Paradoxical association between chronotype and academic achievement: eveningness reduces academic achievement through sleep disturbance and daytime sleepiness

Abstract

There are conflicting reports about the association between chronotype and academic achievement. Eveningness persons tend to have lower academic achievement, but have higher cognitive abilities. We hypothesized that sleep disturbance and daytime sleepiness, which are known to affect academic achievement, will interact with this association. To investigate the association, a sleep survey and covariance structure analysis was performed on high-school students. Among a total of 344 first-year high-school students, 294 students validly completed the questionnaire. The association between the recent change in their academic achievement, chronotype, daytime sleepiness, and sleep disturbance were analyzed. A simple comparison demonstrated that not chronotype but sleep disturbance and excessive daytime sleepiness were significant associated factors. Chronotype affects academic achievement through sleep disturbance and daytime sleepiness. Chronotype did not have a significant total effect on the reduction in academic achievement, whereas morningness had a significant direct effect and a significant indirect inverse effect through better sleep and less daytime sleepiness. This model accounted for 13.0% of the variance of the reduction in academic achievement. When discussing the association between chronotype and academic achievement, the effect of sleep disturbance and daytime sleepiness should be considered. Reducing sleep disturbance and daytime sleepiness with consideration to the chronotype of each person would be beneficial for the improvement of academic achievement.

Introduction

Academic achievement is a public health concern that goes beyond personal issues. It is not only associated with the mental and physical health of individuals [1] but also with welfare expenses, crime costs, and unemployment rates [2]. In adolescents, associations between sleep problems and academic achievement, and between chronotype and academic achievement have been reported previously. Sleep problems in adolescents are very common all over the world [3]. Sleep problems and daytime sleepiness are associated with academic achievement [4–7], and a meta-analysis demonstrated that daytime sleepiness is significantly associated with academic achievement and that sleep quality and sleep duration are the secondary factors [8].

Chronotype is a circadian typology that includes morningness and eveningness, and is an important factor that affects sleep, daytime performance, and physical and mental conditions [9]. Chronotype is known to be affected by age, and adolescents tend to physiologically have the eveningness chronotype [10–12]. Although several studies have analyzed the association between chronotype and academic achievement, the conclusion remains uncertain. Some studies showed an association between higher academic achievement and the morningness chronotype, or an association between lower academic achievement and the eveningness chronotype, in early adolescents [13–16] and high school [15] and university students [17–20]. On the other hand, a review pointed out that such associations appear to depend on the time within the curriculum or on the time of the exams [21]. A large population study revealed the association between social jet lag and academic achievement, and suggested the importance of the viewpoint of chronotype [22]. For exams held at later times of the day, chronotype is not associated with academic achievement and students with eveningness tend to achieve higher scores, whereas students with morningness tend to achieve lower academic scores [23].

Previous meta-analyses [24–26] showed the paradoxical result that the morningness chronotype is associated with lower cognitive ability and higher academic achievement, and the eveningness chronotype is associated with higher cognitive ability and lower academic achievement in adolescents. Recently, it was shown that an early school start time is associated with poor academic achievement [27] and the interventions to start school at later times may improve students’ sleep problems, daytime sleepiness, and academic achievement [28, 29].

Hence, we hypothesized that chronotype does not have a direct effect on academic achievement, but has indirect effects via subsequent sleep disturbance and daytime sleepiness. Therefore, the purpose of this study was to analyze and calculate the direct and indirect effects of the association between chronotype and sleep disturbance, daytime sleepiness, and academic achievement using covariance structure analysis.

Materials and methods

Participants

This research was conducted at a senior high school in Tokyo, Japan, in December 2015, and had the same subject population as a previous study [30]. The high school was a national public school that requires students to pass an entrance exam for entry. Of the 344 first-year high-school students (171 adolescent boys and 173 adolescent girls) and 334 (97.1%) gave informed consent and responded to the questionnaire. The questionnaire was distributed by teachers during school hours. Of these students, 294 (85.5%) validly completed the questionnaire, and the dataset was used for the following analyses. The data of the remaining 40 students with missing values, and 10 students who did not answer the questionnaire were excluded from the study. This survey was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Tokyo Medical University, Tokyo, Japan (approval number: #3158).

