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. 2014 Nov 1;37(11):1777–1786. doi: 10.5665/sleep.4170

Insomnia Symptoms and Risk for Unintentional Fatal Injuries—The HUNT Study

Lars Erik Laugsand 1,2,✉, Linn B Strand 1, Lars J Vatten 1, Imre Janszky 1,3, Johan Håkon Bjørngaard 1
PMCID: PMC4196061  PMID: 25364073

Abstract

Study Objectives:

To assess the association between insomnia symptoms and risk of fatal unintentional injuries.

Design:

Population-based prospective cohort study with a mean follow-up of 14 y, linking health survey data with information on insomnia symptoms to the National Cause of Death Registry.

Setting:

Nord-Trøndelag County, Norway.

Participants:

A total of 54,399 men and women 20-89 y of age who participated in the Nord-Trøndelag Health Study between 1995 and 1997.

Interventions:

N/A.

Measurements and results:

There were 277 unintentional fatal injuries, including 57 fatal motor vehicle injuries during follow-up. There was a dose-dependent association between the number of insomnia symptoms and risk of unintentional fatal injuries (P for trend 0.001) and fatal motor vehicle injuries (P for trend 0.023), respectively. The proportion of unintentional fatal injuries cases that could have been prevented in the absence of difficulties initiating sleep, difficulties maintaining sleep, and having a feeling of nonrestorative sleep were 8%, 9%, and 8%, respectively. The corresponding estimates for motor vehicle injuries were 34%, 11%, and 10%.

Conclusion:

Insomnia is a major contributor to both unintentional fatal injuries in general as well as fatal motor vehicle injuries. Increasing public health awareness about insomnia and identifying persons with insomnia may be important in preventing unintentional fatal injuries.

Citation:

Laugsand LE, Strand LB, Vatten LJ, Janszky I, Bjørngaard JH. Insomnia symptoms and risk for unintentional fatal injuries—the HUNT Study. SLEEP 2014;37(11):1777-1786.

Keywords: accidents, injuries, insomnia symptoms, motor vehicle injuries, sleep, unintentional fatal injuries

INTRODUCTION

Approximately five million people die from injuries each year and fatal injuries contribute substantially to worldwide loss of life years.1 Being awake and alert are likely to be key factors to avoid unintentional fatal injuries. Insomnia, which is a subjective feeling of having difficulties initiating or maintaining sleep or having a feeling of nonrestorative sleep, is a highly prevalent condition in the industrialized world. It has been estimated that the prevalence of at least one insomnia symptom is as high as 33% in the general population.2

Only a few prospective studies have investigated insomnia in relation to risk for injuries. In a large, population-based Swedish cohort, there was a positive association of having difficulties sleeping during the two last week and risk of fatal occupational accidents.3 However, this study did not evaluate the association of individual insomnia symptoms and risk of fatal occupational accidents. In a large cohort of Finnish public sector employees, men and women reporting difficulties initiating sleep, maintaining sleep, and having a feeling of nonrestorative sleep had increased risk of nonfatal occupational injuries over 1-y follow-up.4 Although there is considerable overlap between insomnia and psychological distress,5 only the Finnish study included measures of anxiety and depression. Furthermore, insomnia is very common in several chronic somatic disorders.5 However, none of the previous studies investigated the potential confounding role of common chronic somatic disorders. Moreover, to the best of our knowledge, no large prospective studies have investigated insomnia symptoms and risk of fatal motor vehicle injuries.

In the second wave of the HUNT study (Nord-Trøndelag Health Study), we were able to link health survey data with information on all the major insomnia symptoms among the participants to the National Cause of Death Registry in Norway with a sufficiently long follow-up time for unintentional fatal injuries. Therefore, we prospectively investigated the association of insomnia symptoms with the risk of overall unintentional fatal injuries and with fatal motor vehicle injuries in a large population-based study, taking into account the effects of socioeconomic status, alcohol consumption, psychological distress, use of sleep medication, and common chronic disorders.

METHODS

Study Population

The HUNT study constitutes a large database of clinical, anthropometric, and socioeconomic information collected during a three-phase, population-based health survey in Nord-Trøndelag County in Norway. Details of the study are available on the HUNT website (http://www.ntnu.no/hunt).

