Social Development Theory

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Personality and Individual Differences

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Self-regulation and bedtime procrastination: The role of self-regulation skills and chronotype

Romana Kadzikowska-Wrzosek University of Social Sciences and Humanities Faculty in Sopot, ul. Polna 16/20, 81-745 Sopot, Poland

A R T I C L E I N F O

Keywords: Bedtime procrastination Quality of sleep Self-regulation Morningness-eveningness Health

A B S T R A C T

The present study focuses on examining the role of chronotype in the relationship between self-regulation skills and bedtime procrastination. Two possible models were tested, hypothesizing that eveningness may moderate or mediate the association between self-regulation abilities and bedtime procrastination. A sample of 304 parti- cipants completed measures of self-regulation, morningness–eveningness, bedtime procrastination, and sub- jective indicators of the amount and quality of sleep. The analyses showed that bedtime procrastination was negatively correlated with hours of sleep and positively correlated with frequencies of perceived insufficient sleep and daily fatigue. Moreover, bedtime procrastination was negatively related to self-regulation skills and to morningness. No significant interaction (moderation) effect was found between self-regulation skills and eveningness. The analysis showed that eveningness partly mediated the relationship between low self-regulation skills and bedtime procrastination. The results confirm that low self-regulation skills may account for higher bedtime procrastination, and suggest that this negative effect reveals itself, in part, through enhancing even- ingness.

1. Introduction

Perceived insufficient sleep has recently grown to become a serious social problem. Empirical findings from many countries confirm that the problem affects a large proportion of people, who not only ex- perience discomfort, such as daytime fatigue and sleepiness, but also are exposed to other serious effects of sleep insufficiency (Kroese, De Ridder, Evers, & Adriaanse, 2014; Nowicki et al., 2016; Ogińska, Mojsa- Kaja, Fafrowicz, & Marek, 2014). Just like objective or actual sleep insufficiency, perceived insufficient sleep has a significant effect on physical and mental health, achievement, work safety and efficiency, and social relationships (Hairston & Shpitalni, 2016). Perceived poor quality and insufficient amount of sleep are more common than clinical insomnia and research into this problem may provide important addi- tional information helpful in understanding the causes of sleep pro- blems and the significance of sleep for health in the general population (Benham, 2010; Nowicki et al., 2016).

The prevalence of subjective complaints about poor quality and insufficient amount of sleep, however, calls for looking at sleep pro- blems from a different perspective than just the clinical one. Sleep problems leading to perceived insufficient sleep are often caused by the lifestyle and negative sleep habits. People's behavior is a significant determinant of their health. Apart from the genes, the quality of healthcare, and the environment, it is behavioral factors that account

for 40% of deaths in developed countries (Benham, 2010). This is why a large proportion of studies within health psychology focus on ex- amining the determinants and supporting the positive health effects of such behaviors as physical activity, healthy diet, following doctor's orders, avoiding smoking and drinking, and, recently, also behaviors enabling healthy sleep (Hagger et al., 2014; Kor & Mullan, 2011; Loft & Cameron, 2013).

Preliminary evidence confirms that just like other health related behaviors, the quantity and quality of sleep are related to self-regula- tion skills. Higher self-regulation skills not only facilitate healthy eating, systematic physical exercise, and changing bad habits; in- dividual differences in self-regulation skills are also related to hours of sleep and daytime fatigue (Hagger, 2010; Kroese et al., 2014; Kroese, Evers, Adriaanse, & de Ridder, 2016). Research findings also suggest that poorer self-regulation skills are related to a failure to follow the principles of sleep hygiene (Kor & Mullan, 2011), whereas improving the ability to control one's behavior using intention implementation imagery may be a good way to develop healthy sleep related habits (Loft & Cameron, 2013).

According to Duckworth and Steinberg (2015), in order to fully understand the findings from numerous studies of self-regulation, we need to make a distinction between the external signs of self-regulation and its underlying psychological processes. There are two basic groups of processes that play a significant role in self-regulation: volitional

https://doi.org/10.1016/j.paid.2018.02.015 Received 26 December 2017; Received in revised form 7 February 2018; Accepted 9 February 2018

E-mail address: [email protected].

Personality and Individual Differences 128 (2018) 10–15

Available online 22 February 2018 0191-8869/ © 2018 Elsevier Ltd. All rights reserved.

