Background
Autism spectrum disorder (ASD) affects 0.5 to 1% of children [
1‐
3] and has an early onset, typically before age 2 [
4‐
6]. ASD is often characterised by severe deficiencies in social interaction and communication, accompanied by repetive behaviour. Children with ASD frequently suffer from comorbid psychopathologies [
1,
7‐
9]. Among those, sleep problems, defined as difficulties falling asleep or nightmares, are common [
10] occurring in 40–80% of cases across all ages [
11‐
17] in comparison to 25–50% in normally developing children [
16,
18,
19]. The broad range of prevalence estimates is explained by multiple factors, such as different measures for sleep problems, age of the child, IQ of the autistic children studied, and the heterogeneity of ASD. What is more, there is no clear definition of clinically relevant sleep problems in pediatric populations, resulting in various forms of research questions on sleep problems in ASD (for a review on prevalence of sleep problems in ASD see Richdale & Schreck, 2009) [
16]. As mentioned, the type of sleep problems differs; younger children with ASD exhibit more bedtime resistance, bedtime anxiety, awakenings during the night, and parasomnias (defined as abnormal behavior during sleep, such as sleep walking, sleep talking, and nightmares), whereas older children mainly exhibit insomnia symptoms (defined as the difficulty falling asleep or staying asleep) [
16,
20].
The association between sleep problems and ASD can be of two forms. First sleep problems may precede and worsen the behavioral outcome of ASD [
20‐
22]. Second, sleep problems occur as a consequence of the underlying disorder
. Sleep problems are common in early childhood with prevalence estimates of up to 50% [
18]; prevalence decline in typically developing children but not in children with ASD [
23]. Risk factors or correlates of early childhood sleep problems are, for example, maternal psychopathology, parenting practices, child temperament, difficulties setting bedtime, and feeding patterns [
19,
24,
25]. The influence of these factors diminishes when the child’s sleep patterns become more stable [
24,
25]. Studies have indicated that children with ASD have more sleep problems than typically developing children [
16]. A British cohort study showed that children with and without ASD have similar sleep durations in infancy, but from 30 months onwards, their sleep is characterized by a shorter duration than typically developing children [
26]. Another study showed that children with autistic traits developed more sleep problems in pre-adolescence [
27]. However, most previous studies of sleep problems in children with ASD are cross-sectional and the few longitudinal studies have a lack of baseline measures and diagnosis of ASD [
21,
27‐
29]. Thus, it is difficult to properly asses the course of sleep problems in children with ASD [
28,
30,
31]. To unravel the complex temporal nature of the association between sleep problems and ASD, it is essential to have prospective research that measures autistic traits and associated sleep problems repeatedly throughout childhood.
In this study, we explored the association between the onset of sleep problems and autistic traits and ASD in the general population. An ASD diagnosis must be confirmed by a licensed clinician, while autistic traits are autistic symptoms that do not meet the diagnostic criteria for ASD assessed by questionnaires. Repeated assessments of autistic traits and sleep problems were obtained at several developmental stages. It is important to clarify whether sleep problems precede and worsen autistic traits and ASD or occur after (other) symptoms of ASD become manifest. This enables us to gain more insight in the course over time of sleep problems in children with ASD.
First, we expected that the onset of sleep problems precedes and worsens the early manifestations of autistic traits. Second, we hypothesized that sleep problems in children with autistic traits or with ASD emerge early in life and increase over time.
Results
Characteristics of the children with and without ASD are presented in Table
1. Children with a diagnosis of ASD (
n = 81) were more often boys (86.4%). Their mothers reported more psychopathological symptoms for themselves than mothers of children without ASD. There were no significant differences in the other characteristics, such as maternal age at birth.