Nord-Trøndelag is one of 19 Norwegian counties that is located in the central part of Norway. The county is fairly representative for Norway, although the average educational and income levels are somewhat lower than for the country as a whole. The population is stable and ethnically homogenous, with only a small proportion (3%) of non-Caucasian origin. The size of the population was stable between the HUNT surveys, with a net annual emigration of only 0.3%.6

The adult population of Nord-Trøndelag County was invited to participate in the second phase of the Nord-Trøndelag Health Study (HUNT2) from August 1995 to June 1997. In total, 94,187 individuals were invited, and 65,215 (69%) participated in the study, filled out a questionnaire, and attended a clinical examination at baseline. Details about the study have been published elsewhere.6,7

The study was approved by the regional committee for ethics in medical research, by the National Directorate of Health, and by the Norwegian Data Inspectorate.

Exposure

Insomnia is defined as a subjective feeling of having difficulties initiating or maintaining sleep or having a feeling of nonrestorative sleep. The HUNT questionnaire included three items related to insomnia. One question was related to difficulties initiating sleep (“Have you had difficulties falling asleep in the past month?” with the response options never/occasionally/often/ almost every night), the second question was related to difficulties maintaining sleep (“During the last month, have you woken up too early and not been able to get back to sleep?” with the response options never/occasionally/often/almost every night), whereas the third question was related to having a feeling of nonrestorative sleep (“How often do you suffer from poor sleep?” with the response options never or a few times a year/one to two times per month/about once a week/more than once a week). The HUNT questionnaire had different versions for participants younger and older than 70 y. Questions about difficulties initiating sleep and maintaining sleep were asked of all participants, but the question about nonrestorative sleep was asked only of the younger age group. Thus, a feeling of nonrestorative sleep was only assessed only among those age 20–69 y and could not be analyzed among the older participants in the current work.

We assessed the influence of each insomnia symptom using the original four response categories.

Insomnia symptoms were also dichotomized and the two highest categories, i.e., having difficulty initiating sleep often or almost every night, difficulty maintaining sleep often or almost every night and having nonrestorative sleep once a week or more than once a week, were compared with the rest of the categories. Those in the highest categories were considered to have the respective insomnia symptom in the analysis of the cumulative number of insomnia symptoms with risk of fatal accidents.

Apart from insomnia symptoms, participants age 20–69 y were also asked whether their insomnia symptoms influenced their work situation (“During the past year, have you been troubled by insomnia to such a degree that it influenced your work performance?” with the response options yes or no).

In total, 54,399 participants (83%) answered one or more of the insomnia questions and were included in the analyses. The response rates for the respective questions were between 81% and 83%.

Outcome Ascertainment

Unintentional fatal injuries from baseline to December 31, 2011 were identified from the National Cause of Death Registry. Physicians and public health officers in Norway are required to report all primary and secondary causes of death to the National Cause of Death Registry according to the International Classification of Diseases (ICD). We used ICD-9 chapter XVII (N800-929) and ICD-10 chapter XX (V01- X59) to identify unintentional fatal injuries. Intentional deaths caused by suicide or homicide and motor vehicle-related injuries of passengers were not included as outcomes in the study. The unique 11-digit identification number of every Norwegian citizen enables linkage of data from the Cause of Death Registry with the HUNT study.

During 13.6 y of mean follow-up time, 204 participants left the county, and 10,262 participants died from other causes than unintentional injuries, and were censored at the time of event (emigration or death) in the statistical analysis.

Covariates

Information on health-related variables, lifestyle factors, and medication was collected by a self-administered questionnaire. Participants extensively assessed and reported their medical history regarding common chronic somatic disorders.

Education was categorized as low (≤ 9 y), medium (10–12 y), or high (> 12 y). Marital status was dichotomized to living alone or not. Participants between age 20 and 69 y were asked whether they did shift work. Assessment of shift work was based on self-report of any type of shift work schedule (rotating or night shifts). Occupational status was categorized as paid work, student or military service, retired or on social security, self-employed, full-time housework, and unemployed or laid off.

The participants were asked about their usual intake of wine, beer, and spirits, indicated by their usual number of drinks over a 2-w period. We categorized participants according to their alcohol consumption as abstainers, light drinkers (zero to one drink per day), moderate drinkers (more than one but fewer than two drinks per day) or heavy drinkers (more than two drinks per day).8

Participants were asked about their use of sleep medication/ sedatives (”How often have you taken tranquilizers/sedatives or sleep medication in the past month?”) with the following response options: daily/every week but not every day/less than once a week/never. Type of medication was not available, but based on regional prescription statistics, benzodiazepines were the dominant type.9 Furthermore, if the participants used any medication on a daily basis during the past 12 months, they were also asked about how many months the past year they used sedatives, sleep medication, and antidepressants, respectively.

