How Sleep Influences The Body

Sleep is the most important thing that humans need. Going all the way back to the first lineage of human-like species, sleep would have created a state of vulnerability and thus those who slept would have been naturally selected against, but it was absolutely imperative for their bodies to repair and replenish themselves overnight. Sleep is a highly conserved behaviour across animal evolution, which demonstrates its importance for survival and fitness.

Sleep is characterized by altered consciousness, relatively inhibited sensory activity, reduced muscle activity, as well as the inhibition of nearly all voluntary muscles during deep sleep (John Hopkins Medicine, n.d.). There is a decreased overall reactivity to stimuli, but those who are asleep still show unique and active brain signals. During sleep, most of the body’s systems are in an anabolic state which helps to restore the immune, nervous, muscular, and skeletal systems; these are extremely vital processes that help to maintain a healthy mood, memory, and cognitive functions, and also plays are large role in ensuring proper functions of the endocrine and immune systems (John Hopkins Medicine, n.d.).

The internal circadian rhythm promotes daily sleep at night. This is our “internal clock” that recognizes the difference between day and night and ensures that our bodies are well rested at night in order to function properly throughout the day. The internal circadian rhythm is what causes individuals to become “jet-lagged” when they travel to different time-zones. Humans’ need for sleep varies depending on the individual; sleep is considered to be adequate when there is no daytime sleepiness or dysfunction (John Hopkins Medicine, n.d.). This would indicate that the body has fully replenished overnight and is able to function properly throughout the day. Age also plays a role in how much sleep individuals need on a given night. Younger children require more sleep in order to develop and function properly: up to 18 hours for newborns, with the rate decreasing as the child ages. The recommended amount of sleep for adults is between 7 to 9 hours per night (John Hopkins Medicine, n.d.).

There are several factors that can disturb the balance of our circadian rhythm. One common factor is blue light exposure. Sources of blue light include the sun, digital screens, and LED light. Blue light suppresses the secretion of melatonin, a hormone that promotes sleepiness and shifts circadian rhythms (Harvard News Letter, 2018).

Inconsistent sleep schedules are another factor that disrupts sleep. The body is unable to establish a circadian clock, which leads to difficulty in falling asleep (Kang & Chen, 2009). Stress is also a factor that alters the sleep-wake cycle. Stress triggers the autonomic nervous system to release hormones, such as adrenaline and cortisol. These hormones are responsible for a reaction known as the flight-or-fight response, which raises one’s alertness and promotes arousal. Frequently being in a heightened state of alertness can delay the onset of sleep and adversely affect one’s sleep quality (Johnson, 2018).

Many would assume exercising before bed will also negatively impact sleep quality as it is another activity that stimulates cortisol release. However, scientists found that vigorous late-night exercise has no association with sleep and does not affect sleep quality (Myllymäki, et al., 2011). The effect of caffeine on sleep is another controversial discussion. Over the years, researches showed contradicting findings. One study suggested that consumption of caffeine 6 hours before bedtime has a significant disruptive effect on sleep. On the other hand, another study examined the relationship between caffeine, alcohol, nicotine, and sleep. Nicotine and alcohol use within 4 hours of bedtime was associated with increased sleep fragmentation, but caffeine was not associated with any of the sleep parameters (Spadola, et al., 2019). The effect of caffeine appears to vary among individuals.

Last but not least, eating near bedtime may also undermine sleep. While some food, such as warm milk and chicken breast, may help us relax and contain chemicals that promote sleep, spicy and acidic food like citrus and tomatoes can induce heartburn, which can lead to insomnia (Peters, 2019).

Animal and human studies have shown that the quantity and quality of sleep play a critical role in learning and memory. Sleep helps learning and memory by allowing an individual to focus their attention maximally throughout the day, thereby allowing efficient learning to occur (Fowler, 1973). Sleep also has a profound effect on memory consolidation, which is the process in our brain used to convert short term memory to long term memory (Stickgold, 2005). It is recommended that students get at least 6 hours of sleep for tests and exams in order to retain and recall information. This also partially explains why all-nighters are not recommended to students. Memory consolidation takes place during sleep through the strengthening of the neural connections that form our memories (Stickgold, 2005). During sleep, the brain organizes information and classifies information depending on what we need and what we don’t need (Sleep, Learning, and Memory, 2020).

Sleep is critical for learning as it contributes to the three essential stages of learning: Acquisition, Consolidation, and Recall, in which each of these steps is necessary for proper memory function (Graves, 2001). Acquisition refers to the introduction of new information into the brain. Consolidation represents the processes by which a memory becomes stable. Recall refers to the ability to access the information after it has been stored. Although acquisition and recalling of information occur when an individual is awake, the consolidation, which links the acquisition stage (i.e. learning of new information) to recalling stage (being able to remember and apply the information), occurs when an individual is asleep (Graves, 2001). In other words, recalling of information cannot occur efficiently nor effectively without a proper memory consolidation. Thus, sleep plays a critical role in learning, memory, and concentration.

Getting a good quality of sleep is critical for the endocrine system to operate effectively. Significant hormonal changes affecting the hypothalamic-pituitary-adrenal (HPA) axis activity take place in the setting of sleep deprivation (Sleep and the Endocrine Brain, 2020). The HPA axis plays a critical role in the stress response, a neuroendocrine adaptation component that has been evolutionarily developed. The stress response initiates with the stimulatory release of corticotropin-releasing hormone (CRH) in the hypothalamus. The CRH binds to CRH receptors on the anterior pituitary gland, which stimulates the release of its hormone, or adrenocorticotropic hormone (ACTH). ACTH then binds to receptors on the adrenal cortex and stimulates the release of cortisol. In response to the stressors, cortisol will be released for several hours after encountering the stressor (Medscape, 2020).

