Different systems of the body follow circadian rhythms that are synchronized with a master clock in the brain. This master clock is directly influenced by environmental cues, especially light, which is why circadian rhythms are tied to the cycle of day and night.
When properly aligned, a circadian rhythm can promote consistent and restorative sleep. But when this circadian rhythm is thrown off, it can create significant sleeping problems, including insomnia. Research is also revealing that circadian rhythms play an integral role in diverse aspects of physical and mental health.
In people, circadian rhythms coordinate mental and physical systems Trusted Source National Library of Medicine, Biotech Information The National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information. See Full Reference throughout the body. The digestive system produces proteins Trusted Source National Institute of General Medical Sciences (NIGMS) The NIGMS supports basic research that increases our understanding of biological processes and lays the foundation for advances in disease diagnosis, treatment, and prevention. See Full Reference to match the typical timing of meals, and the endocrine system regulates hormones to suit normal energy expenditure.
The circadian rhythms throughout the body are connected to a master clock, Trusted Source National Institute of General Medical Sciences (NIGMS) The NIGMS supports basic research that increases our understanding of biological processes and lays the foundation for advances in disease diagnosis, treatment, and prevention. See Full Reference sometimes referred to as the circadian pacemaker, located in the brain. Specifically, it is found in the suprachiasmatic nucleus (SCN), which is in a part of the brain called the hypothalamus. At different times of the day, clock genes Trusted Source National Institute of General Medical Sciences (NIGMS) The NIGMS supports basic research that increases our understanding of biological processes and lays the foundation for advances in disease diagnosis, treatment, and prevention. See Full Reference in the SCN send signals to regulate activity throughout the body.
The SCN is highly sensitive to light, which serves as a critical external cue that influences the signals sent by the SCN to coordinate internal clocks in the body. For this reason, circadian rhythms are closely connected to day and night. While other cues, like exercise, social activity, and temperature, can affect the master clock, light is the most powerful influence on circadian rhythms.
Biological clocks help regulate the timing of bodily processes, including circadian rhythms. A circadian rhythm is an effect of a biological clock, but not all biological clocks are circadian. For instance, plants adjust to changing seasons using a biological clock with timing that is distinct from a 24-hour cycle.
In this way, our circadian rhythm aligns our sleep and wakefulness with day and night Trusted Source Division of Sleep Medicine at Harvard Medical School A production of WGBH Educational Foundation and the Harvard Medical School Division of Sleep Medicine. See Full Reference to create a stable cycle of restorative rest that enables increased daytime activity.
While the sleep-wake cycle is one of the most prominent circadian rhythms, these 24-hour internal clocks play a vital role in virtually all systems of the body Trusted Source National Institute of General Medical Sciences (NIGMS) Blog The NIGMS supports basic research that increases our understanding of biological processes and lays the foundation for advances in disease diagnosis, treatment, and prevention. See Full Reference .
Research continues to uncover details about circadian rhythms, but evidence has connected them Trusted Source National Library of Medicine, Biotech Information The National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information. See Full Reference to metabolism and weight through the regulation of blood sugar and cholesterol. Circadian rhythms influence mental health as well, including the risk of psychiatric illnesses like depression and bipolar disorder as well as the potential for neurodegenerative diseases Trusted Source Medline Plus MedlinePlus is an online health information resource for patients and their families and friends. See Full Reference like dementia.
There are indications that circadian rhythms have an important influence on the immune system as well as processes of DNA repair that are involved in preventing cancer Trusted Source National Library of Medicine, Biotech Information The National Center for Biotechnology Information advances science and health by providing access to biomedical and genomic information. See Full Reference . Early-stage research indicates that circadian cycles can influence the effectiveness of anti-cancer drugs Trusted Source National Cancer Institute (NCI) The NCI is the federal government's principal agency for cancer research and training. See Full Reference and that new medications may be able to harness biological clocks to kill cancer cells.
Disruptions to circadian rhythm can occur over the short- or long-term. Experts have identified a number of types of circadian rhythm sleep-wake disorders (CRSWD) based on their characteristics and causes.