Questionnaire

The questionnaire consisted of demographic items, sleep-associated variables, and academic achievement. Regarding the participants’ sleep disturbance, the Pittsburgh Sleep Quality Index (PSQI) [31, 32] was used. The PSQI comprises the following seven components: sleep quality, sleep latency, sleep duration, habitual sleep efficiency, frequency of sleep disturbance, use of sleep medication, and daytime dysfunction. A higher PSQI score indicates severer sleep disturbance, and a total score of 6 or more points in the Japanese version indicates the existence of sleep disturbance. To evaluate students’ chronotypes, the diurnal type scale [33, 34] was used. A higher diurnal type scale score means that the participants have the morningness chronotype, and a lower score means eveningness chronotype. In this study, participants who had a diurnal type scale score of 16 (the median) or less were classified as having the eveningness chronotype, and participants with a diurnal type scale score of 17 or more were classified as having the morningness chronotype, according to the previous study [34]. Participants with a diurnal type scale score of 17 or more were classified as having the morningness chronotype. Students’ daytime sleepiness was also assessed using the Pediatric Daytime Sleepiness Scale (PDSS) [35, 36]. Higher PDSS scores mean that the participants have severer daytime sleepiness, and a score of 21 or more indicates the existence of excessive daytime sleepiness in accordance with the original criteria. A reduction in academic achievement was detected based on the answer to a question asking whether the student’s current academic achievement is lower than that of a several months ago: “In the previous several months, in total, how did your academic achievement change in school, cram school, or preparatory school?”, and thus, academic achievement in this study was assessed as changes from one’s recent baseline. Whereas a student’s absolute or relative state of academic achievement is considered to reflect his/her own intelligence, the baseline score, and the past efforts, these factors are expected to be adjusted to measure the changes in academic achievement and will hence clarify the effects of sleep and circadian rhythm on academic achievement. Furthermore, as the students in this study population had passed a uniform high-school entrance exam, it was assumed that their basic academic achievement would be distributed in a narrow range, with low variation.

Data analysis

First, a comparison of demographic variables, PSQI scores, PDSS scores, and diurnal type scale scores regarding reduction in academic achievement was performed using the Chi-square test and Student’s t test. Second, logistic regression analyses were performed to assess the factors associated with a reduction in academic achievement. Third, to determine the association between chronotype, sleep disturbance, daytime sleepiness, and reduction in academic achievement covariance structure analysis was performed. The structural equation model (SEM) was analyzed by robust maximum-likelihood estimation in covariance structure analysis.

IBM SPSS Statistics version 23 and MPlus [37] version 7.11 were used for the above analyses. A p value of less than 0.05 was considered to indicate a statistically significant difference between 2 groups.

Results

A total of 294 students, comprising 129 adolescent boys (43.9%) and 151 adolescent girls (51.4%), were analyzed. The remaining 14 students (4.8%) did not answer about their sex. The mean age was 15.8 years [± standard deviation (SD) = 0.41]. Of these students, 173 (58.8%) were in a school club that involved physical exercise, and 82 (27.9%) were in a school club that did not involve physical exercise, and 39 (13.3%) were not in any school clubs.

Table 1 shows a summary of the demographic characteristics, sleep-associated variables, and a comparison of the reduction in academic achievement of the students. The mean score of the PSQI was 5.94 (SD ± 2.31, Cronbach’s alpha = 0.716), with mean scores of C1: 1.29 (± 0.74), C2: 0.63 (± 0.78), C3: 1.73 (± 0.72), C4: 0.11 (± 0.47), C5: 0.51 (± 0.54), C6: 0.03 (± 0.26), and C7: 1.63 (± 0.85). The mean score of PDSS was 19.0 (± 5.8, Cronbach’s alpha = 0.647), and that of the diurnal type scale was 16.7 (± 3.4, Cronbach’s alpha = 0.538). The existence of sleep disturbance (PSQI ≥ 6) and excessive daytime sleepiness (PDSS ≥ 21) were significantly associated with a decrease in academic achievement. Chronotype (based on the diurnal type scale) was not a significant factor.

