and night. In general, it is worth adapting your own habits to the natural light conditions: In winter, make conscious use of every hour of sunlight and in summer, make sure it is dark in good time in the evening.
Avoid blue light in the evening
In the modern world, screens in particular put a strain on our circadian rhythm. The high blue content of light from LED lamps, smartphones, tablets and computer screens inhibits natural melatonin production in the evening. Experts therefore recommend turning off all electronic devices at least one hour before going to bed or using a blue light filter (night shift mode). Instead of bright ceiling lighting in the late evening, dimmable lamps with warm white light are better suited to signaling the approach of night to the brain. A consciously designed evening routine - for example with reading, journaling or gentle stretching exercises - also helps to calm down. In the office, a rhythmic work schedule can help: Bright light and demanding tasks in the morning, subdued light and routine work in the late afternoon. This conscious play with light and darkness can stabilize the circadian rhythm and regulate energy levels in a natural way.
Biorhythm in the course of life
The circadian rhythm changes over the course of a person's life. Babies do not yet have a stable 24-hour rhythm and have yet to develop one - which is why they sleep around the clock in many short stages at the beginning. Toddlers need significantly more sleep than adults and benefit enormously from fixed bedtimes and evening rituals. During puberty, the chronotype of many adolescents shifts backwards: Teenagers become "owls" who are wide awake for longer in the evening and find it difficult to get out of bed in the morning. Starting school later in the morning could accommodate this natural rhythm in order to improve young people's performance. In old age, the rhythm shifts forward again for many people - senior citizens often wake up very early and get tired earlier in the evening. In addition, the metabolism slows down in old age, which makes an adapted rhythmic diet and regular exercise all the more important. Each stage of life therefore requires slightly different strategies to keep the internal clock in balance.
Chrono-Fitness and NEAT: exercise at the right time
Chrono-fitness means aligning physical activity with your own biorhythm. Early chronotypes (larks) often benefit from morning endurance sessions or yoga, while late chronotypes (owls) tend to reach their peak performance in the late afternoon or evening and can then do strength training, for example. It is important to listen to your body and choose training times that do not interfere with sleep or regeneration. Studies suggest that regular exercise stabilizes the circadian system and reduces stress at the same time【7】.
In addition to planned workouts, everyday exercise also plays a major role. In this context, we talk about NEAT(non-exercise activity thermogenesis) - this refers to all the small activities that burn additional calories without being classic sport. These include, for example, climbing stairs, going for a walk, doing housework or even working standing up. Even simple habits like these keep your metabolism more active. So incorporate as many small exercise sessions as possible into your day: get up from your desk regularly, stretch briefly or go out into the fresh air for a few minutes. This also helps to smooth out the ultradian performance peaks and troughs, as your body and brain receive a short energy boost every now and then.
Social timers: the influence of social life
Humans are social beings - and our social environment also acts as a timer for our internal clock. Regular joint activities give structure to the day and support mental health. For example, having breakfast together as a family, fixed lunch breaks with colleagues or a daily evening walk with your partner can signal to your body what time of day it is. Social interactions stimulate the brain and influence hormones such as oxytocin, which ensures relaxation and connection. Sharing with others also helps to reduce stress and promote healthy habits.
At the same time, it is important to pay attention to your own needs and not overextend yourself. If you notice that everything is getting too much for you, you should consciously plan breaks and be able to say "no" once in a while to take time to relax. A balanced daily routine that includes social, physical and mental activities makes a significant contribution to keeping the circadian rhythm stable and avoiding circadian disruptions.
Technology to support the internal clock
Modern activity trackers and smartwatches can help you to better understand your own biorhythm. They record sleep phases, pulse and movement and provide insights into your personal sleep-wake cycle. Special apps analyze this data and try to determine the ideal time to get up or go to sleep. Light alarm clocks are also popular: they simulate a sunrise in the early hours of the morning and enable a gentler awakening that is less abrupt than a shrill alarm. For people who find it difficult to get going in the morning during the darker months, there are mobile light showers or glasses with built-in LEDs that emit bright light.
As useful as these technical aids are, they should only be seen as a supplement. No one should slavishly follow the instructions of an app if their own body feeling tells them otherwise. In the case of serious sleep problems or pronounced circadian disorders, it is advisable to seek medical advice anyway instead of relying on technology alone. The best support for the internal clock remains a conscious lifestyle with regular habits, sufficient daylight and a healthy balance between activity and rest.
Frequently asked questions about the circadian rhythm
How can I find out my chronotype?
To determine your own chronotype, it is worth paying attention to your personal sleep patterns. As a rule of thumb, if you wake up without an alarm clock, the time roughly corresponds to your natural need to get up. If you sleep significantly longer at the weekend or on vacation than on working days, you probably have a late chronotype (owl). There are also special chronotype questionnaires and apps that provide an assessment based on sleep and activity times. These tools ask, for example, when you normally get tired in the evening and wake up by yourself in the morning. Genetic tests are also available, but are not absolutely necessary for everyday life - honest observation of your own preferences is usually enough.
Can you change your sleep-wake cycle permanently?
Only to a certain extent. The chronotype has a genetic component that cannot be completely changed. However, you can shift your rhythm somewhat with targeted measures. For example, a night person can try to gradually go to bed a little earlier each day and get up earlier in the morning to get used to an earlier schedule. Consistent times (including at weekends) and supportive signals such as bright light in the morning and darkness in the evening are important here. However, the internal clock cannot be completely "reprogrammed" - an owl will never become an absolute lark. However, you can learn to better harmonize your own rhythm with everyday life.
How long does the internal clock need to adapt to a new time zone?
