Good sleep is critical to the quality of life but also in managing diseases and co-morbidities. Despite the increasing recognition of the importance of good sleep in enhancing overall well-being and improving the efficacy of medical treatments, there is a persistent timeout in public awareness regarding the importance of good sleep. Moreover, limited efforts have been made toward sleep education. It is both important and essential to provide people with the most critical knowledge in sleep health to enhance their awareness, understanding, and compliance while also preventing the irrational spread of opportunistic and deceptive information (A. Espie, 2021).
Good sleep health is a critical component of general health and well-being across the lifespan. Sleep is fundamentally restorative, promoting physical and mental health. Sleep is required on a daily basis, but it is also a complex, actively regulated brain‐based behavior. Sleep is, therefore, at once a biological necessity and a behavioral choice, both important for health and yet still not widely understood. Data from large‐scale studies demonstrate that sleep is very much a choice, highlighting the relative flexibility of sleep when lifestyle choices are considered. Good sleep health is misunderstood, neglected, and mismanaged by modern digital lives, driving a wide and inequitably distributed sleep deficit globally.
Understanding the Science of Sleep
Sleep is a complex physiological state characterized by reduced metabolic activity, decreased responsiveness to external stimuli, and inhibition of voluntary motor activity. However, in humans, sleep is not a passive state; it consists of a series of well-defined cycles. During periods of sleep, the autonomic nervous system is active, influencing heart rate and blood pressure.
Stuart C. Y. Liu’s sleep cycle summary highlights that humans go through five stages of sleep: four non-REM sleep stages (NREM sleep, S1-S4) and one REM sleep stage. Sleep cycles (approximately 90-120 minutes) repeat several times during sleep. All stages of sleep are important for physical and mental recovery, making sleep quality necessary for a healthy lifestyle.
The first stage, S1, is the transition from awake to asleep. Typically lasting less than five minutes, this stage accounts for about 2% of the total sleep time in adults. Phase resetting, drowsiness, blinking, and slowed body movement frequently occur during this stage. Muscle activity decreases and relaxation begins, allowing the body to “let go and fall asleep.”
The second stage, S2, is about light sleep. There is no eye movement, heart rate slows down, and body temperature decreases. S2 is a sleepy state often witnessed in fathers during childbirth. Light sleep accounts for 40-60% of the total sleep time in adults. Sleep spindles and K-complexes in EEG occur only in S2, helping muscle control by suppressing information that is not necessary for the body to fall asleep.
The third stage, S3, is moderate sleep. In this stage, eyes move slowly from side to side, and it is difficult to wake the body. S3 is the deepest sleep, accounting for 4.5-20% of total sleep time in adults. Very slow brain waves, called delta waves, occur in this stage.
The fourth stage, S4, is described as deep sleep. There is “no body movement,” and the body is sound asleep for about 30 minutes. S4 has a high percentage of delta waves and is indicative of very deep sleep.
The final stage, S5, is REM sleep, or vivid dreaming. The first episode of S5 occurs approximately 1.5 hours after falling asleep, and it is usually 10-20 minutes in duration. REM sleep accounts for about 20-25% of total sleep time. Heart rate increases to a maximum, averaging as high as the heart rate while awake. The eyes move rapidly under the eyelids, and breathing is irregular. Most of the dreams occur in this stage.
Stages of Sleep
Sleep is composed of a cycle of distinct stages, with four stages of non-Rapid Eye Movement (non-REM) sleep designated as stages N1 to N4, and a fifth REM stage, occurring in cycles approximately every 90 minutes throughout the night. Transitions between these stages occur naturally without the sleeper being aware of them. Non-REM sleep is further divided into light sleep (stages N1 and N2) and deep sleep (stages N3 and N4). Light sleep is characterized by a slower frequency of brain waves (theta waves at 6–8 cycles per second) and task unrelated thoughts (hypnagogic imagery), while deep sleep is characterized by the slowest frequency brain waves (delta waves at less than 4 cycles per second), muscle atonia, and a high threshold to external stimuli. Unlike deep sleep, light sleep is easy to rouse from and is characterized by either continuity with wakefulness or transition to stage N2 sleep.
The first REM sleep stage occurs approximately 70–90 minutes after lights out and is similar to wakefulness in terms of brain activity, heart rate, and breathing, hence it is sometimes referred to as paradoxical sleep. It has a low arousal threshold, where awakening due to external stimuli occurs less frequently than in non-REM sleep. If aroused during REM, dream recall is more likely than in non-REM sleep. Cycles of REM sleep lengthen with the duration of sleep, from about five minutes in the first cycle to up to one hour towards the end of sleep, when it comprises 20–25% of total sleep time in healthy adults. As sleep continues, the duration of stage N2 and REM sleep increases while the duration of stages N3 and N4 decreases. The architecture of a normal night’s sleep is stable and relatively consistent, with individual variation, despite differences in sleep schedules and sleep environment.