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processes that facilitate self-regulation, and impulsigenic processes that undermine self-regulation. When examining the determinants of effec- tive self-regulation, psychologists show a much stronger focus on vo- litional skills than impulsigenic tendencies. For instance, from the de- velopmental perspective, it is often presumed that impulsigenic tendencies remain constant throughout life and that what changes over time is volitional skills. In fact, understanding the full picture of factors determining the efficiency of self-regulation requires taking into ac- count both processes that facilitate self-regulation and those that make it difficult (Duckworth & Steinberg, 2015). Self-regulation failure may result both from weak volitional processes and from powerful im- pulsigenic processes.

In the context of sleep related behavior, the evening chronotype (or eveningness) is one of the factors undermining self-regulation. In other words, sleep problems may result not only from the individual's low self-regulation skills (or volitional skills), but also from the fact that their preferred bedtimes and rise times (related to eveningness) differ from normative social schedules such as work or school hours. For example, sleep problems which are common among adolescents are strongly linked their circadian rhythm shift toward eveningness (Randler, Fabi, & Kalb, 2017). The delayed sleep-phase syndrome is significantly more frequent among adolescents (7%–16%) than in the general population (0,13–0,17%) (Bittencourt, Santos-Silva, De Mello, Anderson, & Tufik, 2010). Thus, it may be concluded that, in a sense, it is an internal biological mechanism that makes adolescents go to sleep late.

1.1. Self-regulation, bedtime procrastination, and insufficient sleep

Self-regulation involves the ability to manage one's goal-directed activities. It activates processes through which people manage their thoughts, emotions, and behavior in ways consistent with their goals. Every effort to change one's own internal states and responses may be regarded as a sign of self-regulation (Baumeister, Schmeichel, & Vohs, 2007; Karoly, 1993; Kuhl, 2000).

Self-regulation skills play a crucial role in the transition from an intention to behavior consistent with this intention. Hence, poor self- regulation is considered to be an important factor responsible for the behavioral gap. One manifestation of intention-behavior inconsistency is procrastination or delaying an intended action (Howell, Watson, Powell, & Buro, 2006; Lay, 1986; Stainton, Lay, & Flett, 2000). Pro- crastination means that the individual has formed an intention, but is unable to proceed from intention to action. Thus, procrastination does not result from low motivation, but rather from poor behavior control mechanisms or low self-regulation (Kuhl, 2000).

It seems that many people have the intention and possibility to sleep well and sleep enough hours, but they cannot implement this intention successfully. The failure to successfully move from the intention to sleep well to taking specific steps that would ensure good sleep may be seen as one of important factors contributing to chronic sleep depri- vation in many individuals (Loft & Cameron, 2013).

According to Kroese et al. (2014) delaying bedtime is a relatively common bad habit contributing to perceived insufficient sleep. Re- searchers refer to this habit as bedtime procrastination. Delaying bed- time may be regarded as an example of procrastination, because just like general procrastination it involves putting off an intended action (in this case, going to bed), a lack of clear reason for the delay (the individual could go to sleep but fails to do so), and the procrastinator's ability to predict some negative consequences of the delay (such as feeling tired when having to get up the next morning) (Nauts, Kamphorst, Poortvliet, Sutu, & Anderson, 2016).

Research shows that this specific type of procrastination, i.e. bed- time procrastination, is linked to low perceived quality and quantity of sleep. Delaying bedtime turns out to be a prevalent habit in the general population, having a significant effect on perceived insufficient sleep and daytime fatigue. Studies have also confirmed that this particular

form of procrastination (delaying bedtime) is associated both with general procrastination and with self-regulation (Kroese et al., 2014, 2016; Nauts et al., 2016).

Kroese et al. (2014, 2016) have found that people with poor self- regulation skills usually go to bed later than they intended and that this behavior has a significant effect on their subjective experience of in- sufficient sleep. These negative consequences raise a question: Why do people engage in bedtime procrastination? Why do they delay bedtime despite being aware of the negative consequences of this behavior? Just like research into the causes of general procrastination, studies looking for the causes of bedtime procrastination have not yet provided a sa- tisfactory answer (Nauts et al., 2016; Steel, 2007). Low self-control making the individual give in to current temptations without con- sidering the long-term consequences of such behavior, does not seem a satisfactory explanation in this case, because the negative con- sequences, such as perceived insufficient sleep, will be experienced by the bedtime procrastinator relatively soon. So perhaps our answer to the question about why people engage in bedtime procrastination cannot be limited to low self-regulation skills, but should also include individual differences in chronotype?

