Sleep restriction
J. Sleep Res. (2008) 17, 395–405 doi: 10.1111/j.1365-2869.2008.00677.x
Chronic sleep reduction, functioning at school and school achievement in preadolescents ANNE MARIE MEIJER Department of Education, University of Amsterdam, Amsterdam, The Netherlands
Accepted in revised form 2 May 2008; received 11 January 2008
SUMMARY
This study investigates the relationship between chronic sleep reduction, functioning at school and school achievement of boys and girls. To establish individual consequences of chronic sleep reduction (tiredness, sleepiness, loss of energy and emotional instability) the Chronic Sleep Reduction Questionnaire has been developed. A total of 436 children (210 boys, 214 girls, 2 missing; mean age = 11 years and 5 months) from the seventh and eight grades of 12 elementary schools participated in this study. The inter-item reliability (CronbachÕs alpha = 0.84) and test–retest reliability (r = 0.78) of the Chronic Sleep Reduction Questionnaire were satisfactory. The construct validity of the questionnaire as measured by a confirmative factor analysis was acceptable as well (CMIN ⁄ DF = 1.49; CFI = 0.94; RMSEA = 0.034). CronbachÕs alphaÕs of the scales measuring functioning at school (teacherÕs influence, self-image as pupil, and achievement motivation) were 0.69, 0.86 and 0.79. School achievement was based on self-reported marks concerning six school subjects. To test the models concerning the relations of chronic sleep reduction, functioning at school, and school achievement, the covariance matrix of these variables were analysed by means of structural equation modelling. To test for differences between boys and girls a multi-group model is used. The models representing the relations between chronic sleep reduction – school achievement and chronic sleep reduction – functioning at school – school achievement fitted the data quite well. The impact of chronic sleep reduction on school achievement and functioning at school appeared to be different for boys and girls. Based on the results of this study, it may be concluded that chronic sleep reduction may affect school achievement directly and indirectly via functioning at school, with worse school marks as a consequence. k e y w o r d s academic performance, chronic sleep reduction, preadolescents, school functioning, sleep duration, sleep quality
A considerable number of studies shows sleep duration and sleep quality to be related to academic or school performance and functioning at school (Curcio et al., 2006; Fallone et al., 2002; Wolfson and Carskadon, 2003). Academic or school performance relates to cognitive functioning of children, such as learning-memory processes and self-, parent- or schoolreported academic or scholastic achievement. In this study, we Correspondence: Anne Marie Meijer, Department of Education, University of Amsterdam, Nieuwe Prinsengracht 130, 1018 V2, Amsterdam, The Netherlands. Tel.: +31 20 525 1572; fax: +31 20 525 1200; e-mail:
[email protected] 2008 European Sleep Research Society
will use the term school achievement for studentsÕ self-reported and actual marks. Functioning at school refers to attitudes and behaviour that are supposed to promote scholastic achievement. Studies concerning sleep and functioning at school are scarce. Meijer et al. (2000) showed that sleep quality, but not time in bed, appeared to be related to functioning at school of preadolescents, such as teacherÕs influence, self-image as student and achievement motivation. However, indirect evidence for the influence of sleep on functioning at school may be derived from studies concerning learning and behavioural problems. Significant negative effects on learning, especially in
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the domain of higher-level cognitive functions and behavioural disorders, such as attention problems, might be due to sleep disturbance and daytime sleepiness (Fallone et al., 2002). Relations between sleep and functioning at school in connection with academic achievement are not examined yet. In this study, we will consider the differential relations of these concepts. As opposed to functioning at school, there are many studies that relate sleep to academic performance. Concerning the influence of sleep on academic performance, it is not clear yet whether sleep duration and sleep quality relate simultaneously or apart to academic performance. Generally, sleepiness, irregular sleep-wake patterns and earlier school starting times (Carskadon et al., 1998; Drake et al., 2003; Trockel et al., 2000; Wolfson and Carskadon, 1998) are assigned to a deteriorated academic performance. In this respect, sleepiness has been seen as a consequence of poor sleep quality (Curcio et al., 2006) or due to shortness of sleep (Wolfson and Carskadon, 2003). Research in which effects of sleep quality (or fragmentation of sleep) and sleep duration, (as measured by time in bed or actigraphy) were investigated simultaneously, displayed a predominant role of sleep quality, or fragmentation of sleep, on academic and school performance (Buckhalt et al., 2007; Eliasson et al., 2002; Howell et al., 2004; Sadeh et al., 2000, 2002; Steenari et al., 2003). However, experimental studies, which systematically restricted sleep duration, did show clear evidence for deteriorated cognitive performance of children as a consequence of shorter sleep duration due to sleep restriction. The results of these laboratory studies suggest that sleep restriction especially affects higher cognitive functions, such as verbal creativity, verbal