Table 1
Characteristics of the study population
Child characteristics |
Gender (% girls) | 5143 | 50.1 | 13.6* |
Gestational age at birth (weeks) | 5102 | 39.80 (0.03) | 39.36 (0.26)* |
Ethnicity (%) |
Dutch | 3386 | 69.7 | 75.6 |
Other-Western | 461 | 9.5 | 7.7 |
Non-Western | 1007 | 20.8 | 16.7 |
Sleep problem score |
At 1.5 years | 3875 | 1.51 (0.03) | 1.75 (0.26) |
At 3 years | 3837 | 1.50 (0.03) | 1.77 (0.24) |
At 6 years | 4981 | 1.02 (0.02) | 1.85 (0.25)* |
At 9 years | 4003 | 0.82 (0.02) | 1.92 (0.28)* |
Trajectories of sleep problems (%) |
Increasing sleep problems | 622 | 15.6 | 33.8* |
Decreasing sleep problems | 2432 | 54.8 | 43.7* |
Stable medium sleep problems | 1318 | 29.6 | 22.5 |
Autistic traits |
PDP score––1.5 years | 3840 | 1.77 (0.03) | 2.26 (0.23)* |
PDP score––3 years | 3823 | 2.03 (0.03) | 4.81 (0.44)* |
SRS score––6 years | 5130 | 0.22 (0.00) | 0.97 (0.08)* |
Abdominal pain (%) | 5064 | 7.5 | 5.7 |
Functional constipation (%) | 4894 | 3.5 | 7.5 |
Maternal Characteristics |
Age at inclusion (years) | 5143 | 31.4 (0.1) | 31.0 (0.5) |
Educational level (%) | | | |
No education/primary school | 275 | 5.7 | 2.6 |
High school/lower vocational training | 1885 | 38.6 | 51.3* |
Higher vocational or academic training | 2699 | 55.7 | 46.2 |
Psychopathology score | 5295 | 0.24 (0.00) | 0.38 (0.07)* |
We first tested the cross-sectional association between sleep problems and autism. At all ages, they were significantly associated (e.g., sleep problems at age 1.5 years and autistic traits, B = 0.27, 95% CI 0.23 to 0.31, p < 0.01).
The longitudinal association of sleep problems with autistic traits and ASD
Table
2 shows the longitudinal associations of sleep problems with autistic traits and ASD adjusted for covariates and baseline autistic traits. Children who presented sleep problems at 1.5 and 3 years were more likely to have autistic traits. However, after adjusting for baseline PDP score, no longitudinal association was observed between sleep problems and autistic traits.
Table 2
The longitudinal association of sleep problems with autistic traits and autism spectrum disorder
1.5 years |
| Model 1 | 0.12 | 0.08–0.15 | < 0.01 | 0.08 | 0.05–0.11 | < 0.01 | 1.11 | 0. 87–1.42 | 0.41 |
| Model 2 | 0.03 | − 0.01–0.06 | 0.13 | 0.03 | − 0.02–0.06 | 0.07 | 1.05 | 0. 81–1.35 | 0.73 |
3 years |
| Model 1 | 0.20 | 0. 17–0.23 | < 0.01 | 0.07 | 0.04–0.10 | < 0.05 | 1.11 | 0.86–1.43 | 0.43 |
| Model 2 | – | – | – | 0.01 | − 0.03–0.04 | 0.70 | 0.95 | 0.72–1.24 | 0.70 |
The longitudinal association of autistic traits and ASD with sleep problems
Table
3 shows the longitudinal associations of autistic traits at ages 1.5, 3, and 6 years, and also that of ASD at age 6 years with sleep problems at age 9 years, after adjustment for covariates and baseline sleep problems. We found a significant association between autistic traits at age 1.5 and 3 years were related to more sleep problems at 6 years, both unadjusted and adjusted for baseline sleep problems. Furthermore, children with autistic traits and children with ASD at 6 years had more sleep problems at 9 years.
