Sleep is a mechanism for restoring bodily functions and for maintaining energy, health, and essentially a balance between sleep and activity. The amount of sleep recommended for adults is 7~8 hours. If sleep duration is decreased in adults, chances of cardiovascular diseases, stroke, and metabolic syndrome increase, while fasting insulin sensitivity and inflammatory markers might reduce [1, 2]. Lack of sleep in young adults has a negative effect on their attention [3]. Young adults are active in social life and work, hence they are physically active. If their sleep is insufficient, their concentration decreases and problems in physical and mental health management and normal social life may occur [3]. Despite its importance, insufficient sleep is an increasing problem throughout the world.

The average amount of sleep in adults, aged 19 years or older, in Korea is 7.85 hours [4]. This is the lowest among all the member countries of the Organization for Economic Co-operation and Development [4]. Unpleasant changes in sleep, brought about by aging, are not only seen in older adults but also young adults and middle-aged people. The sleep quality of young adults is usually low [5, 6]. Fifty to seventy million adults in the United States have sleep disorders, and 37.9% of adults reported experiencing unexpected sleepiness at least once during the day [7]. Since most research studies on sleep have focused primarily on people in their 40s and above, or older adults, there has been insufficient research on the subject of young adults.

Factors that influence quality of sleep in adults include gender, emotion, an increase in body mass index (BMI) and abdominal circumference, obesity, physical activity, sedentary lifestyle, stress, coping style, drinking, smoking, and pain [8]. Emotions have both, positive and negative effects on quality of sleep. Stress level, which has a negative effect on quality of sleep, is the highest among adults aged 19 to 38 years [8, 9]. Moreover, in some studies, stress coping styles such as hobbies, exercise, and optimistic thinking had a positive effect on quality of sleep; exercise and hobbies, in particular, were effective in improving the subjective quality of sleep among young adults [10, 11]. For young adults, recently, smartphone addiction and binge drinking are the most influential factors of sleep [3, 12, 13, 14, 15], but caffeine consumption is the highest among adults aged 18 to 30 years, and high intake of caffeine also has a negative effect on quality of sleep [16].

Sleep hygiene means having both a bedroom environment and daily routines that promote consistent, uninterrupted sleep. Sleep hygiene includes diet; nicotine, caffeine, and alcohol intake; stress; emotional status and physical activity; and physical factors [8]. Binge drinking and night eating are common dietary factors in young adults [15, 17]. Binge drinking is defined as consuming five or more drinks by men and four or more drinks by women within 2 hours. Rates of binge drinking rate in Korea are over 50.0% for men in their 30s to 50s, and 44.1% for women in their 20s [9]. In other countries, binge drinking is common in college students and young adults [14, 15]. It is closely related to sleep disorders associated with psychological problems [15]. Another factor that is unique to young adults is night eating. Night eating syndrome is more commonly reported among young adults in their 30s and college students, than in older adults [17]. Night eating syndrome causes several health-related problems-including sleep disorders, stress, depression, and obesity-as it is related to the discrepancy between sleep cycles and food intake rhythms.

Korea has the highest penetration rate of smartphones worldwide, and 83.6% of its total population uses smartphones [18]. The number of individuals in potential-risk or high-risk groups for smartphone addiction is 3~4 times higher among those in their 20s and 30s, than in their 60s and older. Women are also more susceptible to smartphone addiction than men. Moreover, these trends are increasing [18]. Although smartphones have many advantages, their excessive use negatively affects sleep and health. Excessive smartphone use among young adults leads to a more sedentary lifestyle, problems and dependence in personal relationships, anxiety, loneliness, depression, and insomnia [5]. In particular, the blue light emitted by smartphones has negative effects on sleep [13, 19].

Another important factor related to sleep is social networks, a multidimensional concept that encompasses the size and components of social relationships; the frequency with which one contacts family, friends, and spouse; the degree to which help is given or received; and the degree of satisfaction with one’s social relationships [20]. Among older adults, emotional support and bonding provided through social networks, reduces the effect of stress-induced depression, while weak social networks lead to high levels of stress and depression, which indirectly have a negative effect on sleep [4]. Social networks for young adults, however, are closely related to addictive behaviors such as drinking and smoking, which also have negative effects on sleep [12].

When the factors affecting sleep were examined, physical health status had a greater effect on sleep in middle-aged and older adults, whereas emotional factors, drinking, work hours, stress and coping styles, and smartphone usage had a greater effect on sleep in young adults with relatively good health status [5, 12, 15]. Additionally, sleep hygiene includes overuse of smartphone and lack of a social network [5, 12, 21]. Physical and emotional aspects such as hormones, pain, and depression usually have negative effects on the sleep of older adults or middle-aged individuals. However, in young adults, the increase in social gatherings, smartphone addiction, and alcohol abuse, delay the start-up time and negatively affect sleep [21, 22]. Therefore, we analyzed to identify the effects of sleep hygiene (caffeine intake, alcohol drinking, night eating syndrome, stress coping styles, sedentary lifestyle), social networks, and smartphone-related factors on the quality of sleep in young adults, to provide basic information for developing interventions for young adults with sleep problems.