fluency, memory tasks and abstract thinking, and that the impact of mild sleep restriction over several nights is comparable with one night sleep restriction of 5 h (Fallone et al., 2001; Randazzo et al., 1998a,b; Sadeh et al., 2003; Steenari et al., 2003). The effect of mild sleep restriction over several days on cognitive performance is an important point with respect to the influence of sleep on school achievement, which constitutes of a sequence of school marks over a longer period. This led Meijer and Van den Wittenboer (2004) to recognize the importance of the chronicity concept in cross sectional sleep research. In general, chronic sleep reduction due to bad sleep quality or too short sleep duration shows to have comparable effects (called ÔconsequencesÕ in this paper) on physiological and psychosocial functioning, such as sleepiness, tiredness, loss of energy or emotional instability (Dahl, 1999; Meijer et al., 2000; Pilcher et al., 1997; Sadeh et al., 2003). Symptoms of insufficient sleep, such as sleepiness and tiredness, may also be seen as an indication of a too short sleep with reference to oneÕs sleep need. Van Dongen et al. (2003) relate sleep debt to sleep need. They defined sleep debt as the cumulative hours of sleep loss with respect to a subject-specific daily need for sleep. Chronic sleep reduction or sleep restriction of oneÕs Ôsleep lengthÕ may lead to sleep debt, but due to inter-individual differences in sleep need (Van Dongen et al., 2004), it is
difficult to determine whether a particular sleep length is sufficient or not for a specific child. Moreover, differences in sleep duration as a function of development add to the complexity of determining sleep need in children (Wilkoff et al., 2003). Because manifestations of sleep debt in relation to sleep need may be evidence in symptoms of insufficient sleep, these symptoms may give an indication of the extent of chronic sleep reduction. So, an additional advantage of focussing on consequences of chronic sleep reduction is that the mostly overlooked aspect of individual sleep need will implicitly be taken into account. In agreement with the preceding argumentation, the positive results of sleep restriction studies as opposed to studies that examined time in bed might be explained by the increase of sleep debt in relation to sleep need in these studies. So, shortening of childrenÕs usual sleep, as has been done in sleep restriction laboratory studies and in a naturalistic situation by Sadeh et al. (2003), may lead to sleeping below oneÕs individual sleep need and subsequently to deteriorated cognitive performance. To gain a better understanding of the influence of shortness of sleep on functioning at school and academic achievement it seems thus important to study consequences of chronic sleep reduction due to both impaired sleep quality and sleeping below oneÕs sleep need. Starting from the assumption that bad sleep quality and shortened sleep over longer time show comparable effects on human functioning, we developed a questionnaire with items referring to symptoms of chronic sleep reduction, such as sleepiness, tiredness, loss of energy and emotional instability and we evaluated the reliability and validity of this questionnaire. To examine the construct validity of the questionnaire, a confirmative factor analysis was carried out on the items. After that we correlated the scale with sleep variables that are supposed to be connected with chronic sleep reduction, such as time in bed, sleep quality, variability of bedtimes and circadian preference (Russo et al., 2007). Next, we examined the relationship of chronic sleep reduction with school achievement and after that, we examined this relation with control for the effect of functioning at school (Meijer et al., 2000). Chronic sleep reduction is measured twice with an interval of 2 weeks to ascertain the chronicity aspect. A high correlation between the two measurements is expected assuming that chronic sleep reduction in the same way as chronic sleep restriction, leads to long-time-constant adaptive changes and consequently shows slow recovery to baseline levels of performance when sleep durations are extended to normal levels (Belenky et al., 2003). School performance and functioning at school were measured at the second measurement. Since functioning at school is supposed to be related to consequences of chronic reduction of sleep and functioning at school is an important factor for results at school, a decreasing rate of relationship may be expected between chronic sleep reduction and school achievement if school functioning will be partialled out. Because of gender differences in sleep variables, we performed the analyses for boys and girls simultaneously (multi-group analysis). 2008 European Sleep Research Society, J. Sleep Res., 17, 395–405
Chronic sleep reduction, functioning at school and school achievement METHOD Participants Participants were 436 children (210 boys, 214 girls, missing = 2) from the seventh (195 children) and eight grades (239 children) (missing = 2) of 12 elementary schools in the Netherlands. Ages of the children varied from 9 years and 5 months to13 years and 7 months (M = 11 years and 5 months). The schools were situated in middle-class neighbourhoods and the starting times were on 8:30. Most of the children were white and lived in families with two parents (80%). In 74% of the families both parents were employed, in 24% of the families one parent was employed and in 2% neither parent was employed. From the fathers and ⁄ or mothers 16% was born abroad.