Table 3
The longitudinal association of autistic traits and autism spectrum disorder with sleep problems
Autistic traits* 1.5 years |
| Model 1 | 0.13 | 0.09–0.16 | < 0.01 | 0.10 | 0.06–0.14 | < 0.01 |
| Model 2 | 0.07 | 0.03–0.10 | < 0.01 | 0.08 | 0.03–0.12 | < 0.01 |
Autistic traits* 3 years |
| Model 1 | 0.13 | 0.10–0.16 | < 0.01 | 0.06 | 0.04–0.08 | < 0.01 |
| Model 2 | 0.05 | 0.02–0.08 | < 0.01 | 0.04 | 0.03–0.06 | < 0.01 |
Autistic traits** 6 years |
| Model 1 | 0.13 | 0.10–0.16 | < 0.01 | 0.14 | 0.10–0.18 | < 0.01 |
| Model 2 | – | – | – | 0.11 | 0.07–0.14 | < 0.01 |
ASD 6 years |
| Model 1 | 0.46 | 0.24–0.68 | < 0.01 | 0.84 | 0.58–1.10 | < 0.01 |
| Model 2 | – | – | – | 0.74 | 0.49–0.99 | < 0.01 |
Sleep problem trajectories
Children with a trajectory of increasing sleep problems and children with stable and moderate sleep problems had higher levels of autistic traits than those with decreasing sleep problems (Additional file
1: Table S2). We found that an increasing course of sleep problems was consistently associated with ASD at age 6 years (Additional file
1: Table S2).
Sensitivity analyses
All analyses were adjusted for gender, ethnicity, gestational age, maternal education, and maternal psychopathology, and, if possible, baseline measures of respectively sleep problems or prevalent autistic traits. There was no significant interaction between gender and sleep problems on the risk of autism or between gender and ASD in the analysis of sleep (data not shown).
Sensitivity analyses indicated our findings were robust. The results of all regression analyses remained unchanged after the children with ASD were excluded (data not shown).
Discussion
In this large population-based cohort, we found that sleep problems in toddlerhood were associated with autistic traits in mid-childhood, but this association disappeared when adjusting for early autistic traits. In contrast, autistic traits and a diagnosis of ASD in childhood were associated with sleep problems at later ages. Consistently, children with increasing sleep problems across development were more likely to have autistic traits and ASD. Our findings suggest that sleep problems are part of the construct of ASD, however do not predict severity of autistic traits over time. We showed that there is no bidirectional relation between sleep problems and ASD.
Our finding that sleep problems are associated with more autistic traits is in line with previous studies [
21,
28,
31]. However, these previous studies lacked the repeated measurements of sleep problems and autistic traits across ages [
21,
31]. When we adjusted for baseline autistic traits in the current study, the association between sleep problems at younger ages and later autistic traits disappeared. We found no evidence for sleep problems preceding autistic traits at baseline. This implies that sleep problems do not predict autistic traits and ASD over and above symptoms such as diminished social and communicative abilities, which are measured by the PDP scale. Moreover, sleep problems do not worsen ASD.
Autistic traits and ASD were associated with more sleep problems in accordance with previous studies [
28,
49,
50]. This association remained even after adjusting for baseline sleep problems. Sleep problems in young children are relatively common and can be considered part of normative development in the general population [
26,
51]. Yet, as supported by our trajectory analyses, the severity and frequency of sleep problems decreases in typically developing children [
47], whereas sleep problems worsen over time in children with ASD. This strongly suggests that the pathology underlying ASD on the behavioral sequelae determines the development of sleep problems.
The course of sleep problems over time in these children is poorly understood. Previous studies have been unable to determine the temporal association [
31]. By using trajectories of sleep problems and relating the trajectories to autistic traits and ASD, we show that sleep problems tend to decrease and disappear in the general population but increase in children with ASD. These trajectories are a further indication that the longitudinal course of sleep problems is a symptom and consequence of ASD, rather than worsen ASD symptomatology [
9]. Thus, sleep problems are prevalent in children with ASD and should be considered part of the disorder.
