Original ArticleShort sleep duration is associated with poor performance on IQ measures in healthy school-age children
Introduction
Accumulating evidence indicates that sleep has beneficial effects on attention, behavior, and academic success. Previous studies have identified a significant relationship between sleep fragmentation and decreased cognitive capacity, particularly in the domains of executive control, working memory and attention [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. In pediatric research, most of these studies were conducted in clinical populations of children suffering from sleep-disordered breathing, a clinical condition that might have adverse effects on the brain, potentially leading to deficits in neuropsychological functioning (e.g., as reviewed in Halbower & Mahone) [11]. A few other studies [12], [13], [14], [15] investigating the impact of sleep duration on cognitive functioning were conducted in healthy children. One such study showed that short sleep duration approximately tripled the risk of low performance on school-entry tests in young children [16], whereas another study did not confirm these findings in 7-year-old participants [17]. It is important to note that most of these studies were either based on a single night rather than a longer evaluation of sleep duration (which would have better represented the child’s habitual sleep duration above and beyond intra-individual variations) or were based on subjective parental reports of the children’s sleep and attentional functioning rather than objective measures.
Furthermore, the specific impacts of sleep duration on different aspects of cognitive processing have not been investigated thus far. Intellectual functioning has been separated into many related abilities, such as the capacities to reason, plan, and problem-solve, think abstractly, comprehend ideas, use language, and learn. No published study, however, has yet addressed whether these systems are differentially affected by an individual’s sleep duration. In the literature on adults it has been shown that learning, [18], [19], [20], [21], [22] sustained attention, [23], [24], [25] divided attention, [26] inhibitory efficiency, [27] decision making, [28] and emotional responses to pictures [29] are significantly affected by sleep deprivation. Determining which aspects of intellectual functioning are specifically affected by sleep duration in children could aid in the development of interventions that use sleep as a means to improve these aspects of children.
There is little information regarding the association between sleep and cognitive functioning in children with habitual long sleep durations versus those with short sleep durations, but there is no evidence of sleep deprivation. It is not yet clear whether a short natural sleep duration, in the absence of daytime sleepiness indicating sleep deprivation, is associated with deficits in the same aspects of cognition that are sensitive to sleep deprivation. Given that sleep duration varies considerably between individuals, it is important to define the difference between sleep deprivation and short sleep duration.
From both a clinical and experimental standpoint, the most reliable observable manifestation of insufficient sleep (i.e., sleep deprivation) is excessive daytime sleepiness. Objective studies designed to assess fatigue/alertness have used the Multiple Sleep Latency Test (MSLT), which measures the speed of falling asleep under standard conditions and takes a shorter sleep latency period to indicate a higher level of physiological sleepiness [30]. None of the prior studies examining the association between sleep duration and cognitive performance in children have objectively evaluated the children’s levels of sleepiness. Thus, it is unclear whether the findings of these studies were related to habitual sleep duration versus the consequences of sleep deprivation.
The purpose of this study was to examine the association between habitual sleep duration and cognitive functioning in healthy, non-sleep-deprived participants using objective measures of sleep and cognition. The Wechsler Intelligence Scale for Children-Revised (WISC-R) was used to measure intellectual functioning. In addition, multiple measures were used to assess the participants’ behavior and daytime functioning at school. Each child’s habitual sleep duration was objectively measured for four consecutive week-nights using actigraphy. The MSLT was used to determine the child’s daytime level of physiological sleepiness. Physiological confounders such as puberty and Body Mass Index (BMI) were measured and controlled for. Our hypothesis was that participants with longer sleep duration would perform better than those with short sleep duration on the objective cognitive measures.
Section snippets
Participants
The study population consisted of 39 participants (age range 7–11 years; mean = 8.62, SD = 1.28). Most of the participants were Caucasian (65%), with the remainder classified as African American (7%), Asian (10%), and “other” (mixed ethnicity; 18%). Their average IQ was 104.49 (SD = 16.95). Subjects were excluded from the study if they had (1) an IQ less than or equal to 80, as measured by the Wechsler Intelligence Scale for Children-IV (WISC-IV); (2) a history of psychiatric, developmental disorder,
Sleep duration
Table 1 presents the demographic and sleep characteristics (means and standard deviations) of the enrolled participants. Parental reports indicated that the studied participants spent a mean time of 10:03 h (SD 43 min; range 8:56 to 10:51 h) in bed per night. The mean reported bedtime was 21:25 (SD 57 min), and the mean rising time was 07:23 (SD 52 min). The apparent time spent in bed, as measured by actigraphy, had a mean of 9:28 h (SD 39 min; range from 8:24 to 10:16 h); this was an average of 35 min
Discussion
The main goal of the study was to examine the association between habitual sleep duration and cognitive functioning in healthy school-age participants who showed no evidence of sleep deprivation. The study extended previous research in the following ways: by using objective methodology for the assessment of sleep, cognition and daytime fatigue, rather than more subjective parental reports; by assessing sleep in the participants’ natural home environment for multiple nights in a row, thereby
Acknowledgments
This study was supported by grants to Dr. Reut Gruber from the Canadian Institutes of Health Research (CIHR; Grant number 153139) and the Fonds De La Recherche en sante (FRSQ; Grant number 10091). The authors are grateful to the children and families who participated in this study.
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