Summary: This is the first systematic review of the literature on sleep and its relation to cognition and behaviour in preschool‐aged children. In comparison with the literature focused on school‐aged children, knowledge involving preschoolers is rather sparse. A total of 26 studies was included in this review, which revealed a high degree of heterogeneity regarding the type and means of measuring sleep variables and behavioural and cognitive variables, as well as the statistical methods employed. Amongst the 13 articles with the largest sample sizes (top 50% of the included studies, 12 different populations), 12 found that a higher quantity or quality of sleep was associated with better behavioural and/or cognitive outcomes. Results point to an association between sleep, behaviour and cognition as early as preschool years, but the strengths of associations reported in the articles were relatively small. Studies with a smaller sample size were less concordant. It is consistent with our findings that the strengths of association are small, and thus require large sample sizes to ensure statistical detection power. Different aspects of sleep were not associated with all cognitive or behavioural features in the same way, which underscores the need for specific measures rather than general ones such as ‘sleep problems’ or ‘behaviour problems’ to be able to decipher the relationships. There is also a need for large longitudinal studies using objective measures and accounting for confounding factors. The child's genotype has recently been shown to have a moderating role in the association between sleep and behaviour, and should be further explored.

Actigraphy (ACT) is a non-invasive objective assessment tool for the study of sleep–wake rhythms. It is of particular interest in children with autism spectrum disorder (ASD), as sleep disorders are highly prevalent and have a significant impact on both cognitive and behavioral functions. As polysomnography (PSG), the gold standard for the assessment of sleep, is difficult to perform in children with ASD, ACT has become a tool of choice but has not yet been validated against PSG using state-of-the-art methodology. The main objective of this study was to assess, for the first time, the validity of ACT compared to PSG for the measurement of sleep in children with ASD. During the same night of hospitalization, PSG and ACT were conducted in 26 children (6 girls and 20 boys; mean age 5.4 years ± 1.6) diagnosed with ASD according to DSM-5 criteria and standardized diagnostic scales. Sleep parameters were total sleep time (TST), sleep latency (SL), wake after sleep onset (WASO), and sleep efficiency (SE). To compare PSG and ACT, we conducted sleep parameter agreement analyses including: intraclass correlation coefficient (ICC), Bland-Altman plots, and equivalence tests. The comparison also included an epoch-by-epoch (EBE) agreement analysis to determine sensitivity (ability to detect sleep) and specificity (ability to detect wake). According to equivalence tests, the difference between ACT and PSG measures was clinically acceptable for TST (<30 min, p < 0.01), SL (<15 min, p < 0.001), and SE (10%, p < 0.01), but not for WASO (<15 min, p = 0.13). There was a good agreement between methods for SL (ICC = 0.79) and TST (ICC = 0.85) and a moderate agreement for WASO (ICC = 0.73) and SE (ICC = 0.68). The EBE agreement analysis revealed a high sensitivity (0.94 ± 0.06) and moderate specificity (0.5 ± 0.2). Since sleep disorders are one of the most common comorbidities within the ASD population and are highly prevalent, it is essential to validate objective tools of assessment. To our knowledge, our study is the first to validate ACT compared to PSG, using a state-of-the-art methodology, in children with ASD. The results suggest ACT to be a valid method to evaluate sleep within this population, with a good reliability for most sleep parameters.

Sleep disorders are among the most common comorbidities in children with Autism Spectrum Disorder (ASD), and subjectively defined sleep disturbances have been related to ASD symptom severity. However, no study has investigated the differential impact of objectively measured sleep and circadian rhythm disturbances on behavioral difficulties in this population. Fifty-two children with ASD aged 3–10 years underwent assessments of sleep and circadian rest–activity rhythms objectively with actigraphy and subjectively with the Children’s Sleep Habits Questionnaire. Behavioral difficulties were assessed using the ABC-C. Group comparison analyses were used to compare sleep and circadian rhythm parameters of children with higher and lower behavioral difficulties and dominance analysis to rank predictors and address multicollinearity. Children with high irritability had a shorter continuous sleep period compared to those with lower irritability (−60 min, p = 0.04), as well as those with high stereotypic behaviors compared to children with less stereotypies (−75 min, p = 0.006). Objective circadian and sleep disturbances accounted together for, respectively, 17%, 18% and 36% of the variance in social withdrawal, irritability and stereotypic behaviors. The identification of both sleep and circadian rhythm disturbances as explanatory factors for behavioral difficulties warrants their inclusion in the existing behavioral management strategies for children with ASD.

The Seasonal Pattern Assessment Questionnaire (SPAQ), by Rosenthal et al. (1984), is by far the most used questionnaire to evaluate seasonal effects on mood and behavior. It includes a general seasonality score (GSS), composed of 6 items, from which cutoffs have been established to screen for seasonal affective disorder (SAD). However, it has never been validated in French and associations with circadian rhythm and symptoms of depression and bipolarity remain unclear. In this study, including 165 subjects (95 controls and 70 patients with depression or bipolar disorder), we confirmed the validity of the French version of the SPAQ, with a two-factor structure (a psychological factor: energy, mood, social activity and sleep length; and a food factor: weight and appetite) and a good fit was observed by all indicators. Mood and social activity dimensions were significantly affected by seasons in the depressed/bipolar group and a stronger global seasonality score (GSS) was associated with more severe phenotypes of depression and mania. Subjects meeting SAD and subsyndromal-SAD criteria also showed a delayed circadian rhythm compared to controls. Simple tools, such as the SPAQ, can aid the identification of significant seasonal changes and have direct implications on therapeutics including the use of bright light therapy in order to enhance personalized treatments, but also to prevent adverse seasonal effects.