1.2. Self-regulation skills, chronotype, and bedtime procrastination

The term “chronotype” refers to individual differences in circadian rhythms or, in other words, in the functioning of the human biological clock. At the psychological level, chronotype is a preference for a spe- cific time of the day, reflected by objective effectiveness of action and by mood oscillations (Jankowski, 2014; Stolarski, Ledzińska, & Matthews, 2013). Chronotype is also linked to individual preferences for early or late bedtimes and rise times (Jankowski, 2017;Vollmer et al., 2017; Wittmann, Dinich, Merrow, & Roenneberg, 2006).

Chronotype is determined by biological factors (specific genes), individual factors (such as gender or age), and environmental factors (e.g., changes of light and dark) (Randler et al., 2017). However, the circadian rhythms are also influenced by social synchronizers, such as social activity, including school schedules or the family life, as well as other social factors. For example, a study of adolescents which ex- amined the links between morningness–eveningness, pubertal devel- opment, and family relationships showed that a strong drive toward autonomy (independence) and frequent conflicts in the family were the most significant predictors of eveningness. The effect of family re- lationships on eveningness turned out to be stronger than the effect of factors, such as age and pubertal development (Díaz-Morales, Escribano, Jankowski, Vollmer, & Randler, 2014). Other studies also confirm that factors related to family functioning contribute to chron- otype. Women's chronotype is influenced by pregnancy and by having children (Leonhard & Randler, 2009), and children's chronotype is in- fluenced by the rules of family life, parental attitudes, and sleep pro- blems experienced by the parents, especially the mother (Bajoghli, Alipouri, Holsboer-Trachsler, & Brand, 2013; Brand, Gerber, Hatzinger, Beck, & Holsboer-Trachsler, 2009; Urfer-Maurer et al., 2017). Other data suggest that following the principles of sleep hygiene or shaping healthy patterns of sleep related behavior may be effective in pre- venting the shift toward eveningness in adolescence (Vollmer et al., 2017). It turns out that regular, though not necessarily very intensive, physical activity, adhering to earlier bedtimes, and following the basic rules of sleep hygiene may improve the quality of sleep even in ado- lescents suffering from serious sleep disorders, such as the delayed sleep-phase syndrome (Brand et al., 2010; Dewald-Kaufmann, Oort, & Meijer, 2014; Richardson, Gradisar, Short, & Lang, 2017).

Social jetlag or misalignment of the biological and social clocks is considered to be the main cause of both sleep problems and other health issues of evening types. Evening types have to deal with a dis- crepancy between their preferred times for work and study and morning-oriented schedules imposed by social rules (Jankowski, 2017; Vollmer et al., 2017; Wittman et al., 2006). This misalignment makes

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task fulfilment particularly challenging for evening types' self-regula- tion capacities. As a consequence, their functioning seems to be largely dependent on their self-regulation skills. The lower the skills, the more difficult it is for evening types to regulate their sleep related behaviors. Evidence shows that evening types are likely to experience such sleep problems as insufficient sleep, irregular sleep patterns, low sleep quality, and daytime fatigue (Hairston & Shpitalni, 2016; Vollmer et al., 2017).

The existing data suggest that perceived sleep quantity and quality are strongly influenced by bedtime procrastination, which in turn is linked to self-regulation skills. The lower self-regulation skills, the stronger the tendency to delay bedtime, and ultimately the higher the level of perceived sleep insufficiency (Kroese et al., 2014, 2016). An- other important factor may be eveningness, which undermines self- regulation of sleep related behaviors.

Consequently, a question arises about whether chronotype may be a boundary condition of the effect of self-regulation skills on bedtime procrastination. In other words, is bedtime procrastination dependent on the chronotype? That would mean that low self-control leads to bedtime procrastination in evening types, whereas individuals with the evening chronotype and high self-control do not demonstrate bedtime procrastination. This idea is supported by research evidence showing that not all evening types have sleep problems (Tavernier & Willoughby, 2014).