Children with ASD suffer from more sleep problems than children without ASD, but the pathophysiology of sleep problems in children with ASD has not yet been fully understood. Some studies point to underlying deficits in endogenous melatonin secretion [
52], others to alterations in hypothalamic-pituitary adrenal-axis function and cortisol secretion [
53,
54], alterations in neurodevelopmental pathways [
55], and some to polygenetic variations in circadian rhythm and clock genes related to ASD pathology [
56]. As ASD is highly heritable [
57‐
61], it would be worthwhile to study whether there are shared underlying genetic factors between sleep problems and autistic traits. Another mechanism could be that social problems associated with ASD may worsen the day-night rhythm in these children [
14] and play a crucial role in the development of sleep problems. Socialization of day-night rhythm, such as bedtime routines, nighttime rituals, and family regularity [
62‐
64], are important in young children as they can act as social zeitgebers and thereby contribute to the development of a healthy sleep pattern and the prevention of the occurrence of sleep problems [
14,
56,
65]. Children with ASD have difficulty to adequately respond to the social zeitgebers and therefore struggle to develop a healthy sleep pattern [
56]. More research is needed to unravel the socialization of day-night rhythm in children with ASD and the linkage with the development of sleep problems. Future studies should emphasize bedtime routines and family regularity when investigating children with ASD and sleep problems.
Our findings indicate that sleep problems do not contribute to an exacerbation of autistic traits but rather that sleep problems manifest as part of the broad ASD symptomatology. This is important information for parents who worry sleep problems may precipitate or perpetuate autistic symptoms. Based on our findings, we underline the importance of addressing sleep problems in children with ASD, possibly in the context of ASD treatments. Whereas sleep problems do not have a direct effect on autistic traits, sleep problems are known to negatively affect daytime functioning, such as attention processes and executive functioning [
66‐
68]. Executive dysfunction co-occurs with the core symptoms of ASD [
69‐
71], but the pathways remain unclear. We speculate that sleep problems can contribute to the development of executive dysfunction in children with ASD, but this needs further research. Future research should also investigate the effect of treatment of sleep problems and its concurrent effects on neurocognitive outcomes in children with ASD.
Strengths and limitations
The current study has relevant strengths. First, the large sample size and longitudinal design enabled us to study sleep problems and autistic traits at multiple ages across a broad time span and to control for baseline characteristics and confounders. Our longitudinal design also enabled us to account for reverse causality. Second, we were able to complement parent-reported autistic symptoms with a diagnosis of ASD.
The current study, however, also had some limitations. First, we used a mother-reported questionnaire to assess sleep problems. It would have been ideal to use actigraphic or polysomnographic measures for studying sleep problems. However, mothers are known to be reliable reporters of children’s sleep at younger ages [
72]. Second, our earliest assessment of autistic traits was performed at 1.5 years. Although measuring autistic traits in the general population in younger children is not very reliable [
73], mostly retrospective studies documented that many children who develop ASD will have had symptoms prior to age 1.5 years. As such, our earliest measurement may not represent the pathophysiological onset of the disorder. However, our study characterizes the nature of the longitudinal association between sleep and autistic traits in early childhood. Third, children with ASD were slightly more likely to be lost to follow-up than typically developing children. Nevertheless, our sensitivity analyses yielded similar results after excluding all ASD cases and were therefore likely not affected by the lost to follow-up. Fourth, we used different age-appropriate measures of autistic traits that had a low to moderate correlation at different ages. The effect of adjustment for baseline autistic traits may be influenced by using different measures. Nevertheless, when we adjusted for baseline autistic traits, the association between sleep problems and autistic traits disappeared. If anything, if adjustment with the same measure had been possible, this would have further attenuated any observed association.
Acknowledgements
We gratefully acknowledge the contribution of children and parents, general practitioners, hospitals, midwives, and pharmacies in Rotterdam. The general design of Generation R Study is made possible by the financial support from the Erasmus Medical Center, Rotterdam; the Erasmus University Rotterdam, ZonMw; the Netherlands Organization for Scientific Research (NWO); and the Ministry of Health, Welfare and Sport.