Common Sleep Disorders and Their Impact
Many adults experience a disrupted sleep schedule at some point in their lives. However, for some, disrupted sleep has become a chronic condition. Sleep disorders (SD) occur in a large proportion of the adult population. The International Classification of Sleep Disorders currently lists more than 80 distinct sleep disorders divided into eight major categories, including insomnia, sleep disordered breathing (SDB), parasomnias, hypersomnia, sleep-related movement disorders, circadian rhythm sleep disorders, isolated symptoms, and other sleep disorders (A Hargens et al., 2013). Of these categories, insomnia, SDB, and sleep-related movement disorders are the most common. Insomnia has been diagnosed in 25%–35% of patients seeking treatment. Those individuals with chronic insomnia, which refers to insomnia occurring at least three times per week for at least three months, are estimated to make up 10%–13% of the adult population. Sleep disordered breathing (SDB) encompasses a variety of sleep disorders all characterized by partial or complete obstruction of the airway during sleep. Of the various forms of sleep apnea, obstructive sleep apnea (OSA) is the most prevalent form of SDB. Untreated SDB has been estimated to occur in approximately 2%–4% of the adult population. However, a large proportion of individuals with clinically significant OSA remain undiagnosed. Changes as minor as snoring are associated with increases in weight. Finally, restless legs syndrome (RLS) is a prevalent form of sleep-related movement disorder with prevalence estimates ranging between 4% and 29%. These three common disorders all result in a decrease in sleep duration and quality, and the concomitant reductions in sleep duration and depth have been associated with increases in body weight and adiposity. Inadequate or poor quality sleep has been shown to lead to increased caloric intake and decreased physical activity in both animals and in humans. Common sleep disorders are of paramount importance to sleep health and where there is a need to find solutions for. Additionally, there is the potential that increased awareness of more common sleep disorders may help to address some of the issues associated with decreased acceptance of less common sleep disorders (C. Abad and Guilleminault, 2003).
Healthy Sleep Habits and Sleep Hygiene
Cultivating healthy sleep habits and maintaining good sleep hygiene are essential components of managing sleep disturbance conditions in the general population. Good sleep hygiene encompasses practices that can promote quality sleep and prevent sleep disturbances. People with good sleep hygiene are more likely to experience better-quality sleep and fewer sleep disturbances. This includes adhering to a consistent sleep schedule, engaging in a relaxing bedtime routine, sleeping in a comfortable bedroom setting, and avoiding caffeine, nicotine, alcohol, and screen time before bed. Good sleep hygiene can also involve keeping a sleep diary and avoiding common sleep disruptors, such as engaging in stimulating activities before bed. Establishing and upholding good sleep habits and sleep hygiene is fundamental to improving sleep quality and managing common sleep disturbance conditions, such as insomnia and daytime sleepiness (A. Espie, 2021).
Unfortunately, the cultivation of good sleep habits and sleep hygiene can be difficult, particularly in busy modern lifestyles. Nonetheless, the importance of engaging in good sleep habits and sleep hygiene cannot be underestimated, particularly in light of the growing number of adults with sleep disturbance conditions due to the contemporary 24/7 lifestyle.
Creating a Sleep-Friendly Environment
To improve sleep quality, healthy sleep habits, also known as sleep hygiene, are encouraged. Healthy sleep habits include creating a sleep-friendly environment, following a consistent sleep schedule, being mindful of what you eat and drink, and developing a bedtime routine (A. Espie, 2021).
Creating a sleep-friendly environment is the first step toward improving sleep quality. Important factors to consider include the comfort of your sleep space, ambient noise and light, and room temperature. A comfortable sleep space is key to enjoying uninterrupted sleep and rest. Mattresses and pillows should suit individual preferences in softness, firmness, and material type. If sleep space is shared, noise, light, and differing temperature preferences may present challenges. Sleeping with white noise or earplugs can help neutralize outside sounds. To combat disruptive light, block-out curtains and eye masks create darkness in the room. In general, a cooler room temperature is more conducive to sleep.
Nutrition, Exercise, and Other Lifestyle Factors Affecting Sleep Quality
Nutrition and sleep are closely related. Researchers and health professionals have recently come to appreciate the impact of diet and nutrition on sleep. Dietary intake of protein, fats, and carbohydrates can influence sleep–wake patterns, particularly as meal timing interacts with circadian rhythms (Frank et al., 2017). Foods high in protein and fat can lengthen sleep latency and sleep duration. By contrast, diets rich in carbohydrates can enhance sleep quality and increase the total duration of rapid eye movement (REM) and slow-wave sleep (SWS). Dietary habits characteristic of the western style, particularly high-fat/fried and high-sugar food intake, have detrimental effects on sleep because of associated disruptions in metabolism and inflammatory responses. Therefore, identifying sleep-friendly diets and encouraging dietary modifications are necessary to curtail the current epidemic of social jetlag and short sleep durations, especially among adolescents. Despite the apparent relationship between sleep and eating, the basic physiology underlying food intake and sleep remains largely unexplored. Animal studies reveal complex interactions among circulating hormones connecting food intake and sleep regulation. For example, administration of the gastrointestinal hormone ghrelin increases NREM sleep in mice, with an increase in sleep duration positively correlated with total food intake. In contrast, leptin, an appetite-suppressing hormone secreted by fat tissue, decreases daily NREM sleep. Also, the downregulation of leptin signaling is associated with diet-induced obesity and excessive sleep. Although the effects of several specific food components on sleep patterns have been documented, the direct impact of composition and timing of the overall dietary intake on sleep health in humans is poorly understood. Physical activity is generally perceived as a favorable lifestyle factor contributing to health and well-being, including sufficient sleep (Louwagie, 2019). Similar observations in population-based studies showed associations with an overall healthier lifestyle and a better sleep quality. Studies have implied that the relationship can be bidirectional, but it seems apparent that sleep can also be an important neighboring lifestyle factor. Sleep disturbances, either insufficient or excessive sleep, are linked to increased obesity and chronic disease risk, including diabetes and cardiovascular disease. Information on mechanisms underlying the associations between physical activity/sleep and health has been explored in multiple complex pathways, mainly focusing on separate sides while disregarding the effects of diet or mood. Current knowledge suggests that both sleep and physical activity can influence health through parallel neuroendocrine pathways, affecting metabolism, autonomic nervous system regulation, stress levels, and inflammation. Furthermore, sleep deficiencies are linked to negative lifestyle changes, including a decrease in physical activity and dietary changes.