Good quality of sleep maintains and regulates the HPA activity to a body’s optimum level, establishing homeostasis. However, activation of the HPA axis by a stressor is known to disrupt normal sleep patterns (Trayhurn, 2005). Hence, lack of sleep can heighten an individual’s sensitivity to stress.

Studies show that sleep loss is associated with an elevation of sympathovagal balance, activating and stimulating the sympathetic system (i.e. fight or flight) while deactivating the parasympathetic system (i.e. rest or digest). Studies demonstrate that the HPA axis that controls the release of the stress hormones (i.e. cortisol in primates and corticosterone in rodents) is reciprocally connected to sleep (Sam & Dhillo, 2010). Therefore, the lower the number of hours of sleep an individual gets, the higher the activation of the HPA axis accompanied by greater secretion of stress hormones, thereby increasing an individual's sensitivity to stress, fatigue, and pressure.

Sleep and the circadian system exert a strong regulatory influence on immune functions (Besedovsky & Born, 2012). There is general consensus in the literature that adequate “restorative” sleep is needed to maintain good immunity. Immune parameters like leukocyte numbers, function, proliferation and cytokine production are altered by lack of sleep. Sleep deprivation is linked to increased daytime levels of inflammatory mediators such as interleukin-1 (IL-1), IL-6, and tumor necrosis factor (TNF) (AlDabal & BaHammam, 2011). Several other studies have addressed increased pro-inflammatory cytokine levels in response to sleep deprivation (AlDabal & BaHammam, 2011).

Young, healthy women who demonstrated disturbed sleep have shown signs of increased inflammatory markers along with specific chronic inflammatory diseases (AlDabal & BaHammam, 2011). Chronic sleep deprivation might be linked to increased all-cause morbidity and mortality (AlDabal & BaHammam, 2011). Studies show that if inflammation is present during waking, it can cause malaise, fatigue, immobility, pain and other aspects of sickness behaviour that are incompatible with the demands of mental and physical activity required for functioning effectively and carrying out daily functions (Besedovsky & Born, 2012).

Improper sleeping patterns can harm the activity of the digestive system. The body’s circadian rhythm is responsible for maintaining a regular sleep-wake schedule and is responsible for contributing to homeostasis (Rhoads, Levitsky, & Tavakkoli, 2016). The origin of the circadian rhythm is rather interesting. It exists as a result of evolutionary progression since survival is dependent on an individual’s ability to obtain food and sleep without being hunted (Rhoads, Levitsky, & Tavakkoli, 2016). It is, therefore, necessary to maintain a pattern of when to perform daily activities, and when it is time to sleep.

The circadian rhythm when disturbed can result in severe metabolic changes and an increased risk of developing diabetes and obesity (Rhoads, Levitsky, & Tavakkoli, 2016). This is because the physiological processes in the body follow the circadian rhythm and normally perform the same activities at the same time each day, and improper sleep patterns can, therefore, disrupt the mechanisms of metabolism. Individuals who receive less sleep than required tend to have lower levels of leptin, and higher levels of ghrelin which lead to an increased appetite, ultimately increasing body mass index (BMI) (Taheri et al., 2004). Improper sleep can also result in an increased prevalence of gastrointestinal tract diseases, particularly those involving the inflammation of the intestines (Manzar, Zannat, & Hussain, 2015). Some of these include an increased risk of developing mucous colitis and irritable bowel syndrome (Manzar, Zannat, & Hussain, 2015).

The reproductive system can be affected by improper sleep, as with the other systems in the body. Fewer hours of sleep contributes to decreased levels of follicle-stimulating hormone (FSH), and increasing luteinizing hormone (LH) amplitude in women (Manzar, Zannat, & Hussain, 2015). FSH is responsible for stimulating the growth of follicles just as the name implies, and a decrease in its activity can result in a lack of maturation of follicles, and possibly implying a lower sex drive (or libido). The luteinizing hormone spikes can be harmful because it compromises the integrity by which the hormone acts on the body and this can result in the detrimental release of the hormone.

For pregnant women, sleep deprivation has been associated with an increase in levels of pro-inflammatory serum cytokines (Chang et al., 2010). This change can promote the incidence of postpartum depression (after giving birth), along with negative birth outcomes such as preterm delivery (Chang et al., 2010). Sleep deprivation for 4 to 7 days has been shown to result in significant decreases in testosterone levels (Choi et al., 2016). Studies have shown that decreased levels of testosterone have been linked with low libido (Travison et al., 2006). Sperm viability also significantly decreases with sleep deprivation (Alvarenga et al., 2015).

The cardiovascular system can be impacted by sleep deprivation leading to complications such as an increased risk of cardiovascular disease (Manzar, Zannat, & Hussain, 2014). Sleeping less than 5 hours a day has been shown to increase the risk of developing hypertension (Gangwisch et al. 2006). Conditions such as sleep apnea and insulin resistance can also promote the incidence of developing cardiovascular disease and atherosclerosis (Manzar, Zannat, & Hussain, 2014). Improper sleep patterns has been associated with increased levels of insulin resistance (which has shown to increase the risk of developing cardiovascular disease), along with increased sympathetic activation and levels of cortisol (a stress hormone) (Bopparaju & Surani, 2010). The strongest contributors to cardiovascular disease are increased insulin resistance, CRP (C-reactive protein), and leptin levels (Bopparaju & Surani, 2010). Studies have shown that leptin plays a role in the etiology of insulin resistance, therefore increased levels of it would increase the risk of cardiovascular disease (Bopparaju & Surani, 2010). CRP has been shown to be a good biomarker for the development of cardiovascular disease when measuring its levels in blood tests (Karakas & Koenig, 2009).