The Hospital Anxiety and Depression Scale (HADS) was used to assess symptoms of anxiety and depression. The HADS consists of seven depression-related and seven anxiety-related questions, each with response options on a scale of 0 to 3. Scores on both the anxiety and depression subscales ranged from 0 to 21, and increasing score indicated increasing symptom load. No somatic items or items regarding sleep difficulties were included. HADS has shown good test properties in the assessment of symptom severity of anxiety and depression both in the primary health care and in hospital settings (Cronbach α 0.83 and 0.82 for HADS-anxiety and HADS-depression, respectively).10 It has been shown that optimal balance between sensitivity and specificity for identifying anxiety and depression disorders was achieved by a score of 8 or higher on both HADS-A and HADS-D.10 The psychometric properties of the scale have previously been validated as part of the HUNT Study (Cronbach α 0.80 and 0.76 for HADS-anxiety and HADS-depression, respectively).11

At baseline, a clinical examination was conducted by trained nurses and included standardized assessment of blood pressure, weight, height, and waist and hip circumference. Systolic and diastolic blood pressure were measured using a Dinamap 845XT (Critikon) sphygmomanometer based on oscillometry, and the average of the second and the third of three measurements was used in the analysis. Height and weight were recorded with participants wearing light clothes without shoes; height was measured to the nearest 1 cm and weight to the nearest 0.5 kg. Waist circumference was measured to the nearest centimeter at the level of umbilicus. The body mass index (BMI) was computed as weight (in kilograms) divided by the squared value of height (in meters).

Statistical Analysis

Comparisons of continuous variables among different groups were made by Student t-tests, and chi-square tests were used to compare categorical data. We used Cox proportional hazard models to examine the association of insomnia symptoms with subsequent risk of unintentional fatal injuries and motor vehicle injuries, respectively. We calculated the number of events, person-years at risk and hazard ratios (HR) with 95% confidence intervals (CIs). Each category of insomnia symptoms was compared to having no insomnia complaints. For tests of trend, we assigned a numeric value of 0–3 to the different categories, with 0 having no insomnia complaints, and the categories were treated as a continuous variable.

In a separate analysis, we calculated the risk associated with an increasing number of dichotomized insomnia symptoms, compared to the risk of participants without any insomnia symptoms. Participants with missing data on any of the insomnia symptoms were excluded from this analysis. Also, the assessment of having a feeling of nonrestorative sleep was restricted to participants age 20–69 y, and the analysis related to the cumulative number of insomnia symptoms was therefore restricted to this age group.

We used age at risk as the time dimension, which allows a very precise adjustment for age,12 and we included sex, education, shift work, alcohol intake, and marital status as potentially confounding factors in our models. It is not clear whether psychological distress is a cause or a consequence of sleep disorders. In a separate analysis, we therefore also adjusted for depression and anxiety. One way of treating complaints of insomnia involves the prescription of hypnotic medication, which may impair psychomotor performance and lead to increased risk for accidents. However, it is unclear whether the use of hypnotic medication constitutes a surrogate variable for insomnia, whether it is an intermediating variable between insomnia and unintentional fatal injuries, or whether it constitutes an independent risk factor for unintentional fatal injuries. Thus, in separate analysis, we additionally adjusted for use of sleep medication. As a supplementary analysis, we also assessed the association of use of sleep medication, sedatives, and antidepressants and subsequent risk of unintentional fatal injuries and motor vehicle injuries, respectively.

We assessed the public health importance of the association between insomnia complaints and risk of unintentional fatal injuries by calculating population attributable proportions. Thus, we estimated the proportion of unintentional fatal injuries cases that could have been prevented if these individuals had not suffered from difficulties initiating sleep, maintaining sleep, or feeling of nonrestorative sleep. Population attributable proportions were estimated using models where we controlled for the potentially confounding factors in our models.13

Several studies have suggested that women are more prone to insomnia and insomnia increases in prevalence with age,2 and there are also age and sex differences related to fatal injuries.14 Therefore, we performed stratified analyses to assess whether the association of insomnia symptoms with unintentional fatal injuries could be modified by age (dichotomized at age 50 y) and sex. We also conducted several stratified analyses to assess whether the association of insomnia symptoms with unintentional fatal injuries could be modified by other factors. Thus, we investigated the potential effect modification by education (dichotomized at 10 y), shift work (yes/no), use of sleep medication/sedative (daily versus less than daily), anxiety symptom score (HADS-A ≥ 8 versus < 8), and depression symptom score (HADS-D ≥ 8 versus < 8). We also formally tested the homogeneity of stratum specific relative risks. For these tests of interaction we used the previously defined insomnia trend variables.

