Understanding the Cognitive and Health Effects of Sleep Duration

Why Do We Need Sleep? 

Healthy sleep — defined by good quality, appropriate timing and regularity, adequate length, and absence of disorders or disturbances — leaves individuals feeling refreshed and alert. Healthy sleep also prevents disease development and helps keep the body healthy. Because sleep makes humans feel more energetic, more alert, and happier, the resulting feeling of going without sleep or a good night of sleep begins to suggest its vital importance to humans. 

While genetic, behavioral, environmental, and medical factors influence the amount of sleep an individual may need, more research is necessary for establishing the precise biological mechanisms essential for determining an individual’s sleep need. 

Sleep Theories

Scientists have developed several theories suggesting why humans need sleep. However, no single theory has been proven correct. Understanding these theories can help deepen our understanding of why humans spend a third of their lives sleeping. Scientists have studied what happens when humans are deprived of sleep, yet the impetus behind sleep remains unclear.

Adaptive (Inactivity) Theory

One of the earliest theories formed about sleep, the adaptive or inactivity theory, operates under the premise that inactivity during the night is an adaptation that served a previous survival function by reducing animal vulnerability. This theory implies that animals with the ability to lie quiet and still for long periods of time had a significant survival advantage over animals who could not. Through natural selection, it is assumed that this behavioral strategy evolved into the practice we call sleep today. However, a common counterargument to adaptive theory is that remaining unconscious during a potential attack is a major disadvantage if safety is paramount. 

Energy Conservation Theory

Although people living in societies with abundant food sources may forget that effective energy utilization is essential in times of food scarcity, this theory suggests sleep’s primary function is to reduce energy demand and expenditure during part of the day and night — the least efficient times to hunt for food. 

Studies have shown a significant decrease in energy metabolism during the sleep cycle because the body’s temperature and caloric demand decrease during sleep, hinting at sleep’s primary function in allowing organisms to conserve energy resources. 

Restorative Theories

These theories indicate that sleep is vital for restoring physiological functions in the body by repairing and repleting cellular components. Many bodily functions occur during sleep cycles such as tissue growth, muscle repair, protein synthesis, and growth hormone release. 

Brain Plasticity Theory

The brain plasticity theory suggests that sleep is vital for neural reorganization and growth of the brain’s function and development. This is why infants sleep upward of 14 hours a day — to allow for brain development. 

While these theories are not exhaustive or all-inclusive of established viewpoints, they prove that experts do not fully comprehend nor agree on the need for sleep. In the scientific community, a widely accepted belief prevails that a combination of these theories is more likely to explain the need for sleep. 

Circadian Rhythm

A circadian rhythm is a person’s sleep–wake pattern over 24 hours. Most living organisms have circadian rhythms influenced by light–dark cycles and other factors. The brain receives signals based on environmental factors and activates specific hormones, alters the body’s temperature, and regulates the metabolism to promote sleepiness or alertness. While external factors or sleep disorders can disrupt circadian rhythms, maintaining healthy habits can support this natural physiological and behavioral bodily rhythm. 

Hormone levels of cortisol and melatonin may increase or decrease based on circadian rhythms. For example, the body produces cortisol in the mornings to promote alertness and releases melatonin to promote sleepiness during the night but suppresses it during the day. 

Sleep Disorders

Although sleep disorders like insomnia, restless leg syndrome, and sleep apnea have always existed, they have been underdiagnosed. There are approximately 80 different sleep disorders affecting 70 million Americans every year. 

In the United States, 33–50% of the adult population experience insomnia symptoms, and 10–15% experience chronic insomnia disorder, which is associated with distress or impairment. Approximately 75% of adults with depression suffer from insomnia. 

Obstructive sleep apnea, the most common sleep-related breathing disorder characterized by repeatedly starting and stopping breathing, is more common in individuals with Alzheimer’s disease. While sleep apnea is more common in men, other risk factors include age and obesity. In the US, around 2–5% of women and 3–7% of men suffer from sleep apnea, respectively. 

Because sleep disorders can affect emotional and physical health, insufficient sleep has been linked to the development of several chronic conditions including hypertension, heart disease, stroke, obesity, type 2 diabetes, weight gain, depression, and even the risk of death. 

A recent Cleveland Clinic study found that people with certain sleep disorders have more severe outcomes from COVID-19, including a 31% higher rate of hospitalization and mortality. 

Cognitive and Health Effects of Sleep

Although the amount of sleep people need changes significantly over their lifetime, adults are recommended to get 7 or more hours of healthy sleep a night for optimal health. Chronic sleep deprivation can have detrimental effects on the brain by decreasing reasoning skills and cognitive abilities, impairing judgment, and hampering a person’s ability to focus and perform tasks. Sleep deprivation can also lead to potentially serious health problems like hypertension, diabetes, and cardiovascular problems. 

According to a study conducted by Soomi Lee, assistant professor at the School of Aging Studies at the University of South Florida, in the Annals of Behavioral Medicine, consecutive sleep loss is related to degraded mental and physical well-being. It only takes three consecutive nights of sleep loss to cause the deterioration of psychological and physical health. 

Cardiovascular disease is a leading cause of death worldwide, and increasing evidence demonstrates that poor sleep quality and short sleep duration are associated with cardiovascular disease. In 2018, a study published in the Journal of Clinical Sleep Medicine reported that both short sleep duration and poor sleep quality are related to the risk of coronary heart disease, though data associated with a long sleep duration does not reach statistical significance. 

Additionally, studies suggest that a lack of sleep is associated with an individual’s increased risk for dementia. Because individuals with dementia frequently have poor, interrupted sleeping patterns that worsen as the disease progresses, 25% of people with mild-to-moderate dementia and 50% of people with severe dementia experience sleep disturbances. 

While sleeping more than 9 hours a night regularly is recommended for young children and adults recovering from a lack of sleep or an illness, it is not clear if longer sleeping periods are associated with adverse health risks. 

Where Is Sleep Science Heading?

Because chronic sleep loss is now viewed as a growing epidemic in the US, scientists will continue to study the effects of insufficient sleep. “Each year, Americans are shaving more and more time off of the sleep they get per night,” says Charlene Gamaldo, MD, medical director of Johns Hopkins Center for Sleep at Howard County General Hospital. 

“More individuals are doing shift work, and people are distracted in the evening hours by cable TV with 1,000s of channels, computer streaming, binge-watching, not to mention our growing dependence on mobile phones and social media,” she continued. 

Each mobile device generation results in greater exposure to blue light, interfering with the body’s clock and causing sleeping problems. 

Researchers will continue to study how lack of sleep and poor-quality sleep impacts other health conditions such as diabetes and heart disease, Gamaldo says. 

A full night of sleep is vital for optimal health and permits the mind and body to function properly, and a Behavioral Medicine study discovered that people who are the most optimistic tend to sleep better and have better health outcomes. Additionally, a Northwestern University and Rush University Medical Center study published in Sleep Science and Practice found that having a purpose in life results in a better night’s sleep with fewer disturbances.

Given what the pandemic has taught the healthcare industry and society at large about mental health, integrated care should provide a stepping stone toward treating sleep disorders.