It should be noted, however, that many studies have found a re- lationship between chronotype and self-control conceptualized as a personal disposition. Within the Big Five model of personality, con- scientiousness is regarded as a trait related to self-regulation or self- control skills; at the same time, it is conscientiousness that has received the strongest empirical support as a correlate of chronotype (Stolarski et al., 2013). What is more, evidence confirms that conscientiousness is the strongest single predictor of chronotype. Morning types are more conscientious than evening types (Hogben, Ellis, Archer, & von Schantz, 2007). These findings are consistent with data showing that morning- ness is also linked to higher perseverance (Tonetti, Fabbri, & Natale, 2009), better self-control (Digdon & Howell, 2008; Milfont & Schwarzenthal, 2014), and the future temporal orientation, whereas eveningness turns out to be related to the hedonic present orientation (Stolarski et al., 2013). Research has also found a negative relationship between morningness and the so called avoidant procrastination (Díaz- Morales, Ferrari, & Cohen, 2008) and has demonstrated that in evening types procrastination may lead to sleep problems (Hairston & Shpitalni, 2016).

The above cited data on the relationship between chronotype and self-regulation come from correlational studies that cannot determine the direction of causality. Although most researchers emphasize the biological determination of chronotype, this does not rule out the in- fluence of other factors (Díaz-Morales et al., 2014; Randler et al., 2017). It appears that individual differences in self-regulation skills may be a significant factor shaping and strengthening people's chronotype. Poor self-regulation, on the other hand, may strengthen evening types' bed- time and rise time preferences inconsistent with social norms, thus leading to bedtime procrastination. This would mean that chronotype is not a moderator, but rather a mediator of the relationship between self- control and bedtime procrastination.

Whereas the previous research supported the relationship between self-control skills and bedtime procrastination, the current study is in- tended to complement this evidence by highlighting the role of even- ingness as a moderator or mediator of the relationship between low self-control skills and bedtime procrastination.

1.3. The current study

The current study has two major goals: first, to find out whether the relationship between bedtime procrastination and perceived sleep quantity and quality, demonstrated by Kroese et al. (2014, 2016), can

be replicated in a Polish sample; second, to examine whether chron- otype is a moderator or rather a mediator of the relationship between low self-regulation skills and bedtime procrastination. It was expected that, similarly to the studies by Kroese et al. (2014, 2016), bedtime procrastination would contribute to a lower average number of hours of night-time sleep, and higher perceived sleep insufficiency and daytime fatigue. It was also expected that including chronotype in the analyses would shed some new light on the link between self-regulation and bedtime procrastination.

2. Method

2.1. Measures

2.1.1. Sleep descriptive and indicators of insufficient sleep Indicators of sleep, including sleep deprivation, were measured

using measures developed by Kroese et al. (2014, 2016). The partici- pants were asked to indicate, on average, how many hours they typi- cally slept during week nights. Hours of sleep were measured by asking “On average, how many hours a night do you sleep on weekdays?”, which was answered on a 7-point scale ranging from “ < 5 h”, with increments of 1 h (5–6 h, 6–7 h,…) to “ > 10 h.”

A number of indicators of sleep insufficiency were assessed: ex- periencing insufficient sleep (“In an average week, how many days do you feel you have had too little sleep?”), and daytime fatigue (“In an average week, how many days do you feel tired during the day”). Those two questions were answered on 5-point scales (never; 1–2 days; 3–4 days; 5–6 days; always).

2.1.2. Bedtime procrastination Bedtime procrastination was assessed with a 9-item scale that was

created by Kroese et al. (2014)) (Cronbach's alpha = 0.85). Sample items are “I go to bed later than I would like to,” and “I find it easy to quit doing other activities in the evening when it is time to go to bed” (reverse coded). Questions were answered on a scale from 1 (almost never) to 5 (almost always).

2.1.3. Morningness-eveningness The Polish version of the Composite Scale of Morningness (CSM;

Smith, Reilly, & Midkiff, 1989) was used (Jankowski, 2015) to asses morningness/eveningness. The CSM consists of 13 questions about the time individuals get up and go to bed, their preferred times for physical and mental activity, and subjective alertness. Five of the elements of the scale refer to different times of the day. Sample items are “How alert do you feel during the first half hour after having awakened in the morning?” or “At what time in the evening do you feel tired and, as a result, in need of sleep?”. Each item is rated on a 4- or 5-point scale. The morningness score is obtained by adding up the item scores and ranges from 13 (extreme eveningness) to 55 (extreme morningness).

