Intraindividual variability of sleep/wake patterns in relation to child and adolescent functioning: A systematic review

Abstract

Substantial research attention has been devoted to understanding the importance and impact of sleep in children and adolescents. Traditionally, this has focused on mean sleep variables (e.g., a child's "typical" or average sleep duration), yet research increasingly suggests that intraindividual variability (IIV) of sleep/wake patterns (sometimes referred to as sleep variability or night-to-night variability) regularly occurs and may have implications for adjustment. A systematic search of five electronic databases identified 52 empirical studies published between 2000 and 2015 that examined correlates of sleep IIV in children and adolescents, with a recent increase in the publication rate of such studies. Identified studies were often atheoretical and included post hoc analyses, though IIV in select aspects of sleep does appear to be associated with increasing age/pubertal status, non-White race, physical and neurodevelopmental conditions (e.g., attention-deficit/hyperactivity disorder; autism), psychopathology symptoms (e.g., anxiety, depression, inattention), body weight, stress, aspects of cognitive functioning, and poorer sleep functioning/habits. The limited intervention work examining sleep IIV in adolescents is promising, though studies are needed using more rigorous intervention designs. Clinical sleep recommendations may not only need to address overall sleep duration and sleep habits but also the stability of sleep duration and timing. It will be important for future research examining sleep IIV in children and adolescents to use a developmental framework in advancing theory pertaining to the causes, mechanisms, moderators, and outcomes of sleep IIV in youth, and a conceptual model is proposed to help guide such efforts.

Keywords: ADHD; Day-to-day; Development; Fluctuation; Intraindividual; Night-to-night; Sleep; Variability; Variation; Youth.

Figure 1

Flow diagram of study selection. Note. n = number of studies, IIV = intraindividual variability.

Figure 2

Summary of quality assessment for included studies. Note. 1. A priori aim/hypothesis: whether there were aims/hypotheses specific to intraindividual variability (IIV) of sleep/wake patterns; “++” indicates specific aims/hypotheses related to IIV; “+” indicates IIV implied but not explicitly stated, e.g., difference in sleep patterns; “-” indicates no aim/hypothesis specific to IIV. 2. Sample size justification: whether sample size was justified; “++” indicates justified, “+” indicates justified based on outcomes other than sleep/wake variability, “?” indicates unjustified. 3. Sample representativeness: representativeness of samples for the intended study population and for conclusions drawn, with “++”, “+”, and “-” indicating “good”, “fair”, and “poor”. 4. Number of days: the number of continuous days variability was based on; “++” indicates ≥ 14, “+” indicates ≥7 and <14, and “-” indicates <7. 5. Quality of sleep measures: “++” indicates well validated, “+” indicates not well validated. 6. Quality of correlates measured: “++” indicates well validated, “+” indicates not well validated. 7. Rates of missing in daily data: “++” indicates ≤10%, “+” indicates ≤20% and >10%, “-” indicates >20%, and “?” indicates not reported. 8. Inferences and conclusions: quality of inferences and conclusions drawn, with “++”, “+”, and “-” indicating “good”, “fair”, and “poor”.

Figure 3

Number of articles included in review by year of publication. Note. One of the 2015 articles is an advance online publication awaiting pagination in a journal issue.

Figure 4

Proposed theoretical model for intraindividual variability (IIV) of sleep/wake patterns in children and adolescents.