Sleep problems are among the most common complaints in children, and have been linked to a range of negative consequences, including reduced daytime functioning, academic and cognitive deficits as well as increased risk of emotional and behavioural problems [1, 2]. Children with chronic illness are at increased risk for sleep problems, and several cross-sectional studies have found an increased rate of sleep problems in children with specific chronic illnesses, including cerebral palsy [3], epilepsy [4], asthma [5], headaches [6], and migraine [7]. In one of the few population-based studies assessing sleep problems among children with chronic illness, Hysing et al. [8] found that these children reported more problems falling asleep and had more night-time awakenings compared to their healthy peers.

Few longitudinal studies of children in the general population have explored the stability of sleep problems, and with mixed findings. In a Swiss study [9] following children from infancy to 10 years, night-time awakenings were found to be both frequent and persistent over time. In contrast, Gregory et al. [10] found a reduction of sleep problems from early childhood to mid-adolescence, and Laberge et al. [11] found a similar reduction in sleep onset problems in children from 10 to 13 years. However, little is known with regards to the chronicity of sleep problems in children with chronic illness, and to the best of our knowledge, no longitudinal population-based studies have investigated the stability of sleep problems over time in this group of children.

The increased rate of sleep problems in children with chronic illness may have several potential pathways, some of them suggesting a higher likelihood of chronicity. For example, chronic illness may affect the sleep physiology and sleep systems in disorders with impaired central nervous system (CNS) functioning. Other factors contributing to a chronic trajectory of sleep problems in chronic illness may include higher rates of upper-airway obstruction and BMI (body mass index), as well as emotional and behavioural disorders, which previously has been linked to sleep problems in children with chronic illness [8]. It is also possible that parental stress related to managing their child's chronic illness might contribute to poor implementation of sleep schedules, and thus sleep problems.

Based on the same study population as the study by Hysing et al. [8], the current paper linked two waves of the Bergen Child Study (BCS), assessing all children at two time points (7-9 and 11-13 years of age) in order to explore the chronicity of sleep problems in children with chronic illness. We hypothesized that children with chronic illness would report higher rates of both acute and chronic sleep problems than their peers, and that sleep problems would differ between specific subgroups of chronic illnesses. We expected both sleep problems and behavioural and emotional problems to predict subsequent sleep problems.

The aim of the current study was to examine the chronicity and predictors of sleep problems in children with chronic illness compared to their healthy peers. Overall, the prevalence of sleep problems in both children with and without chronic illness increased from wave 1 to 2. Children with chronic illness had a higher rate of both chronic and acute sleep problems. Sleep problems at wave 1 was the strongest predictor of subsequent sleep problems. In addition, hyperactivity and emotional problems were smaller but significant risk factors.

While prospective studies of sleep problems in children in general have yielded mixed results on chronicity [9–11], the current study indicates that children have more problems initiating and maintaining sleep as they enter early adolescence, both in the chronic illness and non-chronic illness group. Being the first study to explore the course of sleep problems in children with chronic illness, the current findings show that both persistent and transient sleep problems are significantly more common in children with a chronic illness compared to healthy children. As such, the current study extends on previous cross-sectional evidence of sleep problems being more common in children with chronic illness [18].

There are several potential factors that may explain the increased persistency of sleep problems in the chronic illness group. Having a neurological disorder greatly increased the risk of developing chronic sleep problems. General risk factors, such as sociodemograhic factors and BMI, were found to be more prevalent in the chronic illness group, but only slightly reduced the risk of sleep problems at wave 2, and could hence not account for the high rate of sleep problems in the group as a whole. [20]. In a previous report from the same study [18], behavioural and emotional problems were found to account for most of the sleep problems in children with chronic illness. However, due to the cross-sectional nature of that study, no conclusions could be drawn about directions of causality. In the current study, we show that emotional and behavioural problems are independent risk factors for later sleep problems. As emotional problems was one of the strongest predictors of later sleep problems one potential mechanism of this association may be through increased worry at bedtime, which may delay sleep onset and increase night-time awakenings in the child. In sum, these findings emphasize the need for early detection of emotional and behavioural problems in this population.

There are several limitations to the present study. Chronic illness was assessed by parent report only, without medical verification of the diagnosis. Difficulties initiating or maintaining sleep were assessed by a joint variable, making it difficult to examine each construct separately and to assess the importance of the finings, and we also had no measure of the severity and duration of the sleep problems. Although not a validated measurement of sleep problems, we still consider that its inclusion in the present study design adds valuable information in a field and age cohort in which the focus on sleep problems has been virtually non-existing in epidemiological research. Unfortunately, the operationalization of insomnia and sleep problems has been extremely diverse in the general sleep literature, causing problems when comparing results across studies [21, 22]. Therefore, future studies should seek to employ validated instruments based on agree-upon diagnostic criteria when assessing sleep problems to facilitate study comparisons. Also, we had no measure of symptoms of obstructive sleep apnoea (OSA), which previously has been linked to obesity in children in general. OSA may be one potential mechanism through which obesity may contribute to increased sleep problems in this group. Another limitation is number of dropouts from wave 1 to 2, and we unfortunately have no information as to why these families did not participate in the longitudinal study. Also, several of the potential factors that could affect the relationship between sleep and chronic illness were only assessed in the second wave, and hence could not be used as predictors of chronicity of sleep problems. Finally, children with mental retardation were included in chronic illness group, the reason being the high degree of overlap between parent-reported mental retardation and having another neurological disorder. As such, excluding mental retardation from the CI-group would both have considerably reduced the sample size as well placed a substantial amount of children with CI in the healthy comparison group.

Increased awareness of the course of sleep problems over time is important for both clinicians, as well as to caregivers. In contrast to the common belief that children often will outgrow their sleep problems, the current study shows that this may not be the case, especially in children with chronic illness, thereby emphasizing the need to develop treatment strategies for this group of children. In addition, previous studies in the adult population have shown that even small improvements in sleep quality may yield noticeable relief in other co-existing symptoms (such as pain or fatigue) [23]. There is now substantial evidence that behavioural interventions are efficacious in treating sleep problems in children [24], with more than 80% showing clinically significant and lasting improvements. In addition, pharmacological interventions may be beneficial in subgroups of children with CI. In cases where mental retardation and hyperactivity co-exist with other chronic illnesses, circadian rhythm sleep disorders plays an important role in the aetiology of the sleep problems, in which adequately timed melatonin has shown to effectively relieve chronic sleep problems [25–28]. When also considering that improved sleep may have positive effects on both psychological, academic and possibly physiological variables [29], we consider it especially important that sleep problems in children with chronic illness are detected and managed adequately. Because disrupted sleep in children also influences other members of the family and remains a primary concern for many parents and caregivers [30], the quality of life for the child as well as her or his family as a whole may improve following treatment of sleep problems.