2.1.4. Self-regulation skills Self-regulation skills were measured using the Action Control Scale

(ACS-90; Kuhl, 1994), the Polish adaptation by Marszał-Wiśniewska (2002). The study used 2 subscales: AOD/SOD (decision-related action versus state orientation) and AOF/SOF (failure-related action versus state orientation). The individual items of the AOD/SOD scale of Kuhl's questionnaire describe situations of moderate everyday challenge-re- lated stress (i.e., related to conflicting, difficult, or tedious goals). The participant chooses one of two proposed responses that better describes his or her typical behavior in each situation. A sample item describing challenge-related stress (AOD/SOD) is: “When I am getting ready to tackle a difficult problem: a) It feels like I am facing a big mountain that I don't think I can climb. b) I look for a way that the problem can be approached in a suitable manner.” The items of the AOF/SOF subscale of Kuhl's ques- tionnaire, in turn, describe situations of moderate everyday threat-re- lated stress. The individual selects one of two proposed responses that

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better describes his or her typical behavior in each situation. A sample item describing threat-related stress (AOF/SOF) is: “When I am told that my work has been completely unsatisfactory: a) I don't let it bother me for too long. b) I feel paralyzed.” The higher the scale score, the higher the individual's self-regulation skills.

2.2. Participants and procedure

A sample of 304 (71.7% of females) students of B.A., M.A., and postgraduate courses at two universities in Gdańsk (Poland) vo- lunteered to participate in the study (mean age = 28.54 years; SD = 7.97, age range = 18–38).The study was conducted in small groups and the participants were invited to complete the questionnaires in a classroom environment. It took them 20 min to fill in all the questionnaires. After the survey the participants' questions were an- swered and the researchers thanked them for taking part in the study. Participation in the survey was voluntary, anonymous, and unpaid.

3. Results

Frequencies of hours of sleep, indicators of insufficient sleep and daily fatigue are reported in Tables 1 and 2. > 20% of the subjects reported that during the week they slept 6 or fewer hours per night, on average, and about 30% of the sample said they experienced in- sufficient sleep and daily fatigue 3 to 4 days a week.

Table 3 contains the means, standard deviations, internal con- sistency coefficients (Cronbach's α) and intercorrelations of all the variables measured. The pattern of correlations between bedtime pro- crastination and hours of sleep, and insufficient sleep indicators was in line with expectations. Higher bedtime procrastination is linked to a lower average number of hours of sleep, higher frequencies of experi- enced insufficient sleep, and more frequent daytime fatigue. Bedtime procrastination was negatively related to both morningness and self- regulation skills. As regards, morningness, it was positively related to self-regulation skills. In addition morningness and self-regulation skills were negatively correlated with insufficient sleep indicators. Morn- ingness and high level of self-regulation skills are linked to a lower frequencies of experienced insufficient sleep, and less frequent daytime fatigue. Results of correlation analyses indicated also positive correla- tion between morningness and age. Results showed also that female participants reported a higher score on morningness than their male counterparts, t(146.85) = 2.24, p < 0.05, M = 34.28 (SD = 6.64) and M = 32.15 (SD = 8.01) respectively (Cohen's d = 0.29).

Table 4 provides information on the results of regression analyses for bedtime procrastination. Hierarchical multiple regression analysis was performed to assess the impact of gender, age, chronotype, self- regulation skills, and also the interaction between chronotype and self- regulation skills. In the first step, the demographic variables (gender and age) were entered, followed, in the second step, by self-regulation skills. In the third step chronotype was added and in the final fourth step the interaction term (between chronotype and self-regulation skills) was included.

The results indicated that the demographic variables did not ac- count for a statistically significant proportion of the variance in bedtime procrastination. Entering self-regulation skills in step 2 resulted in a statistically significant increment in the explained variance, R2 = 0.05, p < 0.001; F(3,301) = 15.43. Similarly, adding chronotype in step 3 significantly improved the model, R2 = 0.14, p < 0.001; F (4,300) = 51.18.The change of the explained variance from the third to fourth model was not significant. Although self-regulation skills and chronotype yielded significant main effects, the interaction term did not reach statistical significance. In other words, the results did not support chronotype as a moderator of the relationship between self-regulation skills and bedtime procrastination.