We performed sensitivity analyses to assess the robustness of our findings. To address the possibility that chronic disorders could be a confounder for the association of insomnia and risk for unintentional fatal injuries, we adjusted for known chronic disorders, such as stroke, asthma, angina pectoris, diabetes mellitus, goiter, hypothyroidism, hyperthyroidism, fibromyalgia, arthritis, rheumatism, ankylosing spondylitis, cancer, epilepsy, diabetes mellitus, or osteoporosis. Sleep apnea is also a possible confounder for the association between insomnia and unintentional fatal injuries, because several studies have revealed a clear positive relationship between sleep apnea and accidents.15 Although daytime sleepiness is the most characteristic symptom for sleep apnea, patients with apnea often complain about difficulty initiating or maintaining sleep, and they often suffer from early awakenings.16 We had no information on the prevalence of sleep apnea syndrome in our study. In a sensitivity analysis, we therefore additionally adjusted for blood pressure and BMI, i.e., two very strong correlates of sleep apnea syndrome. In another set of analyses, we assessed whether occupational status could be an explanation for the observed associations, and adjusted our models for occupational status.

We tested the proportionality of hazards using log-log curves and formal tests of interaction with time or log-time. There was no evidence against the proportionality assumption (all P values higher than 0.10).

The statistical analyses were conducted using Stata 10.1 for Windows (Stata Corp., College Station, TX).

RESULTS

The prevalence of having difficulties initiating sleep almost every night, having difficulties maintaining sleep almost every night, and having nonrestorative sleep more than once a week were 3%, 3%, and 8%, respectively.

During a mean follow-up of 13.6 y (standard deviation = 4.0 y), a total of 277 unintentional fatal injuries occurred among a total of 54,399 participants. Causes of unintentional fatal injuries included falls (n = 169), motor vehicle crashes (n = 57), drowning or submersion (n = 12), exposure to smoke, fire, or flames (n = 10), accidental poisoning (n = 8), airway obstruction (n = 7), struck by a thrown, projectile, or falling object (n = 4), exposure to high and low air pressure and changes in air pressure (n = 2), contact with machinery (n = 2), caught, crushed, jammed, or pinched in or between objects (n = 2), bitten or struck by other mammals (n = 1), bitten or struck by a dog (n = 1), exposure to unspecified factor (n = 2).

Table 1 displays characteristics of the study population at baseline, by unintentional fatal injuries at follow-up. Having difficulties initiating sleep, difficulties maintaining sleep, and nonrestorative sleep were more frequent among those who died from injuries during follow-up than the rest of the population (P < 0.05). Fatal injuries were also more frequent among elderly participants and among men, and participants who died from injuries had a lower level of education, they were more often alcohol abstainers or heavy drinkers, and they more often used sleep medication/sedatives than the other participants (P < 0.001).

Table 1.

Baseline characteristics of the participants according to unintentional fatal injuries during follow-up.

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Table 2 presents the age- and sex-adjusted HRs and several multivariable-adjusted HRs for risk of all unintentional fatal injuries and motor vehicle injuries in relation to the individual insomnia symptoms. Individuals having difficulties initiating sleep, difficulties maintaining sleep, and having the feeling of nonrestorative sleep were associated with an increased risk for both unintentional fatal injuries overall and for fatal motor vehicle injuries compared to those who reported never or almost never to have insomnia symptoms. Among the different exposure variables, difficulties initiating sleep appeared to have the strongest and most robust association with both total unintentional fatal injuries and motor vehicle injuries. Adjustment for socioeconomic status did no attenuate the association, but the estimates of effect were attenuated after adjustment for psychological distress and use of sleep medication.

Table 2.

Hazard ratios (95% confidence intervals) for unintentional fatal injuries according to insomnia symptoms.

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In supplementary analysis, we compared the use of psycho-pharmacological drugs with no such use, and found a similar increase in the risk for unintentional fatal injuries and fatal motor vehicle injuries associated with drug use (shown in Table S1, supplemental material). These estimates were attenuated after adjustment for cumulative insomnia symptoms.