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well) (Meijer and Van den Wittenboer, 2004). CronbachÕs alpha for this study was .70. In addition to sleep quality, the questionnaire also contained questions about bedtimes and wake-up times during school days and in the weekend. Time in bed was based on the time children usually went to bed during school days and the moment of getting up the next morning. A question concerning circadian preference was included in order to validate the chronic sleep reduction questionnaire. The question consists of five items ranging from extreme morningness (1) to extreme eveningness (5). The children had to select the item that best applied to them. The sleep log consisted of questions concerning bedtimes and wake-up times during the week and in the weekend. Besides, the children were asked to fill in a sleep quality questionnaire of six yes or no items each morning. Data of 12 nights were analysed.
Procedure and measures Procedure
Functioning at school
After getting informed consent from schools and parents, the researchers made appointments for the data collection with the teachers of the classes. The questionnaires were completed in the classroom in a non-demanding situation. The questionnaires were administered at two Times with a delay of 14 days. During this delay the participants kept a sleep log. At Time 1 questionnaires pertaining to sleep and chronic sleep reduction were administered and instructions were given with respect to the sleep log. At Time 2 the participants completed questionnaires concerning functioning at school, school marks and chronic sleep reduction and they handed in their sleep logs. The data collection took place in the last month of the first trimester of the school year.
Functioning at school was measured with two scales of the School Perception Questionnaire (Meijer et al., 2000) and an abbreviated form of the Achievement Motivation Scale (Hermans, 1983). Examples of questions of the Perception of TeacherÕs Behaviour (eight items) and Self-image concerning School Achievement (eight items) scales are: My teacher is happy if we are doing well and I can keep up with the others at school. Response categories ranked from 1 (completely true) to 5 (absolutely not true). CronbachÕs alpha values in this study were .69 and .86. The Achievement Motivation Scale consists of 12 close-ended questions with three two- and 12 three-response categories (e.g. Teachers think that: A: I am lazy; B: I am not doing the best I can; C: I am doing the best I can). Minimum score of this scale is 12 and maximum score is 33. CronbachÕs alpha in this study was .79.
Measures Sleep Chronic sleep reduction questionnaire. This questionnaire consists of 20 close-ended questions with three ordinal response categories ranking from 1 to 3. Minimum score is 20 (no indication of chronic sleep reduction) and maximum score 60 (strong indication of chronic sleep reduction). The Chronic Sleep Reduction Questionnaire is developed for this study. A description of the factor model, the reliability of the scales and the construct validity is given in the Results section. Quality of sleep. The children were assessed on aspects of sleep latency, number of awakenings at night, sleep latency after awakenings and perception of sleep quality and feeling rested after sleep. These are also criteria for insomnia as provided in the DSM IV (American Psychiatric Association (APA), 1994). The scale consists of seven close-ended questions with three ordinal response categories ranking from 1 to 3. Minimum score is 7 (sleeping badly) and maximum score 21 (sleeping 2008 European Sleep Research Society, J. Sleep Res., 17, 395–405
School achievement To measure school achievement a questionnaire was constructed consisting of six close-ended questions. The children indicated whether their mark on their last report was unsatisfactory, satisfactory or good for Dutch language, English language, Mathematics, Biology, Geography and History. This resulted in a minimum score of 6 (very bad marks) and a maximum score of 18 (very good marks). CronbachÕs alpha for this scale was .65. For approximately half of the participants we received the actual marks from the schools. Unfortunately, these were not equivalent in schools. Moreover, schools differed in the description of school subjects, e.g. only Language or separate marks for Language, Reading and Spelling. After transformation into z-scores we correlated the self-reported marks with the real marks of the trimester. These ranged from .36 (geography) to .59 (mathematics), which seems not unreasonable according to empirical guidelines (Hemphill, 2003). There were no differences in means of
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children from schools that offer actual marks and children from schools who did not offer these marks. Statistical analyses To test the factor model of the Chronic Sleep Reduction Questionnaire and the models concerning the relations of chronic sleep reduction, functioning at school and school achievement, the covariance matrix of these variables were analysed by means of structural equation modelling with the computer program AMOS (Arbuckle, 1995). In this study, we tested the structure of the Chronic Sleep Reduction Questionnaire with a confirmatory factor analysis (CFA-model) and the models concerning the relations of chronic sleep reduction with school achievement and functioning at school with a Full Latent Variable Model. This model consists of a measurement model [to be seen as a confirmatory factor analysis for the latent (unobserved) variables] and a path model in which relations between latent variables and observed variables can be specified. Although such a model, in fact, consists of many regression equations, it will mostly be represented by a graph in which arrows (paths) designate the direction of the relationship. An outgoing arrow from a latent or an observed variable indicates an independent variable in a regression equation. An incoming arrow points at the dependent variable of a regression equation. A good fit of the model is indicated by a non-significant v2 statistic or at least a v2 to degrees of freedom ratio (CMIN ⁄ DF)