Table 1.

Description of demographic characteristics, sleep-related variables, and group comparison

			Reduction of academic achievement
		Total: n or mean	Yes: n or mean	No: n or mean	χ2 or t value
Sex
Male		129	39	90	χ2(df=2) = 3.173 (n.s.)
Female		151	57	94
No response		14	7	7
Belonging to a club
Exercise		173	62	111	χ2(df=2) = 2.501 (n.s.)
Non-exercise		82	24	58
Not belong		39	17	22
Age		15.8 (± 0.4)	15.8 (± 0.5)	15.8 (± 0.4)	t = − 0.920 (n.s.)
Sleep-related variables (categorical)
Sleep disturbance (PSQI ≥ 6)	Yes	156	65	91	χ2(df=2) = 6.424**
	No	138	38	100
Excessive daytime sleepiness (PDSS ≥ 21)	Yes	129	61	68	χ2(df=2) = 15.162***
	No	165	42	123
Morningness (diurnal type scale ≥ 17)	Yes	157	58	99	χ2(df=2) = 0.539 (n.s.)
	No	137	45	92
Sleep-related variables (continuous)
PSQI global score		5.94 (± 2.31)	6.3 (± 2.35)	5.74 (± 2.28)	t = 1.999*
PSQI C1		1.29 (± 0.74)	1.38 (± 0.79)	1.25 (± 0.7)	t = 1.476 (n.s.)
PSQI C2		0.63 (± 0.78)	0.63 (± 0.84)	0.63 (± 0.74)	t = − 0.026 (n.s.)
PSQI C3		1.73 (± 0.72)	1.69 (± 0.74)	1.75 (± 0.71)	t = − 0.673 (n.s.)
PSQI C4		0.11 (± 0.47)	0.14 (± 0.51)	0.10 (± 0.45)	t = 0.631 (n.s.)
PSQI C5		0.51 (± 0.54)	0.59 (± 0.51)	0.46 (± 0.55)	t = 2.002*
PSQI C6		0.03 (± 0.26)	0.03 (± 0.22)	0.04 (± 0.28)	t = − 0.237 (n.s.)
PSQI C7		1.63 (± 0.85)	1.84 (± 0.96)	1.51 (± 0.77)	t = 3.235**
PDSS		19.0 (± 5.8)	20.6 (± 5.9)	18.3 (± 5.7)	t = 3.282**
Diurnal type scale		16.7 (± 3.4)	17.1 (± 3.5)	16.5 (± 3.4)	t = 1.623 (n.s.)

PSQI Pittsburgh Sleep Quality Index, PDSS Pediatric Daytime Sleepiness Scale

*p < 0.05, **p < 0.01

Table 2 shows the results of logistic regression analysis of the associated factors with reduction in academic achievement. Univariate logistic regression showed that sleep disturbance and daytime sleepiness were significantly associated with, but chronotype was not significantly associated with a reduction in academic achievement. Daytime sleepiness was the most effective statistically significant factor for the reduction in academic achievement [existence of excessive daytime sleepiness (PDSS ≥ 21); adjusted odds ratio in multivariate analysis (adjusted OR) = 2.57; 95% confidence interval (95% CI) = 1.55–4.26 (p < 0.001)]. When the confounding factors were adjusted in a continuous variable model, not eveningness, but morningness (higher diurnal type scale score) was associated with the reduction in academic achievement [1 point increase on the diurnal type scale (more likely to be morningness): adjusted OR 1.09; 95% CI 1.00–1.19, p = 0.043].

Table 2.