That depends on the number of time zones crossed. As a rough rule of thumb: approximately one day of adjustment time for every hour of time difference. For example, if you fly 6 hours to the east (time advance), it can take around 6 days to get back into the rhythm. If you fly west (if you "gain time"), the body often copes a little faster - here the internal clock can manage up to 1.5 time zones per day. With the right strategies - for example, making some adjustments before the trip and looking for plenty of daylight at the new local time - the adjustment can be accelerated. Nevertheless, the body usually needs several days to completely overcome jet lag.
Are short naps during the day useful or harmful?
If used correctly, short naps - so-called power naps - can be very useful. An afternoon nap of 10 to a maximum of 30 minutes can improve concentration for the rest of the day and reduce tiredness. It is important not to fall into a deep sleep: Therefore, a nap should not last longer than half an hour and should ideally take place more in the early afternoon (e.g. around 1 p.m.). If you sleep too late in the day or for too long, you run the risk of being less able to fall asleep in the evening. However, people who get enough sleep at night usually don't need a nap. The key here is to listen to your own body: If you regularly feel tired after lunch, a short nap can help - as long as it doesn't prevent you from sleeping at night.
Can I catch up on sleep at the weekend?
Many people sleep much longer at the weekend than on weekdays to make up for missed sleep. Sleeping in can actually help to alleviate acute sleep deprivation in the short term - you feel more rested afterwards. However, chronic sleep deprivation cannot be completely "slept off". Our body, especially the brain, needs regular sleep in order to function optimally. If you constantly sleep too little during the week, the weekend can only make up part of the deficit. What's more, sleeping very late at the weekend will upset your internal clock again, which often leads to a mini jet lag on Mondays. It is better to plan to get enough sleep every night and maintain a consistent rhythm. If you still need to catch up on sleep, you should try not to stay in bed until midday, but rather take a short afternoon nap.
Conclusion
The circadian rhythm is a complex, fascinating system that influences almost all aspects of our health. Fortunately, the internal clock can be positively supported by simple means: fixed bedtimes, regular meals, as much daylight as possible, physical exercise, relaxation techniques and a balanced alternation between work, rest and social contact. If you respect your sleep-wake rhythm and adapt it to your personal chronotype, you can not only improve your performance, but also prevent various illnesses and have a more balanced life overall. Much of the knowledge about the internal clock comes from scientific studies - but there are still unanswered questions, especially when it comes to individual differences and new technologies. It is important to always observe the EFSA rules when making health-related statements and to consult experts if you have specific problems or questions. If we succeed in gaining the circadian rhythm as an ally, nothing stands in the way of a healthy and fulfilling life in time with our internal clock.
- Czeisler, C.A., Allan, J.S., Strogatz, S.H., Ronda, J.M., Sánchez, R., Ríos, D., Freitag, W.O., Richardson, G.S., & Kronauer, R.E. (1986). Bright light resets the human circadian pacemaker independent of the timing of the sleep-wake cycle. Science, 233(4764), 667-671. DOI: 10.1126/science.3726555.
- Roenneberg, T., & Merrow, M. (2016). The circadian clock and human health. Current Biology, 26(10), R432-R443. DOI: 10.1016/j.cub.2016.04.011.
- Duffy, J.F., & Czeisler, C.A. (2009). Effect of light on human circadian physiology. Sleep Medicine Clinics, 4(2), 165-177. DOI: 10.1016/j.jsmc.2009.01.004.
- Bass, J., & Takahashi, J.S. (2010). Circadian integration of metabolism and energetics. Science, 330(6009), 1349-1354. DOI: 10.1126/science.1195027.
- Buxton, O.M., Cain, S.W., O'Connor, S.P., Porter, J.H., Duffy, J.F., Wang, W., Czeisler, C.A., & Shea, S.A. (2012). Adverse metabolic consequences in humans of prolonged sleep restriction combined with circadian disruption. Science Translational Medicine, 4(129), 129ra43. DOI: 10.1126/scitranslmed.3003655.
- Albrecht, U. (2012). Circadian clocks and mood-related behaviors. Handbook of Experimental Pharmacology, 217, 227-239. DOI: 10.1007/978-3-642-25950-0_10.
- Kalsbeek, A., la Fleur, S., & Fliers, E. (2014). Circadian control of glucose and lipid metabolism. Progress in Brain Research, 199, 193-218. DOI: 10.1016/B978-0-444-59427-3.00012-8.
- Garaulet, M., Gómez-Abellán, P., Alburquerque-Béjar, J.J., Lee, Y.C., Ordovás, J.M., & Scheer, F.A.J.L. (2013). Timing of food intake predicts weight loss effectiveness. International Journal of Obesity, 37(4), 604-611. DOI: 10.1038/ijo.2012.229.
- Mather, M., et al (2020). Circadian rhythms and neurodegenerative diseases: a cause or effect? Neurobiology of Disease, 134, 104707. DOI: 10.1016/j.nbd.2019.104707.
- Kalsbeek, A., Garidou, M.-L., & Circadian Biology Team. (2014). Circadian control of the daily plasma glucose rhythm: a multifaceted role for the suprachiasmatic nucleus. Journal of Neuroendocrinology, 26(2), 114-125. DOI: 10.1111/jne.12148.
- National Institute of General Medical Sciences (2024). Circadian rhythms fact sheet. Retrieved July 25, 2025, from https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx.
- National Sleep Foundation (2025). Understanding Circadian Rhythms. Retrieved July 25, 2025 from https://www.thensf.org/what-is-a-circadian-rhythm/.
- National Sleep Foundation (2025). How to Prepare for the Start and End of Daylight Saving Time. Retrieved July 25, 2025 from https://www.sleepfoundation.org/circadian-rhythm/how-to-prepare-for-daylight-saving-time.