With the rapid development of Internet technology, electronic products such as mobile phones have become one of the main tools for individuals to access and supply information [
1]; conduct interpersonal communication; obtain entertainment, diversion, and relaxation; receive monetary compensation (such as finding bargains on product and services to save money, getting profitable financial information, or working and doing tasks to make money) [
2]; and pursue other activities (such as education and health management), by virtue of their devices’ convenience, accessibility, and powerful functions [
3,
4]. However, in the process of using these electronic products and the functions they enable, human beings often experience a variety of problematic behaviors, including overuse and dependence [
5], which have gradually become the focus of academic attention. Among groups that typically have access to their own phones, adolescents and young adults, especially college students, are more likely to experience problematic mobile phone use (PMPU) because they have more free time, lower levels of self-control, and increased identity and lifestyle needs (such as online learning, social interaction, games, and shopping) [
6‐
8]. A meta-analysis pointed out that the prevalence of PMPU among Chinese college students was as high as 23% [
9]. In a previous study, PMPU was defined as uncontrolled or excessive use of mobile phones by individuals that causes problems in daily life [
10]. In some studies, it was also referred to as mobile phone dependence [
11], mobile phone addiction [
12], and smartphone addiction [
13]. Similar to the symptoms of substance use disorder, uncontrolled and excessive use was the important symptoms and characteristics of PMPU [
14]. In this concept, uncontrolled use was considered a core feature of PMPU, meaning, although aware of the adverse effects, individuals still used and had difficulty controlling the use of mobile phones [
10]. Further, excessive use means that an individual’s mobile phone use exceeds a certain time and range. Obviously, this requires a demarcation point to determine whether an individual has excessive mobile phone use. Based on the existing research, it is not feasible to determine the cut-off point by using quantitative methods such as time and frequency, because the motivations and natures or modes of mobile phone use of different individuals are very heterogeneous [
15]. For example, when mobile phones were used to contact families and friends, provide social support, or when learning or working with an aim to increase productivity or self-improvement (such as participating in online work conferences, browsing online learning resources, information retrieval, and schedules) [
16], the use time of mobile phone can be long and may not have negative consequences; such situations should not be categorized as excessive mobile phone use [
17]. Therefore, on the basis of considering the motivation, nature or mode of mobile phone use, current studies tend to use evaluation from the perspective of others (for example, my classmates say that I use mobile phones for too long and too often), rather than quantitative methods to judge excessive use. Specifically, when evaluating excessive use, it need to declare to participants that the mobile phone usage in the evaluation refers to mobile phone use patterns or content are uncontrolled online games, social media, or entertainment (e.g., watching movies and listening music)—and the reason or motivation for use is evasion of reality, failure to regulate stress and negative emotions, and boredom, fear of missing out [
14] or specific personality traits such as shyness [
10]. In addition, PMPU also showed two characteristics: tolerance (the frequency and duration of mobile phone use by individuals to achieve satisfaction have increased significantly) and withdrawal (individuals experience psychological withdrawal symptoms such as panic, restlessness, and irritability when separated from their mobile phone) [
18]. Therefore, in this study, we define PMPU as an individual’s uncontrolled or excessive use of mobile phones and adverse effects when performing activities with the motivation and purpose of relieving negative emotions, relaxing oneself, and satisfying online social and entertainment needs, rather than activities with the motivation and purpose of self-improvement, increase productivity or search for social support, such as work, study, and communication with families and friends.
The concept of PMPU is somewhat controversial, because the main function of mobile phones was the operation of Internet-based applications [
19], which indicates that PMPU has many similarities with Internet-based addictions, such as gaming disorders, and may have mutual influences. For example, individuals with Internet addiction are more likely to experience PMPU, and vice versa. Previous research has shown that there was a positive correlation between Internet addiction and PMPU [
20]. However, studies have also shown differences between the two in risk factors such as gender and personality characteristics [
21]. For example, men experience more Internet addiction, while women demonstrate more PMPU [
22]. However, it should be noted that mobile phones not only provide functions such as the Internet and games, but also have various other services and functions such as communications, cameras, multimedia playback, painting, and e-book reading. These services may not be related to the Internet. In addition, individual Internet use relies not only on mobile phones but also on desktop laptops, computers, or digital tablets. Therefore, some symptoms of PMPU may be different from those of Internet addiction. In addition, a study found that, compared with using their phones for playing games, individuals with PMPU were more likely to use social networks [
23], which indicates that there may also be differences between PMPU and gaming disorders. Therefore, current research tends to treat PMPU as an independent concept, and has developed some specific assessment tools [
15,
24‐
26], which are widely accepted and recognized by scholars. However, it should be noted that none of the existing scales can fully consider the characteristics of uncontrolled or excessive, tolerance and withdrawal to evaluate PMPU [
15]. Thus, this study used the Mobile Phone Addiction Tendency Scale (MPATS), which is widely used in mainland China, to focus on the evaluation of uncontrolled use, excessive use evaluated by classmates or friends, tolerance, withdrawal symptoms, and negative consequences [
27].
