The current study examined the co-development of pain and insomnia throughout adolescence. We identified four subgroups with different concurrent developmental trajectories of pain and insomnia symptoms. The results suggest that adolescents with consistently high pain and insomnia may constitute a unique subgroup that is particularly vulnerable. Furthermore, the study established that both a late chronotype and pre-sleep behaviors causing cognitive-emotional (but not behavioral) arousal predicted pain and insomnia symptoms.
The current results show that trajectories of pain and insomnia co-occur to a high degree in all subgroups, regardless of the direction of the trajectories. We saw no indication that there were adolescents with only pain problems or insomnia, with no symptoms of the other condition. Given the substantial variability in the trajectories, however, there may be adolescents with high symptoms levels of either pain or insomnia included. This finding also indicates that previous lack of such relationship is not due to pain and insomnia being unrelated in some subgroups. Rather, previous study samples might have included subgroups with both increasing and decreasing trajectories, resulting in a null result when looking at grand means. By utilizing a person-oriented approach, we were able to show that there is a relationship between pain and insomnia in all unobserved subgroups in the general adolescent population. We are not aware of any previous study that has explored heterogeneity in longitudinal trajectories of co-developmental pain and insomnia. Previous studies have primarily focused on associations between variable means, for example that mean levels of insomnia symptoms at one timepoint can predict mean levels of pain at a later timepoint. Such an approach fails to capture continuity of trajectories over time and cannot demonstrate that pain and insomnia continuously co-change within individuals. Previous findings of established longitudinal stability of insomnia in adolescents with chronic pain 9 are in line with the current results.
In line with previous work 2,3,46, we were able to demonstrate a strong relationship between pain and insomnia. Notably, the trajectories in the decreasing pain and insomnia class do not follow each other as well as in the stable or increasing subgroups, as can be seen in Fig. 1. This fits the hypothesis that pain and insomnia follow each other on an upward, but not downward, trajectory 13. Insomnia appears to have an important role in the development of pain in adolescents, but it remains uncertain if also a decrease in insomnia may predict a subsequent decrease in pain.
Most of the adolescents in the current sample had consistently low pain and insomnia levels throughout the measured period (Class 1, 68.7 %), which is to be expected in a sample derived from the general population. About 14 percent of the sample (Class 3, 13.9 %) was characterized by increasing levels of both pain and insomnia symptoms. Although there were no clear-cut characteristics at baseline to discern this subgroup, they showed the largest shift towards a later chronotype and the largest increase in pre-sleep behaviors causing cognitive-emotional arousal. A third subgroup showed decreasing levels of pain and insomnia symptoms (Class 4, 12.5 %), and although the decrease in pain was modest, it is encouraging to find a subgroup of adolescents who show improvement from comorbid pain and insomnia over time. Earlier chronotype and less arousal-causing pre-sleep behaviors could predict these decreasing trajectories, compared to the high stable trajectories. One possibility that deserves further study is that early, proactive strategies that focus on late chronotypes and arousal-causing pre-sleep behaviors may help in preventing the concurrent development of pain and insomnia. For example, experimental studies may manipulate chronotype and pre-sleep behaviors and demonstrate their causal effect on pain and insomnia.
A subgroup of consistently high pain and insomnia symptoms was also identified (Class 2.4.9 %), constituting about 5 percent of the overall sample. Girls made up two thirds of this subgroup. Adolescents in this subgroup showed higher levels of symptoms on most of the baseline characteristics examined, including stress, depression, and anxiety. They also reported the shortest sleep times, the latest average chronotype, as well as the most problematic pre-sleep behaviors. Compared to Class 1 (low pain and insomnia), they reported a more than thirteenfold rate of generalized problematic pain. This indicates that the subpopulation of adolescents with comorbid pain and insomnia is associated with a particularly complex illness-profile, and indeed may benefit from specialized interventions. The current study explicitly proposes that adolescents with comorbid pain and insomnia constitute a distinct subpopulation but, as discussed before, did not reveal subgroups of adolescents with only pain or insomnia problems. Future studies should thoroughly explore potential differences between comorbid pain and insomnia, and on the other hand isolated pain or sleep conditions if observed.
The four classes identified in the current study correspond to previous studies on longitudinal pain trajectories in adults. Most commonly, four or five classes have been identified as the optimal solution in studies on pain, typically including one class with persistent low pain, one with persistent high pain, as well as two or three classes of fluctuating, increasing or decreasing pain 47. Based on the few studies exploring pain trajectories in adolescents, they appear to follow similar patterns as in adults.48. Few studies have been done on insomnia trajectories in adolescents, but one study identified trajectories similar as those found in regard to pain 49. Although the current study identified subgroups with concurrent pain and insomnia, the similarities of the trajectories with previously revealed pain or sleep trajectories add validity to the generalizability of the results.
