We've all been there, a night where you cannot fall asleep — the stress of a presentation or life events endlessly looping through your mind, keeping you awake. Not only does poor sleep exacerbate stress, but it also affects our physiological health and quality of life. From academic performance to recovering from (or preventing) COVID-19, sleep is essential for a healthy, happy life.
The Anatomy and Physiology of Sleep
There are two types of sleep humans alternate between during the sleep cycle: relaxing, non-rapid eye movement (NREM) and excitatory, rapid eye movement (REM) (1). NREM sleep is associated with gamma-aminobutyric acid (GABA) - a neurotransmitter that inhibits the brain's arousal centers - and hormones like melatonin, corticotropin-releasing hormone (CRH), and growth hormone (GH).
Once the phases of NREM are complete, the body enters REM sleep, which is associated with dreams and excitatory neurotransmitters like acetylcholine and histamine, which stimulates the brain's arousal centers (2). Individuals do not stay in REM but cycle between the two types of sleep throughout the night, toggling between relaxed and excitatory states (2). Approximately 75-80% of sleep is NREM, and 20-25% is REM (1). The average length of the first NREM-REM sleep cycle is 70 to 100 minutes; the cycles that follow are longer, lasting approximately 90 to 120 minutes (1). The body also undergoes physiological changes during both phases of sleep. During REM sleep, blood pressure, heart rate, ventilation, and respiratory flow typically increase compared to their NREM levels (1, 3).
What controls the sleep-wake cycle
Circadian rhythms are the daily physiological and behavioral patterns that control the sleep-wake cycle (1). The rhythms also regulate physical activity, food consumption, body temperature, heart rate, muscle tone, and hormone secretion and are generated by neural structures that function as a biological clock (1). Melatonin is a hormone that helps regulate the circadian rhythm and helps to keep it in sync. Cortisol, a hormone typically associated with stress, has an inverse relationship to melatonin, meaning it's at its peak in the morning so the body can wake up and gradually decreases during the day, while melatonin increases at night. However, when the circadian rhythm is out of sync, so too are the hormone levels, resulting in sleep disturbances (2).
The Link Between Sleep, Stress, and Ability to Cope
The link between stress and sleep often seems like a vicious cycle, with stress causing poor sleep and poor sleep negatively affecting the body's stress response (2).
An Israel study assessed the sleep of 36 students during low and high-stress periods. The results demonstrated that how an individual emotionally responds to stress has more to do with their sleep quality than the type of stress they are exposed to. In the study, they looked at the students' different coping styles, including problem-focused coping (PFC), emotion-focused coping (EFC), and disengaged coping. In the PFC style, students tried to manage or alter the problem causing the stress. In contrast, emotion-focused coping involved regulating emotions in response to the problem, and disengaged coping saw individuals dissociate from the stressful stimuli as well as the related thoughts and feelings (4). The results found that a high EFC score (meaning that they had a highly emotional response to stress) was associated with less sleep and poor sleep quality, while low EFC (meaning that they had a more tempered emotional response to stress) scores were associated with longer, improved-quality sleep (4). Additionally, PFC was associated with more hours of sleep regardless of stress levels (4).
Worry about poor performance leads to sleep disturbances
It's no surprise that stress affects sleep quality. A study that looked at how stress affects the sleep of medical students found that those not suffering from stress are less likely to have poor sleep, while students with a cumulative grade point average of less than 4.25 were almost four times more likely to experience poor sleep (5). Another study found that medical students who perform worse on their medical exams are more stressed and suffer from poor sleep; this poor sleep, in turn, negatively affects their test performance "creating a vicious circle" (6). Recommendations include medical schools establishing academic counseling centers that promote good sleep hygiene and strengthen students' study skills and stress management (5).
Night-shift workers are at increased risk
Academic performance is not the only aspect affected by sleep. Studies have found that individuals who work night shifts are more likely to experience jet-lag-like symptoms, including drowsiness, insomnia, or other sleep-related problems (7). The American Sleep Association states that these shift workers have an increased risk of heart problems, digestive disturbances, and emotional and mental problems (7). What's also concerning is that the number and severity of workplace accidents tend to increase during the night shift (7). For example, industrial accidents like the Exxon Valdez oil spill have been partly attributed to errors made by tired night-shift workers, and medical interns working night shifts may be twice as likely as others to misinterpret test results (7). The Sleep Association suggests brighter lights, minimizing shift changes, and scheduled naps as possible solutions to the shift-worker sleep dilemma (7).
Insufficient sleep linked to higher stress levels and dysfunctional parenting
Parents, especially those with a newborn, may also be running on minimal hours of sleep. A 2019 study found that mothers who experienced shorter, later, and more variable sleep experienced higher levels of stress; those with higher levels of stress required longer to fall asleep and perceived more sleep problems (8). The study also found that mothers with poor or insufficient sleep were more likely to report dysfunctional parenting (8). The study concludes that parental sleep may be a valuable intervention for promoting positive parent involvement and responsiveness (8).
Sleep and physiological health
Humans spend one-third of their lives sleeping. It is a universal need that can have severe physiological consequences if out of balance (1). The American Sleep Association states that at least 40 million Americans suffer a chronic, long-term sleep disorder every year, and an additional 20 million experience occasional sleeping problems (7). Poor sleep costs an estimated $16 billion in medical costs each year (7).
Like stress, the relationship between the immune system and sleep is a "two-way street," with the immune system activity triggering sleep and sleep strengthening the body's immune system (9). Feeling sick typically makes us feel fatigued because the neurons that control sleep work closely with the immune system (7).
Cytokines - small proteins produced by the immune system to fight infection - are also sleep-inducing chemicals. Sleep deprivation results in a stress response that increases the production of cytokines, leading to low-grade inflammation throughout the body that activates nociceptive sensory neurons, also called pain receptors, which may initiate or worsen chronic pain (10).
One study found that a single night of only four hours of sleep could lead to chronic disorders and have consequences including reduced natural killer cells, increased cytokines, decreased antibodies, and a higher risk of infection than those who slept regular hours (9). Additionally, an analysis determined that individuals who slept well two weeks before being exposed to the rhinovirus were less likely to develop a cold (9).
Pandemic related sleep patterns
With the close relationship between sleep and the immune system, the interrupted and irregular sleep patterns found in COVID-19 patients should come as no surprise (2). Additionally, there is evidence of insomnia in "long-haulers" - those who continue to experience symptoms after they recover from COVID-19 (2). A study published in The Lancet found that six months post COVID-19 infection, 63% of survivors reported fatigue or muscle weakness, and 26% reported sleep difficulties (11). Poor sleep can cause impaired autonomic tone and endothelial vasomotor dysfunction, risk factors for developing atherosclerosis and heart disease (2, 12). However, these symptoms can be counteracted with regular aerobic exercise (2).
It's not just individuals who have had the COVID-19 virus whose sleep may be affected. Stress-related insomnia caused by the pandemic has increased (13). A UK survey found the pandemic lockdown increased the number of people reporting sleep problems from one in six to one in four (14). The number of women with children under the age of four suffering from sleep loss doubled to 40% in the first four weeks of the lockdown (14). Additionally, according to the American Academy of Sleep Medicine, there was a significant increase in online searches for "insomnia" between April and May 2020 (14).
Poor sleep decreases life expectancy
Sleep not only influences the body’s ability to fight off infections, it also plays a role in our overall mortality. Studies have found a relationship between sle