• In pain clinics, as many as 90% of patients reported at least one sleep disturbance (1,2).
  The intensity of sleep disturbances has also been correlated with greater pain, depression and disability (1, 3-6).
• Pain was found to be an important factor related to sleep problems in community-based studies (7-9).

One Scandinavian community-based study on the elderly (7) showed that being female, pain, depression and hypnotic-sedative use were factors significantly related to sleep problems. Another Scandinavian study on the elderly (9) reported that pain was a strong factor contributing only to early morning awakenings. A Canadian study with subjects 15 years and older (8) found among other things, that pain and activity limitation were significantly associated with insomnia in the multivariate model. However, little information is available about how chronic pain is related to sleep disturbances in the general population.

Consequently, we decided to assess the prevalence of chronic painful physical condition in the general population of five European countries and to examine the place of chronic painful physical condition in insomnia.

METHOD

18,980 participants aged 15 years or older from five European countries (the United Kingdom, Germany, Italy, Portugal and Spain) and representative of approximately 206 millions Europeans were interviewed by telephone.

The interview included questions about sleeping habits, health, sleep and mental disorders. Painful physical conditions were ascertained through questions about medical treatment, consultations and/or hospitalizations for medical reasons and a list of 42 diseases.

A painful physical condition was considered chronic when it lasted at least six months.

Insomnia symptoms were defined as:

• difficulty initiating
• or maintaining sleep
• or non-restorative sleep,

present at least 3 nights per week, lasting at least one month ,and accompanied by daytime consequences.

RESULTS

• The point prevalence of at least one CPPC was set at 17.1% (95% CI: 16.5% to 17.6%) in the sample.
• Difficulty initiating sleep was found in 5.1% (95% Confidence Intervals: 4.8% to 5.4%) of the sample.
• Disrupted sleep in 7.5% (95% CI: 7.2% to 7.9%)
• Early morning awakenings in 4.8% (95% CI: 7.2% to 7.9%).
• Non-restorative sleep in 4.5% (95% CI: 4.2% to 4.8%).
• More than 40% of individuals with insomnia symptoms reported at least one CPPC.
• CPPC was associated with more frequent difficulty or inability to resume sleep once awake and a shorter sleep duration.
• In middle-aged subjects (45 to 64 years of age), CPPC was associated with longer insomnia duration. At any age, insomnia with CPPC was associated with a greater number of daytime consequences (average of four consequences) than in insomnia without CPPC (average of 2.3 consequences).
• In multivariate models, CPPC, especially backaches and joint/articular diseases, were at least as importantly associated with insomnia than were mood disorders with odds ratios ranging from 4.1 to 5.0 for backaches and from 3.0 to 4.8 for joint/articular diseases.

DISCUSSION

This study examines the relationship between insomnia and chronic painful physical conditions in the general population of five European countries. This is the first community-based study that investigates this association using several chronic painful physical conditions and insomnia. Most previous community-based studies were limited to only one type of pain or to a broad definition of pain (7-9). Furthermore, the objective of these studies was not necessarily to examine how the presence of chronic pain related to insomnia, but had a broader objective to find associated factors to one of these two conditions (8,19,20). As a consequence, the importance of pain in insomnia is often overshadowed by a myriad of other factors. Moreover, definitions of insomnia used are often inconsistent and lack crucial information such as the frequency and duration. It is difficult therefore to have a clear portrait of the importance of pain in insomnia.

In this study, each insomnia symptom had to be present at least three times per week for at least one month and to have caused at least minimal consequences on daytime functioning. Our results show that:

• Chronic pain is frequent among subjects with insomnia: More than 40% of them reported at least one chronic painful physical condition.
• Chronic pain can be a factor that contributes to the chronicity of insomnia. We found longer insomnia duration in our middle-aged subjects with chronic pain compared to other subjects with insomnia. This result suggests that, at least in this specific age group, chronic pain contributes to the maintenance of insomnia.
• The relationship between pain and sleep is likely to be bi-directional. Indeed, if pain may cause difficulty initiating and/or maintaining sleep, pain intensity can be as well exacerbated by the lack of sleep. For example, Affleck et al. (21) asked to 50 women with fibromyalgia syndrome to keep a diary about their sleep and pain for 30 days. They found that a poor night's sleep was followed by a day with greater pain and a painful day with a night of poorer sleep. In this study, we found that subjects with pain and insomnia experienced greater daytime consequences of insomnia at any age than when the insomnia subjects had no pain. Their nighttime sleep duration was also shorter We also found that some individuals compensated this shortening of sleep by napping during the daytime.
• Interestingly, apart from sociodemographic determinants, associated factors for each insomnia symptom were about the same. Noteworthy, backaches and joint/articular disease were at least as importantly associated with insomnia as were mood disorders.
• The interactions between insomnia, pain and mental disorders deserve some attention. Interestingly, anxiety disorders did not make independent contributions to any insomnia symptom in the multivariate models. Part of the explanation can be in the high co-morbidity between anxiety and mood disorders, which played a greater role than anxiety disorders. Another possibility lies in the definition of insomnia we adopted. Mostly, epidemiological surveys defined insomnia as the presence of the symptoms at least three nights per week regardless of the duration and daytime consequences. Obviously, this definition increases prevalence of insomnia symptoms. Mildest manifestations of insomnia could be more frequently related to anxiety disorders.

CONCLUSIONS

CPPC is associated with a worsening of insomnia on several aspects: a greater number of insomnia symptoms, more severe daytime consequences and more chronic insomnia situation. CPPC plays a major role on insomnia. Its place as major contributive factor for insomnia is as much important as mood disorders.