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Chronobiology and Stress in Horses Research Paper

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Updated: Apr 27th, 2021

Current Problem

Chronobiology shows that the intensity, duration, and direction of light influence the behavioral and physiological processes of horses. The circadian system enables horses to synchronize their internal environment with the external environment, and thus, making them adapt to their environment. However, unfavorable changes in the intensity, duration, and direction of light affect the physiological and behavioral functions of horses. Essentially, these changes stress horses, for they disturb the circadian rhythm and compel horses to alter their physiological and behavioral activities. According to Murphy, circadian rhythm influences behaviors and physiological functions of horses because it controls gene expression and immune responses (2).

The circadian rhythm controls how horses behave in a given environment and determine the nature of physiological functions that take place in their bodies. For horses to remain healthy, their circadian rhythm should be in an innate state or without much deviation from the native circadian rhythm. Murphy notes that diurnal variation in the secretion of hormones such as melatonin and cortisol is due to the circadian rhythm (4). The decrease in the level of cortisol hormone predisposes horses to arthritis and asthma, which are immuno-inflammatory diseases. In this view, it is evident that light has a major role in the growth and development of horses because it influences physiological and behavioral activities.

Horses are susceptible to the circadian rhythm because it influences their behaviors and physiological activities. Melatonin is an important hormone among horses because it mediates circadian rhythm, influences physiological functions, and causes behavioral changes. The level of melatonin is subject to the circadian rhythm, which regulates diverse physiological functions, as well as behaviors of horses. Normally, horses start to secrete melatonin when darkness commences, and the level peaks as darkness progress. In this case, it implies that the light/dark cycle, which is an integral aspect of chronobiology, influences the secretion of melatonin among horses.

Murphy et al. state that melatonin regulates reproduction among horses because it dictates physiological functions and behaviors of horses, such as fertility and mating. As a hormone, melatonin has an inverse relationship with cortisol because its levels increase as the levels of cortisol decreases. As melatonin and cortisol mediate immune responses, the inverse relationships show their regulation in response to the circadian rhythm. Evidently, melatonin and cortisol influence immune responses among horses, for disruption of circadian rhythm reduces the efficacy of vaccinations (Murphy 5). Therefore, it suffices to say that changes in circadian rhythm have marked changes in physiological and behavioral functions such as fertility, mating, and immune responses among horses.

Possible Solutions

Supplementation of light to manipulate the photoperiod is an appropriate solution to the changes in the circadian rhythm, according to seasons. During winter, when the daylight is too short, chronobiology stress increases as horses experience disturbance of their circadian rhythms. An extended period of darkness implies that melatonin secretion increases and lasts for a long period. Murphy argues that melatonin regulates reproduction among horses because female horses are fertile during summer when the length of the day is long (2). Given that melatonin secretion starts at dusk and stops at dawn, it implies that darkness promotes its secretion by the pineal gland. Hence, supplementation of light during winter increases photoperiod and delay secretion of melatonin, and consequently, promotes reproduction among horses. Moreover, extended daylight is important among horses because it promotes the growth of hair in horses. Therefore, caretakers of horses should ensure that the supplementation of artificial light follows the normal pattern of summer when horses have enhanced fertility.

Hormonal treatment is another intervention that is applicable to the management of stress associated with photoperiod and chronobiology. Injection of serotonin and dopamine decreases the levels of cortisol and increases the levels of melatonin, leading to a reduced predisposition to Cushing’s syndrome. Given that cortisol and melatonin have an inverse relationship, they represent hormones that horses secrete during day and night, respectively. Extended daylight increases cortisol levels, while extended darkness increases melatonin levels. Therefore, horses need to maintain the levels of cortisol to remain within the normal ranges. Aged horses have a problem maintaining cortisol because their circadian rhythm is no longer sensitive to changes in photoperiod. Thus, they require supplementation of dopamine and serotonin to enhance the regulation of cortisol and prevent predisposition to Cushing’s syndrome.


  1. What behavioral changes does the circadian rhythm cause in horses?
  2. What are the physiological processes that circadian rhythm influences in the body?
  3. How does the circadian rhythm influence the expression of clock genes during winter and summer?
  4. What are peak levels of melatonin during summer and winter?
  5. Do melatonin levels vary across different ages of horses?
  6. What are the synthetic hormones that influence circadian rhythm?
  7. Does artificial light determine the level of cortisol and melatonin among horses?
  8. How does the circadian rhythm mediate immune responses?

Interesting Quotes

  • “The circadian system provides animals with a means to adapt their internal physiology to the constantly changing environmental stimuli that exist on a rotating planet” (Murphy 1).
  • “There is extensive evidence for circadian regulation of immune parameters exemplified by rhythmic secretion of the neuroendocrine hormone cortisol and the pineal hormone melatonin, both of which exhibit diurnal variation in the horse” (Murphy 4).
  • “Secondary to light and similar to feeding cues, exercise is a known synchronizer of circadian clocks” (Murphy 6).
  • “Circadian rhythms are thus controlled by an endogenous oscillator that enables organisms to anticipate rhythmic environmental changes and tailor their behavioral and physiological states to the most appropriate time of solar day” (Murphy et al. 1).
  • “In diurnal mammals, there exists an inverse relationship between plasma melatonin and circadian cortisol rhythms with the start of the quiescent period of cortisol production phase-locked approximately to the onset of melatonin production” (Murphy et al. 2).
  • “These findings suggest that under natural conditions, melatonin inhibition in the horse occurs in response to light and not through an endogenous mechanism” (Murphy et al. 4).

Ideas that Come to the Mind

Chronobiology indicates that light is an important environmental factor that influences physiological functions and behaviors. Photoperiod influences physiological functions such as metabolism, immune responses, and hormonal secretion. The most important hormones that photoperiod influences are serotonin, melatonin, and cortisol. In the aspect of behaviors, photoperiod influences the mating season of horses. Essentially, female horses are fertile during summer for the photoperiod is longer than normal. Therefore, caretakers of horses use photoperiod in regulating physiological functions and behaviors with a view of increasing equine productivity and reproduction.

Works Cited

Murphy, Barbara (2009). “Chronobiology and the horse: Recent revelations and future directions.” The Veterinary Journal 1.1 (2009): 1-10. Print.

Murphy, Barbara, Ann-Marrie, Penney Furney, and Jeffrey Elliot. “Absence of a serum melatonin rhythm under acutely extended darkness in the horse.” Journal of Circadian Rhythms 9.3 (2011): 1-8. Print.

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1. IvyPanda. "Chronobiology and Stress in Horses." April 27, 2021. https://ivypanda.com/essays/chronobiology-and-stress-in-horses/.


IvyPanda. "Chronobiology and Stress in Horses." April 27, 2021. https://ivypanda.com/essays/chronobiology-and-stress-in-horses/.


IvyPanda. 2021. "Chronobiology and Stress in Horses." April 27, 2021. https://ivypanda.com/essays/chronobiology-and-stress-in-horses/.


IvyPanda. (2021) 'Chronobiology and Stress in Horses'. 27 April.

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