Logistic regression analysis of factors associated with a decrease in academic achievement

	Univariate odds ratios			Multivariate odds ratios
	OR	95% CI	p	aOR	95% CI	p
Continuous variable model
PSQI (+ 1 pt)	1.15	1.03–1.29	0.017	1.08	0.97–1.22	n.s
PDSS (+ 1 pt)	1.10	1.04–1.15	< 0.001	1.11	1.05–1.17	< 0.001
Diurnal type scale (+ 1 pt)	1.05	0.97–1.13	n.s	1.09	1.00–1.19	0.043
Categorical variable model
Sleep disturbance (PSQI ≥ 6)	1.88	1.15–3.07	0.012	1.74	1.05–2.91	0.033
Excessive daytime sleepiness (PDSS ≥ 21)	2.63	1.61–4.30	< 0.001	2.57	1.55–4.26	< 0.001
Morningness (diurnal type scale ≥ 17)	1.20	0.74–1.94	n.s	1.47	0.88–2.44	n.s

A higher diurnal scale score indicates a higher likelihood of being morningness, and a lower score indicates a higher likelihood of being eveningness

PSQI the Pittsburgh Sleep Quality Index, PDSS Pediatric Daytime Sleepiness Scale

Figure 1 and Table 3 show the results of covariance structure analysis with a saturated structural equation model performed to clarify the mechanism by which chronotype affects academic achievement through sleep disturbance and daytime sleepiness. Morningness (higher diurnal type scale score) did not have a significant total effect on the reduction in academic achievement [standardized total path coefficient (total effect) = 0.119; n.s.]; however, morningness had a significant direct effect and significant indirect inverse effects. Morningness reduced academic achievement directly (direct effect = 0.188; p = 0.032), but counteracted the reduction in academic achievement through better sleep and less daytime sleepiness (total indirect effect =  − 0.069; p = 0.003). This model accounted for 13.0% of the variance of the reduction in academic achievement. This model was built to analyze the direct and indirect effects between all paths. As this was a saturated model, the model fit indices indicated the maximum values with Root Mean Square Error of Approximation (RMSEA) = 0.000, CFI = 1.000, and TLI = 1.000.

Fig. 1.

Structural equation model of the factors associated with academic achievement. Results of covariance structure analysis of the structural equation model regarding the association between sleep-associated variables and the reduction in academic achievement. Rectangles indicate the observed variables. Numbers beside the arrows show the standardized path coefficients. PSQI Pittsburgh Sleep Quality Index, PDSS Pediatric Daytime Sleepiness Scale. A higher diurnal type scale score indicates a higher likelihood to be morningness. Reduction of academic achievement: YES = 1, NO = 0. *p < 0.05, **p < 0.01, ***p < 0.001

Table 3.

Standardized path coefficients between each variable

	Direct effect to
From	Sleep disturbance (PSQI)	Daytime sleepiness (PDSS)	Reduction of academic achievement
Diurnal type scale	− 0.238***	− 0.136**	0.188*
PSQI		0.367***	0.124
PDSS			0.294***

	Indirect effect to
	Sleep disturbance (PSQI)	Daytime sleepiness (PDSS)	Reduction of academic achievement
Diurnal type scale	Via PSQI	− 0.087***	− 0.030
	Via PDSS		− 0.040*
	Total indirect effect		− 0.069**
PSQI	Via PDSS		0.108***

	Total effect to
	Sleep disturbance (PSQI)	Daytime sleepiness (PDSS)	Reduction of academic achievement
Diurnal type scale	− 0.238***	− 0.223**	0.119
PSQI		0.367***	0.232***
PDSS		–	0.294***

A higher diurnal scale score indicates a higher likelihood of being morningness, and a lower score indicates a higher likelihood of being eveningness. Reduction of academic achievement: YES = 1, NO = 0

PSQI Pittsburgh Sleep Quality Index, PDSS Pediatric Daytime Sleepiness Scale

*p < 0.05, **p < 0.01, ***p < 0.001

Discussion

This is the first study clarifying the reason as to why previous studies investigating the association between chronotype and academic achievement demonstrated inconsistent results.

