What are Circadian Rhythms?
The word “circadian” originates from the Latin phrase circa, meaning “about”, and dies, meaning “day, which means the rough translation of circadian rhythm is “about the days rhythm”. The study of circadian rhythms falls under the science known as Chronobiology.
Chronobiology is the study of biological rhythms including, but not limited to sleep/wake cycles, respiration, heart rate, blood pressure, body temperature, mental function, nerve activity, hormone regulation, and activities at the cellular level. Circadian rhythms are a subtype of chronobiological rhythms with 24-hour cycles. Other chronobiological rhythm subtypes include infradian rhythms that are on greater than 24-hour cycles, such as seasonal rhythms and ultradian rhythms that are less than 24-hours.
How Does Light Exposure Affect Circadian Rhythms?
Although circadian rhythms are internally produced, they adjust to the local environment based on external factors including temperature, eating patterns, and exposure to light. Light enters our eyes and then neurons in our eyes send signals to our brains allowing us to perceive vision. Other neurons will send signals to another part of our brain known as the suprachiasmatic nucleus (SCN), which is a cluster of neurons that lies just above the crossing of the optic nerves. The SCN is known as “the master clock” and its regulation by light stimulation will entrain the body’s internal circadian rhythm to coincide with external light/dark cycle cues. This is why we feel awake and are active during the days and feel tired and go to sleep during the nights.
SCN: The Master Clock
The SCN is the body’s control center of circadian rhythms. It generates circadian outputs that affect the rhythmic activity in almost every tissue and cell in the body. The SCN sends hormones and other mediators which can change the expressional patterns of specific genes, known as “clock genes” that affect circadian rhythms of the body’s many tissues and cells.
Why are Circadian Rhythms Important?
Circadian rhythms govern all of our biological processes and we need them in order to function properly. Evidence for this is seen with sleep disorders and aggravated disease states arising from circadian disruptions.
What is pain?
Pain is an unpleasant sensation with both physical and emotional components. Pain comes in many forms and is often used as a symptom relating to a disease state of an individual. We feel pain due to the activation of sensory neurons, known as nociceptive neurons, that send signals to the brain indicating pain. In addition to the nervous system, cells and mediators of the immune system play a role in the sensitization of nociceptive neurons in the cases of inflammation and injury related pain. There are two major types of pain disorders: nociceptive pain which arises due to non-neural tissue damage; and neuropathic pain that is due to damage or altered functioning of the nervous system.
Pain can either be acute (lasting for a short amount of time) or chronic (lasting for longer periods of time – typically 3 months or more). There are many different types of chronic pain including cancer related pain, arthritis, headache, fibromyalgia, low back and neck, and posttraumatic pain.
Many types of chronic pain have been shown to have fluctuations in the intensity of pain reported from patients that follow a circadian (24-hour cycle) rhythmic pattern. This is an emerging field of study where new rhythmic patterns are being described for various types of pain including rheumatoid arthritis, fibromyalgia, and multiple sclerosis related pain. Identifying these pain patterns could provide clues to the underlying cause of this phenomenon as well as help us better understand the modulation of ongoing pain.
The definitive explanation as to why there is an observed circadian rhythmicity in certain chronic pain types of chronic pain is still being uncovered. Since circadian rhythms are involved in so many biological processes it is likely that there is a combined effect of multiple systems involved.
Circadian rhythms have been linked to play a part in the sensitization of peripheral and central nociceptive neurons which mediate the pain response. Additionally, cells and mediators of the immune system, involved in sensitization and/or inflammation causing a pain response, are under circadian control. Some of the cells of interest are microglia and astrocytes, which are found in the central nervous system (brain and spinal cord), as well as lymphocytes, which infiltrate to the site of injury and cause inflammation and nociceptive hypersensitivity.
Circadian rhythms are involved in virtually all biological processes within the body, including some forms of chronic pain. This observation has important implications to future research and treatment of pain. Further investigation into the role that circadian rhythms play in the regulation of neuroimmune cells and mediators of pain is necessary. This may help to better characterize these phenomena and help develop therapeutic strategies to treat chronic pain more effectively.
Written by Bruce M.
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