Pre-sleep behaviors causing cognitive-emotional arousal were the most consistent predictor of change in both pain and insomnia symptoms. This is in line with previous research suggesting that cognitive-emotional arousal may delay sleep onset and lower sleep quality 12, and that worry, rumination and emotional distress are associated with the sleep-pain relationship 3,50. Additionally, stress responses or arousal has been proposed to explain the sleep-pain relationship 5, as well as influencing melatonin secretion26. Pre-sleep behaviors causing arousal may have both a moderating and a mediating effect on comorbid pain and insomnia Moreover, the role of cognitive-emotional arousal in the sleep-pain relationship might be driven by trait negative affect 20, which is closely related to negative mood 51. Negative mood is established as a mediator in the effect of sleep on pain 4 – and worry and rumination have a negative influence on mood 52. This multiple impact of pre-sleep behaviors causing cognitive-emotional arousal would make it a particularly salient predictor of the simultaneous development of chronic pain and insomnia. We therefore encourage future studies to explore the efficacy of hybrid interventions for comorbid pain and insomnia that incorporate a more exclusive focus on pre-sleep behaviors causing cognitive-emotional arousal. Notably, sleep hygiene practices (including pre-sleep behaviors) often are the focus in insomnia-interventions for adolescents 7.
Unexpectedly, pre-sleep behaviors causing behavioral arousal did not consistently predict conjoint trajectories of pain and insomnia. However, pre-sleep behaviors (ASHS full scale score) and somatic pre-sleep arousal (according to the Pre-sleep Arousal Scale) have previously been associated with insomnia in adolescents suffering from chronic pain 12. Plausible explanations for our null finding that require further investigation lie either in that this finding is specific to conjoint pain and insomnia symptoms, that the internal consistency of the behavioral arousal subscale was insufficient in the current study, ranging from .63 to .67, or that the items in this subscale are relatively positive in their valence.
Chronotype’s association to the trajectories of pain and insomnia symptoms may help explain inconsistent results of non-pharmacological treatments for comorbid pain and insomnia 53,54. It may be that social and behavioral factors related to adolescent sleep-wake patterns, which are not a target in psychological treatments, may impede or confound treatment effects. The influence of chronotype indicates that pharmacological, social or psychological interventions may be effective in treating the conjoint development of pain and insomnia, including melatonin 7, synchronizing adolescents’ school time with their biological clock 55, and CBT 56. Although we did not find a late chronotype to directly predict pain in the current study, melatonin used therapeutically has been shown to reduce both hyperalgesia and allodynia in different pain conditions 26. One could speculate that the current results indicate that the effect of melatonin on pain is mediated through insomnia symptoms, practically meaning that advancing adolescents’ sleep phase may lead to a reduction in insomnia symptoms, which, in turn, may reduce pain (but see evidence suggesting a direct effect of melatonin on pain 26).
A strength of the current study is the large population sample of adolescents followed over four yearly measurement occasions. This large set of data allowed us to use class-invariant GMM and account for potential sub-distributions via the class-variable, as well as between-individual differences and within-individual temporal change via the growth factors. Accounting for within-class variance often decreases the risk of identifying spurious classes 16 and reduces the risk of bias in the trajectories 57. To our knowledge, this is the first study to identify trajectories of pain and insomnia with a multidimensional GMM. The use of latent basis and unspecified growth curves is also a strength of the study, since it does not impose any restrictions to the shape of longitudinal change. It has been recommended over linear or polynomial models in studies within the social and behavioral sciences, since longitudinal change in humans rarely follow pure mathematical shapes 58. Another strength is the commonly used and validated measures.
While the current study has many strengths, some limitations need to be considered. First, for current purposes, we summarized the experience of three common types of pain (musculoskeletal pain, headache and abdominal pain) in an average pain grade, taking into account the intensity, frequency, and interference of the experience. Although this composite measure captures covariance across different pain types with insomnia 5, future studies might want to focus on co-developmental patterns in specific pain types, which would also allow interpretation of the pain grades in terms of pain-related disability only 17,59. Indeed, the co-development of pain and insomnia might turn out to be different for specific pain types 60.
A second limitation is that only self-report measures of complex constructs were used. Since objective and subjective sleep measures appear to partly measure different aspects of sleep 9, future studies should include also objective sleep-related measures.
A third limitation is that diverging trajectories were not perfectly overlapping at baseline, which may (although unlikely) have confounded the multinomial analysis of diverging trajectories.
A fourth limitation is that although a class-invariant model, such as the current one, facilitates interpretation and communication of the results, it only allows for an overall within-individual analysis of predictive effects. An optimal model with class-varying variances would have allowed for even more optimized growth curve shapes (since each subgroup would have its own unique shape of longitudinal change) and for an exploration whether chronotype and arousal-causing pre-sleep behaviors had different predictive effects depending on subgroup. However, this would also increase the risk of overfitting the model to the specific sample and thereby impeding generalizability. We recommend future studies to explore such potential subgroup-differences via class-varying growth factors.
In conclusion, the current study brings novel insights into how pain and insomnia symptoms longitudinally co-develop in adolescents. We identified four unique subgroups, showing that longitudinal trajectories of pain and insomnia closely follow each other, and that chronotype and pre-sleep behaviors causing cognitive-emotional arousal could predict the trajectories. A considerable portion of adolescents have either high persistent or increasing levels of problematic pain and insomnia. Adolescence constitutes a melting pot of factors contributing to the development of both insomnia and pain 5, and therefore appears to be an important developmental stage for preventing these conditions from developing and becoming chronic sources of disability and suffering. A sleep-phase focus may be relevant in interventions for comorbid pain and insomnia in adolescents, and pre-sleep behaviors causing cognitive-emotional arousal seem to be of particular importance to focus on in psychological interventions.