More and more studies have demonstrated the importance of a good night’s sleep - nevertheless, we tend to sleep less and less
On average, we spend nearly a third of our lives sleeping. Sleep is a necessary physiological process, and almost all animals sleep in one way or another. Each one of us experiences the immediate effects of sleep deprivation - difficulty concentrating at work or in studies, slower response time and more frequent mistakes, which may even be dangerous. In fact, a slight sleep deficit - losing just an hour or two of sleep per night, can impair our functioning in a way similar to losing an entire night’s sleep. Over time, lack of sleep is related to the development of chronic diseases such as type 2 diabetes, hypertension, atherosclerosis, obesity and depression. The prevailing recommendation is to get seven hours of sleep a night. Nevertheless, the modern lifestyle leads to a chronic shortage of sleep. A survey conducted in Finland reported an 18-minute decrease in average sleep duration over 33 years, while the American Sleep Research Institute reported that the proportion of people sleeping less than six hours a night increased from 12% to 16% between 1985 and 2012.
In mammals (and birds) sleep occurs in two stages: Rapid Eye Movement (REM) sleep and Non-Rapid Eye Movement (non-REM) sleep. Non-REM sleep is further divided into three distinct phases, each characterized by unique patterns of brain activity. Throughout the night we experience several sleep cycles, each encompassing all these sleep stages. The first non-REM stage is a transition from wakefulness to sleep. It is a brief stage, lasting only a few minutes, during which sleep is light, heart rate, breathing and eye movements slow down, and muscles start to relax. The subsequent non-REM stage involves a shorter sleep duration before entering the stage of deep sleep. Here, our heart rate and breathing slow down even more significantly, muscles relax even more, body temperature falls, and eye movements cease. The final non-REM phase is the deep sleep phase that is essential in order to feel refreshed in the morning. At this point, heart rate and breathing are at their lowest levels, muscles are fully relaxed, and waking up during this stage may be especially challenging.
The first REM stage occurs approximately 90 minutes after falling asleep. During this stage rapid side-to-side eye movements begin, and brainwave patterns resemble those of wakefulness. Breathing and heart rate increase, becoming more similar to waking states. This is the stage during which most dreaming takes place; our leg and arm muscles experience temporary paralysis that prevents us from acting out our dreams. As we age, this stage of sleep becomes shorter. Typically, REM and non-REM alternate in a pattern that includes three to five sleep cycles each night.
Mechanisms of Sleep Regulation
The sleep process in our bodies is controlled by two biological mechanisms: circadian control and homeostasis. The circadian control is responsible for our natural tendency to feel tired at night and awake during the day. It is a biological clock based on a 24-hour cycle that responds to environmental stimuli such as light and darkness. In the evening hours, as the environment becomes darker, the hormone melatonin is released in our body. The accumulation of melatonin causes a feeling of fatigue and eventually leads to sleep. Cortisol is another hormone involved in the circadian control of sleep. The levels of cortisol decrease during the day, begin to rise approximately three hours into sleep and peak about half an hour after waking. Exposure to light further stimulates cortisol release.
Homeostatic control, on the other hand, reflects the body’s needs, such as the sensation of tiredness. The factors involved in homeostatic control are not fully understood, but studies have found clear involvement of a substance called adenosine. Adenosine levels in the brain rise during wakefulness, and it binds to specific receptors, thus signaling to the body the need for sleep. In fact, caffeine blocks the activity of adenosine, thereby promoting wakefulness. During sleep, the body breaks down accumulated adenosine molecules.
While its precise role remains unclear, sleep is undeniably essential, and insufficient sleep significantly affects our performance. Difficulty sleeping | Illustration: Shutterstock
Why We Need To Sleep
The purpose of sleep remains a mystery, but several theories attmpt to explain why we need to sleep. Researchers have hypothesized that sleep is needed to clear out metabolic waste from the brain, and have provided evidence by brain scans using functional magnetic resonance imaging (fMRI). Other theories claim that sleep is essential for maintaining the immune system, replenishing the brain’s energy reserves and creating connections between nerve cells.