Contrary to previous studies, the assumption that eveningness simply impairs academic achievement was not supported in our present study. The simple comparison and univariate analysis did not detect a significant association between chronotype and change in academic achievement. Furthermore, multivariate logistic regression analysis with the continuous variables model showed a weak positive correlation between the morningness chronotype and the reduction in academic achievement (aOR: 1.09, 95% CI 1.00–1.19, p = 0.043) when controlling for sleep disturbance and daytime sleepiness. The results of covariance structure analysis also indicated that morningness significantly directly associates with and indirectly inversely associates with the reduction in academic achievement. Rather, our results suggest that eveningness students may only impair academic achievement in limited cases in which such a chronotype was the cause of a student’s sleep problem or daytime dysfunction. A previous meta-analysis [24] indicated the weak correlation between chronotype and academic achievement (r =  − 0.14), and another review [21] pointed out that the effect is unstable and disappeared when the curriculum or the exam was in the afternoon. The present study, which identified two important mediators, namely, sleep disturbance and daytime sleepiness, demonstrated the mechanism of this inconsistency and instability. Some studies indicated that persons with the eveningness chronotype had higher cognitive ability and intelligence functions [24–26, 38, 39]. Thus, there might be a mechanism associated with eveningness persons having higher intelligence functions for some unknown reason, but such persons have poor sleep quality and daytime sleepiness caused by a misalignment of their own internal body clock, and as a result, they have reduced academic achievement.

These results and the suggested mechanism are important for public policymaking. Although sometimes adolescents are recommended to go to bed early and get up early [20], the results of our present study do not be recommend this. Although there is the possibility of some adjustment by lifestyle interventions [30], adolescents tend to inherently have an eveningness chronotype and adolescents tend to physiologically have the eveningness chronotype [10–12], and a person’s chronotype is innate and unchangeable. For instance, a biological study demonstrated that a strong association between cell cycle time and chronotype [40] and a cohort study demonstrated very low changeability of chronotype even when subjects lived together for a long time [41]. Although the association between morningness—“early to bed and early to rise”—and higher academic achievement is sometimes found [15, 18–21, 42], our present study demonstrated that this may be a spurious association and may be explained by sleep disturbance and daytime sleepiness. Thus, a policy that does not address the improvement of sleep disturbance and daytime sleepiness would be meaningless. If “early to bed and early to rise” leads to daytime sleepiness, it will result in an inverse effect. Previous studies on interventions to start school at a later time indicated that “early to bed and early to rise” resulted in sleep disturbance and daytime sleepiness and that rising at a later time reduced sleep disturbance and daytime sleepiness [43–45], and improved academic achievement [27, 28].

This study has some limitations. First, we should point out that academic achievement was assessed by subjective measurements asking about the change in achievement, and not the actual state. Therefore, the accuracy and validity of the measurements remain to be clarified. The absolute or the relative academic achievement of students also remains unclear. The change in academic achievement and the integral of it will determine the state of academic achievement, and hence, it may not be possible to directly compare the results of this study with those of previous studies. Second, this study was a cross-sectional study conducted at a single time point in school life. To confirm the validity of this study, a longitudinal study is necessary. Third, the sample was collected from only one school and only one grade with students from a limited geographical area, and the students were those who had passed the school entrance exam. Hence, further research analyzing a wider range of subjects is required to generalize the results of this study.

In conclusion, discussions such as “eveningness is bad” and “morningness is good” are not very meaningful regarding academic achievement. To improve academic achievement, it will be necessary for individuals to determine and solve the problems causing sleep disturbance and daytime dysfunction, such as sleep disorders, inadequate sleep hygiene, and mismatch between their sleep times and their own chronotype.

Declarations

Conflict of interest

The authors declare that they have no conflicts of interests associated with this manuscript. Akiyoshi Shimura has received fees from Dainihon Sumitomo Pharma, MSD, and Eisai outside of the submitted work. Takeshi Inoue has received personal compensation from GlaxoSmithKline, Mochida Pharmaceutical, Asahi Kasei Pharma, and Shionogi; grants from Astellas; and grants and personal compensation from Otsuka Pharmaceutical, Dainippon Sumitomo Pharma, Eli Lilly, Eisai, Mitsubishi Tanabe Pharma, Pfizer, AbbVie GK, MSD, Yoshitomiyakuhin, Takeda Pharmaceutical, and Meiji Seika Pharma; and is a member of the advisory boards of GlaxoSmithKline, Pfizer, Eli Lilly, Mochida Pharmaceutical, and Mitsubishi Tanabe Pharma outside of the submitted work.