The examination of beta weights indicated that self-regulation skills and chronotype had significant negative beta weights (β = −0.22, p < 0.001 and β = −0.39, p < 0,001, respectively). Thus, low levels of self-regulation skills and eveningness can be predictors of bedtime procrastination.

Since the reduction of self-regulation skills' effect upon adding chronotype to the model suggested possible mediation, a bootstrapping procedure (Preacher & Hayes, 2008) was employed. The bootstrapping procedure based on 5000 bootstrapped resamples to estimate BC95% CI for indirect effect was used in order to examine the indirect effect of self-regulation skills on bedtime procrastination through the mediation of chronotype. The SPSS macro ‘indirect’ (Preacher & Hayes, 2008) was used. The bootstrapped unstandardized indirect effect was −0.011, and the 95% confidence interval ranged from −0.0203 to −0.0033. Thus, the effect of self-regulation skills on bedtime procrastination was par- tially mediated by chronotype. The relationships are illustrated in Fig. 1.

The results demonstrated that a lower level of self-regulation skills was significantly associated with eveningness, which, in turn, predicted higher levels of bedtime procrastination.

It needs to be acknowledged that the conducted mediational ana- lyses do not determine the casual relationship between self-regulation skills and chronotype. In fact, one may also consider the reversed re- lationship in the way that self-regulation skills mediate the relationship between chronotype and bedtime procrastination. Thus, the decision was made to test such a model. The indirect effect was significant, though weaker in comparison to the ones reported above (−0.003, BC95%CI from −0.0068 to −0.0007).

4. Discussion

The main goal of the study was to examine the role of chronotype in the relationship between self-regulation skills and a specific type of procrastination – bedtime procrastination. The results demonstrated that bedtime procrastination was related to fewer hours of sleep and to more frequent experiences of insufficient sleep and daily fatigue. This finding is consistent with previous results obtained by Kroese et al. (2014, 2016). The study also showed that bedtime procrastination was related to both self-regulation skills and chronotype. The analyses de- monstrated that low self-regulation skills were significantly related to

Table 1 Frequencies of hours of sleep.

On average how many hours a night do you sleep during the week?

Frequency % Cum %

< 5 h 9 2.80 2.80 5–6 h 62 19.40 22.80 6–7 h 96 30 52.20 7–8 h 109 34.10 86.30 8–9 h 36 11.30 97.50 9–10 h 5 1.30 99.10 > 10 h 3 0.90 100

Table 2 Frequencies of insufficient sleep indicators.

How many days in an average week do you feel you have had to little sleep

How many days in an average week do you feel tired during a day?

Frequency % Cum % Frequency % Cum %

0 28 8.90 8.90 14 4.40 4.40 1–2 days 121 38.30 47.20 126 39.40 43.80 3–4 days 94 29.70 76.90 103 32.20 75.90 5–6 days 56 17.70 94.60 45 14.10 90 7 days 17 5.40 100 32 10 100

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eveningness, which in turn increased the tendency to delay bedtime. The results support findings from previous studies that suggested a

link between self-regulation skills and chronotype. What is new is the discovery that this relationship partly accounts for the link between self-regulation skills and bedtime procrastination. Low self-regulation skills may contribute to eveningness, which in turn influences the tendency to delay bedtime.

Although chronotype is largely biologically determined, it is also influenced by social and environmental factors (Díaz-Morales et al., 2014; Randler et al., 2017). Negative influences on the circadian rhythm include the evening use of electronic devices such as computers, smartphones, and tablets. Blue light emitted by computer screens is a significant contributor to the worldwide trend toward increased even- ingness and decreased morningness in young people, growing from generation to generation (Tęgowska, Sobocińska, & Maliszewska, 2014). Low self-regulation skills facilitate the development of addic- tions to the Internet, computer games, and social networking sites (Błachnio & Przepiórka, 2016; Kim, Namkoong, Ku, & Kim, 2007). This may lead to the use of blue light emitting devices in the evening and during the night, thus influencing the biological rhythm and enhancing eveningness (Tęgowska et al., 2014).

The analyses did not support chronotype's moderating role in the

relationship between self-regulation skills and bedtime procrastination. Instead, the current study suggests that eveningness is a mechanism through which low self-regulation skills influence bedtime procrasti- nation.