The circadian clock system is necessary to adapt endogenous physiological functions to daily variations in environmental conditions. Abnormality in circadian rhythms, such as the sleep-wake cycle and the timing of hormonal secretions, is implicated in various physiological and psychiatrical disorders. Recent molecular studies have revealed that oscillation in the transcription of specific clock genes plays a central role in the generation of 24h cycles of physiology and behavior. It has been noticed that patients receiving chemotherapeutic agents experience disturbances in their behavioral and physical performances, including circadian rhythms. To explore the underlying mechanism of chemotherapeutic agent-induced disturbance of these rhythms, we investigated the influence of 5-fluorouracil (5-FU), one of the most widely used chemotherapeutic agents for the treatment of cancers, on the expression of clock genes. Treatment of cultured NIH3T3 cells with 5-FU for 48 h resulted in a significant reduction of mRNA levels of Period1 (Per1) and Period2 (Per2) without affecting cell viability; however, treatment with the same amount of uracil, a structural analog of 5-FU, had little effect on the expression of clock genes. Consistent with its inhibitory actions, continuous administration of 5-FU (2 mg/kg/h) to mice attenuated the oscillation in the expressions of Per1 and Per2 in the liver and suprachiasmatic nuclei, the center of the mammalian circadian clock. These results reveal a possible pharmacological action by the chemotherapeutic agent 5-FU on the circadian clock mechanism, which is the underlying cause of its adverse effects on 24-h rhythms of physiology and behavior.
Due to our modern 24-hour society, nearly 15 million people in the United States work full-time night shifts, evening shifts, rotational shifts, or other such irregular schedules. What is more, a poll by American analytics company Gallup showed that almost 4 in 10 adults said they work at least 50 hours every week.
Circadian rhythms run in 24-hour cycles and are significantly influenced by the natural light and dark cycles. Many bodily processes that are active in the daytime slow down at night to prepare for sleep. At night, the circadian pacemaker releases the sleep hormone melatonin from the pineal gland, which causes the body to feel less alert and raises the desire to sleep.
Exposure to light triggers chemical events in the circadian pacemaker that affects your sleep and wake cycles. For example, the body releases melatonin as it gets dark in the evening to instigate drowsiness. By morning light, the body suppresses melatonin and elevates cortisol levels to make the body feel more awake.
People should beware of exposure to blue light emitted from digital devices, such as a smartphone, tablet, or television, before bed after a night shift. Research suggests that blue light knocks the circadian rhythm off-kilter, which signals to the brain that it is daytime, resulting in poorer sleep quality.
As insomnia is reported at nearly twice the rate in women than in men, multiple studies have looked at the sex differences in sleep and wake patterns. These studies have proposed that the variances in our internal clocks may play a substantial role in these differences. For example, one study found that women have a shorter intrinsic circadian period and are more likely than men to have biological days that are shorter than 24 hours, the difference being an average of 6 minutes. While 6 minutes may not seem like a lot, the experiment concludes that the small difference would account for a nearly half-hour earlier onset of melatonin secretion in women.
In animal models, disturbance of circadian timing by either a genetic manipulation or the disruption of the light-dark cycle has resulted in impaired fertility and early gestational complications. A 2016 systematic review demonstrates how mutations of clock genes (CRD) resulted in a disruption of the pre-ovulatory luteinizing hormone (LH) surge and other hormonal profiles, which ultimately impacted fertility in animals. Furthermore, the same review found that bright light exposure/therapy mimicking the natural light-dark cycle influenced the ovulatory cycle in multiple studies, increasing levels of prolactin, follicule stimulating hormone (FSH) and LH secretion.
While we do not have many human studies that have directly looked at how CRD impacts fertility, there is substantial evidence of associations between CRD and impaired reproductive health via analysis of female shift workers. A 2014 meta-analysis of 15 studies with more than 100,000 women included found that shift workers had an increased rate of infertility (11.3%) in addition to an increased rates of menstrual disruptions (16.05%). While there are multiple pathways in which circadian rhythms and sleep may influence fertility, these findings suggest that CRD may have the ability to impact conception. 2b1af7f3a8