This study consisted of a descriptive survey, examining how sleep hygiene (caffeine intake, alcohol intake, night eating syndrome, stress, and coping styles), smartphone-related factors, and social networks affect sleep in young adults aged less than 40 years.

The quality of sleep in our young adult sample was 59.34 points on average, which is below the cut-off score of 65 points, thus indicating a low quality of sleep [23]. The average sleep duration on weekdays was 6.86 hours (range: 4~12), while that on weekends was 7.84 hours (range: 4~16), both of which are lower than the recommended sleep duration on weekdays for healthy adults (7~8 hours) [7]. Our findings are lower than the average hours of sleep for Americans in their 20s (8.2 hours) and the OECD results (7.8 hours) [4, 29]. Domestic results in the OECD statistics are from 2014 and sleep duration has recently decreased. Poor quality of sleep and short sleep duration among young adults in Korea is incresed the incidence of obesity, cardiovascular disease, diabetes, and hypertension of middle-aged or elderly people [1, 2, 4, 29, 30]. Therefore, active interventions should be needed at young adults with low quality of sleep or a sleep duration of seven hours or less to prevent health problems related to sleep deprivation.

We found that the quality of sleep in young adults varied by residence type, caffeine intake, and smartphone addiction. Young adults’ sleep duration on weekdays and weekends was longer for those in their 20s than 30s, and longer for women than men. Sleep duration on weekdays and weekends was shorter for participants who were married or had jobs. Following Matricciani et al. [1], we infer that young adults with jobs tended to have shorter sleep durations due to long working hours and work-related stress, while those who were married had shorter sleep durations due to housework and child-rearing. Sleep duration on weekdays and weekends decreased as daily caffeine intake increased. Recently, young adults in Korea have been consuming an increasing amount of caffeine through coffee, tea, and other kinds of drinks, which may help in improving concentration or staying awake during the day, but causes reduced sleep duration [1, 5, 16, 31]. Health care providers should include in their interventions a appropriate daily intake of caffeine (less than 400 mg) and avoidance of excessive caffeine intake for young adults. The high-risk and potential-risk groups for smartphone addiction were 88.9%. The high risk group and the potential risk group for smartphone addiction were very high at 88.9%, which was lower than in infants and adolescents, but among adults, the smartphone addiction group was the highest [18]. Our result is similar to findings that adults who are addicted to smartphone use have reduced quality of sleep and sleep duration and experience more severe degrees of anxiety and depression [1, 5, 21]. In order to improve the sleep quality of young adults, strategies to prevent smartphone addiction are required.

When the factors affecting the quality of sleep in young adults were analyzed using stepwise multiple regression, sleep mood (a subcategory of the night-eating syndrome), the impact of pain over the last four weeks, and family (a subcategory of social networks) had the greatest impact, in that order, with an explanatory power of 33%. In young adults, Night-eating syndrome is worsened by emotional factors such as stress, which in turn causes irregular sleep patterns, thereby ultimately reducing the quality of sleep [17, 23]. In addition, physical problems such as pain and reduced health status in young adults directly affect the quality of sleep [3, 5]. Finally, young adults value social networks and are forming social networks through direct relationships and indirect resctionships by social network services or smartphone. These social relationships affect self-management such as obesity and sleep. A weak social network can cause perceived social isolation, which can negatively affect cardiovascular diseases, neuroendocrine diseases, cognitive functions, and sleep. Family in one’s social network has a greater effect than friends or local communities, on obesity-related health behaviors. Hence, it has an indirect positive effect on the quality of sleep [5, 18, 20].

The purpose of this study was to identify sleep hygiene factors affecting the quality of sleep in young adults. Factors affecting quality of sleep in young adults included sleep mood (a subcategory of night eating syndrome), the effects of pain over the last four weeks, and social networks. The above factors are similar to the factors affecting sleep in adolescents, except school life and homework. Sleep mood in night eating syndrome has been suggested as a factor affecting sleep in young adults. But the reliability of Korean Version of Night Eating Questionnaire (NEQ) is low, so there are limitations in the interpretation of the results. As young adults are an important period in forming adult lifestyles, it is suggested that occupational classifications, working hours, and work patterns that affect directly sleep, and private meetings, depression, should be added in following study. As life expectancy is increasing, it is important to focus on investigating health status for prevention rather than treating diseases. Sleep hygiene, which is crucial for one’s health, must be identified and diseases caused by sleep deprivation can be prevented through interventions design based on study findings such as ours.