The present study has some limitations. First, the current study does not use a cross-lagged design; hence, statements about causal re- lationships and the mediation effect should be put forward with cau- tion. Considering the dynamic interrelationships between biological and psychosocial factors influencing chronotype, the reverse relation- ship cannot be ruled out (Díaz-Morales et al., 2014). It is worth bearing in mind that an alternative model, suggesting the opposite direction of the relationship, though weaker, was found to be significant, too. Fu- ture researchers may consider conducting cross-lagged and quasi-ex- perimental research designs to refine the findings. Second, the mea- surement of sleep indicators was not based on objective measures, such as actigraphs, but rather on self-report. However, self-report ques- tionnaires are highly correlated with objective indicators of circadian behavior (Di Milia, Adan, & Natale V,& Randler C., 2013; Díaz-Morales et al., 2014). Nevertheless, follow-up studies should take into account previous findings suggesting different patterns of sleep behaviors on weekdays and at the weekend, and incorporate those differences in the measurement of sleep indicators.

Despite the above limitations, the present study confirms that self- regulation skills predict a specific type of procrastination, i.e. bedtime procrastination. Bedtime procrastination is a behavioral factor that significantly contributes to common complaints about the amount and quality of sleep. Thus, the reported results support the previous finding that self-regulation skills play a significant role in healthy, sufficient sleep (Kroese et al., 2014, 2016; Loft & Cameron, 2013; Kor & Mullan, 2011). Additionally, the present findings complement the previous observations suggesting one possible mechanism through which self- regulation skills may contribute to bedtime procrastination and, con- sequently, lead to insufficient sleep. As suggested by the current study, this mechanism can involve the enhancement of eveningness by low self-regulation skills. Thus, the study provides some useful suggestions about possible ways of preventing the increase in eveningness and ne- gative behavior and, as a consequence, reducing sleep problems.

Ethical statement

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/ or national research committee and with the 1964 Helsinki declaration

Table 3 Internal consistency reliability (Cronbach's α), means, standard deviations, and intercorrelations among all study variables.

M SD M-E ACS-90 Bedtime procrastination Hours of sleep Insufficient sleep Fatigue Gender Age

M-E 33.64 7.13 (0.84) 0.16⁎⁎ −0.41⁎⁎ 0.10 −0.31⁎⁎ −0.26⁎⁎ −0.14⁎ 0.22⁎⁎

ACS-90 11.69 4.85 (0.83) −0.23⁎⁎ 0.03 −0.17⁎⁎ −0.30⁎⁎ 0.05 0.09 Bedtime procrastination 3.19 0.84 (0.85) −0.34⁎⁎ 0.44⁎⁎ 0.32⁎⁎ −0.10 −0.02

Note: M-E = Morningness-Eveningness; ACS-90 = Action Control Scale. Diagonal values are the internal consistency estimates for each scale. Gender is coded 0 = female, 1 = male. ⁎ p < 0.05. ⁎⁎ p < 0.01 (all two-tailed significance tests).

Table 4 Results of hierarchical multiple regression of bedtime procrastination on demographic variables, chronotype, self-regulation skills and interaction between chronotype and self- regulation skills.

Bedtime procrastination

Step 1 Step 2 Step3 Step4

β β β β

Gender −0.09 −0.09 0.002 0.002 Age −0.01 0.01 −0.07 −0.07 Self-regulation skills −0.22⁎⁎⁎ −0.15⁎⁎ −0.15⁎⁎

Chronotype −0.39⁎⁎⁎ −0.39⁎⁎⁎

Chronotype X Self-regulation skills 0.005 R2 0.002 0.047⁎⁎⁎ 0.183⁎⁎⁎ 0.181⁎⁎⁎

R 2 change 0.008 0.048⁎⁎⁎ 0.137 0.000

Note. All beta weights are standardized; all R2 values presented in the results are adjusted R2 values.

⁎⁎ p < 0.01. ⁎⁎⁎ p < 0.001.

Fig. 1. The mediating effect of chronotype on the relationship between self-regulation skills and bedtime procrastination. Note: Path values represent unstandardized regression coef- ficients. ***p < 0.001;**p < 0.01.

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and its later amendments or comparable ethical standards. The study had been approved by the Ethical Committee of the University of Social Sciences and Humanities Faculty in Sopot. Participation in the survey was voluntary, anonymous, and unpaid.

Declarations of interest

None.

Acknowledgments

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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