What Happens When We Don’t Get Enough Sleep Or Sleep Too Much?
Lack of sleep (and sometimes also excess sleep) has many negative impacts. Three studies published in 2018 explored a link between inadequate sleep and heart disease. In the first study, a group of researchers monitored sleep duration in about 4,000 volunteers, using electronic bracelets. The researchers found that sleeping less than six hours a night, or experiencing discontinuous sleep is associated with an increased risk of atherosclerosis, which leads to a narrowing of the arteries. Participants were divided into groups according to the quality and quantity of sleep: very short sleep (less than six hours), short sleep (six to seven hours), average sleep (seven to eight hours) and long sleep (more than eight hours). The researchers then examined atherosclerosis in the leg and neck arteries. The highest rate of atherosclerosis was found in the subjects with very short sleep, while the lowest rate of atherosclerosis was found in the subjects with average sleep (seven to eight hours a night). An increased risk of atherosclerosis was also found in the group that slept more than eight hours a night, suggesting that even prolonged sleep may be harmful.
A Swedish study found that people who sleep less than five hours a night are at higher risk of cardiovascular disease. Researchers followed the medical state and sleep habits of about 800 men born in 1943 for 21 years, starting in 1993. They documented significant cardiovascular events, such as heart attack, stroke, hospitalization due to heart failure, and death following a cardiovascular event. They found that sleeping less than five hours doubled the risk of a cardiovascular event by age 71.
The third study employed a meta-analysis, that is, analysis of the findings of many studies, to examine the relationship between sleep duration and cardiovascular disease. The researchers found that people who slept less than six hours and people who slept more than eight hours a night were at higher risk of cardiovascular disease and stroke than people who slept between six and eight hours a night. Previous meta-analyses also indicated a relationship between a lack of sleep and morbidity. Thus, researchers in China used meta-analysis to examine the relationship between sleep duration and elevated blood pressure, and found that both insufficient sleep (six hours or less) and excess sleep (eight hours or more) increased the risk of hypertension. Finally, another study examined the relationship between sleep duration and stroke events in the USA. The study showed that among individuals who sleep less than seven hours or more than eight hours a night, the chance of stroke is higher than in those who sleep between seven and eight hours.
All of these studies clearly demonstrate that insufficient or excessive sleep is linked to health risks. However, these studies do not explain the mechanism by which inadequate or excess sleep lead to increased morbidity. Since sleep is affected by many mechanisms and affects many mechanisms, finding mechanistic and molecular explanations for this morbidity remains challenging.
Nevertheless, some new studies have provided possible paths to answer this question. Thus, for example, a study showed that a lack of sleep affects two hormones responsible for hunger and satiation—leptin, which suppresses the appetite, and ghrelin, which increases the appetite. The researchers demonstrated that a lack of sleep leads to a decrease in the levels of leptin and an increase in the levels of ghrelin. These changes in hormone levels are expected to increase hunger, and thus lead to obesity due to a lack of sleep. Another study, conducted recently, found that a lack of sleep leads to changes in the expression of genes in different tissues. The researchers found that even after one night of inadequate sleep, changes take place in the expression of genes, expressed by symptoms of inflammation and metabolic changes in fatty tissues, which may cause obesity later in life. However, further studies are still needed to understand all the mechanisms leading to morbidity due to inadequate or excess sleep.
Tips For Sleeping Well
The modern lifestyle can disrupt the mechanisms regulating sleep and lead to sleep disorders. For instance, exposure to screens (television, computer screen, smartphone) before sleep acts similar to daylight, thus delaying the release of melatonin and preventing us from falling asleep. Thus, it is important to reduce screen usage before bedtime. Other recommendations include avoiding caffeine before sleep, keeping regular sleeping hours as much as possible, and refraining from heavy meals before bedtime. And of course, first and foremost, we must dedicate enough hours to sleep each night, and maintain a dark, quiet sleeping environment.