PMPU and depressive symptoms
Previous research has shown PMPU to be associated with individual health status and to not only cause physical symptoms such as musculoskeletal pain [
28] and increase the risk of traffic injuries [
29] but also induce mental health problems [
30]. Regarding physical and mental health conditions related to PMPU, depressive symptoms have been widely investigated by scholars. A meta-analysis of 33,650 college students in 40 studies showed that PMPU was significantly positively correlated with depression [
31]. In addition to exploring the direct relationship per se, an important research question is the nature of the potential mechanism between PMPU and depressive symptoms. There have been many studies looking at mediators of the relationship (from self-esteem [
32] and self-determination [
33] to personality traits such as mindfulness [
34], attachment variables [
35], interpersonal relationships [
36], and stress and burnout [
5], among others). However, most of these studies adopt cross-sectional designs, failing to demonstrate the directions of effects between various variables, which makes it difficult to truly understand the associations among PMPU, depressive symptoms, and other potential influencing/mediating factors. It seems premature to investigate possible mechanisms of a relationship of which both the direction and causality have not been properly established. Therefore, before paying attention to the mechanism of this relationship, longitudinal research is urgently needed to determine its direction. Existing studies have focused on the longitudinal relationship between PMPU and depressive symptoms, but their results have been contradictory (i.e., the directionality is inconsistent). For example, a three-year follow-up study of 1877 Korean adolescents using autoregressive cross-lagged model analysis found a bidirectional longitudinal relationship between PMPU and depressive symptoms [
37]; however, a longitudinal study in China found that depressive symptoms at baseline predicted follow-up PMPU, but PMPU at baseline did not predict follow-up depressive symptoms [
38]. In light of these inconsistent findings, it is necessary to conduct a longitudinal study to further explore the association.
Sleep quality
Sleep may be a major influencing factor for PMPU and depressive symptoms, because studies have indirectly shown that there may be biological and psychological connections among the three factors. A German study found adolescents with sleep disorders more likely to use smartphones longer because they used smartphones as a coping mechanism to suppress worries [
39]. This indirectly indicated that poor sleep quality may cause individuals to develop PMPU. Studies have shown that in individuals with PMPU, constant exposure to blue light can inhibit the secretion of melatonin, and cause sleep and circadian rhythm disorders [
40,
41], which might be an important factor in the generation of psychopathological symptoms such as depression [
42]. Previous studies have explored the mediating role of sleep quality between PMPU and depressive symptoms using cross-sectional study designs [
43,
44]. However, there have also been studies with conflicting results, in which depressive symptoms played a mediating role between PMPU and sleep quality [
45]. Moreover, considering the limitations of cross-sectional research, these studies have not confirmed the direction of effects among these factors. Fortunately, previous longitudinal studies have found bidirectional relationships between PMPU and sleep quality [
38]; that is, sleep quality predicted PMPU and vice versa. In addition, longitudinal studies show that insufficient sleep and suboptimal sleep quality can predict subsequent depressive symptoms [
46,
47]. However, these studies also have certain limitations in that they focused on the longitudinal relationship between the two, rather than explored the relationship among the three. Given that the literature on the relationship between PMPU, sleep quality, and depressive symptoms and its direction is still inconclusive and incomplete, more longitudinal studies on the relationship among the three are warranted.
Bedtime procrastination
In addition to sleep quality, there may be other sleep factors associated with PMPU and depressive symptoms. The basic characteristics of PMPU are mobile device overuse and lack of self-control. According to the Displacement Hypothesis of The Internet [
48], everyone’s time is constant. The more time and energy an individual spends using a mobile phone, the less time and energy they spend on other activities and tasks (such as sleep), which leads to the delayed completion of those activities and tasks. Moreover, according to the Strength Model of Self-Control [
49], individuals with PMPU need to access limited and domain-general psychological resources in order to develop self-control. Such resources include the ability to suppress their impulse to use their mobile phone, interpersonal communication, emotion regulation, and judgment and decision-making around online activities. The theory of self-regulation failure suggested that procrastination is the result of the exhaustion of self-control resources and the failure of self-regulation [
50]. These theories hint that PMPU may be closely related to individual procrastination, which has been confirmed empirically in cross-sectional studies of nursing students [
51]. Recently, special procrastination behavior related to sleep has attracted the attention of researchers. Bedtime procrastination refers to the situation where individuals deliberately delay going to bed or refuse to do so without external interference [
52]. Previous research has reported that bedtime procrastination played a mediating effect in the positive correlation between PMPU and sleep quality in Chinese college students [
53]. In addition, studies have also found that trait procrastination and general procrastination may lead to PMPU [
54,
55]. From personality trait theory we know that personality affects behavior [
56]; bedtime procrastination, as a characteristic of the procrastinating personality, may then also affect PMPU, but no research has focused on this possible relationship. Meanwhile, a cross-sectional study in China found that bedtime procrastination was positively associated with depressive symptoms in medical students [
57]. Bedtime procrastination means that college students’ sleep time may be reduced, and a previous study has confirmed that short sleep increases the risk of mental disorders in young adults aged 17–25 years [
58]. Moreover, a survey of 802 young people with clinically diagnosed depression showed that about 18% had habitual delayed sleep onset [
59], and a longitudinal study of adolescents found that individuals with depressive symptoms were more likely to delay bedtime [
60]. These studies provide some support for the bidirectional relationship between bedtime procrastination and depressive symptoms. However, further longitudinal studies are needed to confirm this. In addition, according to the Procrastination-Health Model [
61], habitual procrastinators will experience pressure caused by missed deadlines or completing tasks at the last moment, and may participate in various unhealthy behaviors (such as using mobile phone before bedtime) that can provide immediate satisfaction. However, stress and unhealthy behaviors have been recognized as important risks for shortened sleep time and decreased sleep quality [
62]. A previous cross-sectional study of Chinese college students reported that bedtime procrastination was related to sleep quality [
63]. However, to our knowledge, no previous study has longitudinally investigated the association between bedtime procrastination and sleep quality.
In summary, there were known to be close relationships among PMPU, bedtime procrastination, sleep quality, and depressive symptoms. However, the existing studies have mainly conducted simple correlation analyses of two or three out of these four variables or only performed systematic reviews; none systematically included all four variables. Therefore, this study aims to explore the longitudinal relationships among PMPU, bedtime procrastination, sleep quality, and depressive symptoms in Chinese college students.