Module 4.1 Circadian Rhythms (Psychology)

**Module 4.1 Circadian Rhythms: Our 24-Hour Highs and Lows** **Learning Objectives** LO 4.1 Define *circadian rhythms* and explain how they influence sleep. LO 4.2 Explain the importance of the suprachiasmatic nucleus. LO 4.3 Describe the problems associated with shift work. **What is a circadi...

**Module 4.1 Circadian Rhythms: Our 24-Hour Highs and Lows** **Learning Objectives** LO 4.1 Define *circadian rhythms* and explain how they influence sleep. LO 4.2 Explain the importance of the suprachiasmatic nucleus. LO 4.3 Describe the problems associated with shift work. **What is a circadian rhythm, and which rhythms are most relevant to the study of sleep?** Do you notice changes in the way you feel throughout the day---fluctuations in your energy level, mood, or efficiency? These daily fluctuations, called [circadian rhythms](https://plus.pearson.com/products/e05007dc-f4a1-43fb-be4e-34913aec8599/pages/urn:pearson:entity:1cbd7e13-a2cd-40e9-b8dd-d80233dce1a1?userPreferredType=read), are controlled largely by the brain and play a critical role in the timing of life- sustaining processes in virtually all organisms, from humans and other vertebrates to plants, and even single-cell life forms (Bhadra et al., 2017; Walker et al., 2020). Blood pressure, heart rate, appetite, secretion of hormones and digestive enzymes, sensory acuity, elimination, and even our body's responses to medication all follow circadian rhythms (Bailey, Udoh, & Young, 2014). Moreover, the circadian timing system is involved in the 24-hour variation of virtually every physiological and psychological variable researchers have studied, with light serving as the main regulator of circadian rhythms (Tähkämö, Partonen, & Pesonen, 2018). **\[LO 4.1\]** Sleep is mostly influenced by two important factors that vary according to circadian patterns---alertness and body temperature. Normal human body temperature can range from a low of about 36.1 degrees Celsius between 4:00 and 5:00 a.m. to **The Suprachiasmatic Nucleus: The Body's Timekeeper** **What is the suprachiasmatic nucleus?** So, what part of our brain acts as a biological clock? Studies suggest that it is a tiny piece of brain tissue smaller than the head of a pin located in the brain's hypothalamus called the [suprachiasmatic nucleus (SCN)](https://plus.pearson.com/products/e05007dc-f4a1-43fb-be4e-34913aec8599/pages/urn:pearson:entity:85bc0e9d-ca68-47a9-85e9-5f2d6834028c?userPreferredType=read) (Hastings, Brancaccio, & Maywood, 2014). Neurons in the suprachiasmatic nucleus respond to the amount of light, particularly sunlight (Wright et al., 2013), reaching the eye and then transferred via the optic nerve. **\[LO 4.2\]** Specialized cells in the retina, called *photoreceptors*, respond to the amount of light reaching the eye and relay this information via the optic nerve to the suprachiasmatic nucleus. The SCN acts on this information by signalling the pineal gland, located in the centre of the brain. In response, the pineal gland secretes the hormone melatonin from dusk until shortly before dawn but does not secrete it during daylight (Cajochen, Chellappa, & Schmidt, 2010). Melatonin induces sleep, perhaps through its ability to keep all of the body's tissues aware of both the time of day and the time of year (Benarroch, 2008; Gandhi et al., 2015). But light is not the only factor that influences our biological clock. In normal circumstances, external stimuli---day and night, alarm clocks, job or school demands---cause us to modify our biological clock to a 24-hour schedule. Circadian rhythms are slightly disrupted each year when daylight saving time begins and ends. An even gre **Shift Work: Working Day and Night** **What are some problems experienced by employees who work rotating shifts?** According to the most recent Statistics Canada report (2008), over 4 million workers aged 19 to 64 worked something other than a regular day shift. Of these shift workers, about 3.3 million worked full time. Rotating shifts and irregular schedules were the most common types of shift work, accounting for 2.3 million full-time workers. Service workers, including those employed in sales, health care, and police and fire departments, are more likely to do shift work. When people must work at night, they experience a disruption in the rhythms of many bodily functions that are normally synchronized for daytime, which can cause a variety of physical and psychological problems. **\[LO 4.3\]** What are the physical and psychological effects of disturbing the normal sleep/wakefulness cycle when a person works the night shift? The 2008 report by Statistics Canada indicates that shift workers are more likely to cut back on sleep, to spend less time with their spouse, and to worry about not spending enough time with family compared with regular day workers. A recent meta-analysis found that shift workers have more diagnosed physical health problems, higher blood pressure, greater rates of workplace injury, greater risk for miscarriage of a pregnancy, greater risk for obesity, higher cholesterol levels, higher rates of depression, and higher rates of divorce than non-shift-workers (Bolino, Ke