In relation to total unintentional fatal injuries, we compared insomnia that influences work performance with insomnia that did not have such an influence, and found HRs of 1.43 (95% CI: 0.83–2.47, P = 0.2) and 1.30 (95% CI: 0.72–2.34, P = 0.4) in model 1 and model 2, respectively. The corresponding HRs for fatal motor vehicle injuries were 1.48 (95% CI: 0.62–3.52, P = 0.4) and 1.51 (95% CI: 0.63–3.63, P = 0.4).

Table 3 shows that increasing number of insomnia symptoms was strongly and positively associated with risk of total unintentional fatal injuries and fatal motor vehicle injuries. The HRs after adjusting for socioeconomic status and alcohol consumption for those with three insomnia symptoms were 3.10 (95% CI: 1.40–6.93, P = 0.005) and 3.81 (95% CI: 1.13–12.88, P = 0.03) for total unintentional fatal injuries and motor vehicle injuries respectively, compared to reporting no insomnia symptoms. After further adjustment for psychological distress and use of sleep medication the strength of the associations was attenuated, although the precision of the estimates was also lower.

Table 3.

Hazard ratios (95% confidence intervals) for unintentional fatal injuries according to the number of insomnia symptoms.

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The proportion of unintentional fatal injuries cases that could have been prevented in the absence of difficulties initiating sleep, difficulties maintaining sleep, and having a feeling of nonrestorative sleep were 8%, 9%, and 8%, respectively. The corresponding estimates for motor vehicle injuries were 34%, 11%, and 10%.

Among individuals younger than age 50 y, the relative risk estimates for unintentional fatal injuries associated with difficulties initiating sleep appeared to be higher than for people older than 50 y (Table 4). In relation to other stratified variables, we found no statistical evidence for any effect modification, including sex (Table 5), education, shift work, use of sleep medication/sedatives, anxiety symptom score, and depression symptom score.

Table 4.

Hazard ratios and 95% confidence intervals for unintentional fatal injuries according to insomnia symptoms stratified by age.

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Table 5.

Hazard ratios and 95% confidence intervals for unintentional fatal injuries according to insomnia symptoms stratified by sex.

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After adjustment for prevalent chronic diseases (n = 33,833) the association of insomnia symptoms with unintentional fatal injuries or with fatal motor vehicle injuries remained unchanged from that reported in the main analyses. For example, in Model 2, the HR for unintentional fatal injuries and fatal motor vehicle injuries for those having three insomnia symptoms were 2.88 (95% CI: 1.30–6.40, P = 0.009) and 3.54 (95% CI: 1.13–12.88, P = 0.04), respectively. The associations between the individual insomnia symptoms and that of cumulative insomnia symptoms and risk of overall fatal injuries and fatal motor vehicle injuries did not substantially change after additional adjustment for BMI and systolic blood pressure. For example, in Model 2, the HRs associated with having three insomnia symptoms were 3.09 (95% CI: 1.40–6.82, P = 0.005) and 3.75 (95% CI: 1.11–12.66, P = 0.03), respectively. After inclusion of occupational status in the present multivariable models, the estimates remained essentially the same (shown in Table S2 and Table S3, supplemental material).

DISCUSSION

In this large population-based study, we found that the cumulative number of insomnia symptoms was strongly and positively associated with risk in a dose-dependent manner both for total unintentional fatal injuries and for fatal motor vehicle accidents. Also, those who reported to suffer from all insomnia symptoms were at considerably higher risk than those who had no symptoms or only a few symptoms. Among the insomnia symptoms, difficulties initiating sleep appeared to have the strongest and most robust association with fatal injuries. These results were fairly robust in different multivariable models. After adjustment for psychological distress and use of sleep medication/sedatives, the strengths of the associations were weakened. Our results suggest that a large proportion of unintentional fatal injuries and fatal motor vehicle injuries cases could have been prevented in the absence of insomnia. It is important to emphasize that insomnia is an easily recognizable2 and a potentially manageable condition for most patients. Treatment options include adherence to simple recommendations concerning sleeping habits, often referred to as sleep hygiene, and several non-pharmacological and pharmacological therapies, with the potential to produce reliable and durable changes among persons who suffer from chronic insomnia.17–19

Comparisons with Previous Studies

The association between disturbed sleep and increased risk of occupational injury and traffic accidents has been observed in several cross-sectional and case-control studies.20–32 Only a few prospective studies have investigated insomnia in relation to injuries, and these studies included only few aspects of insomnia or included only work-related injuries.3,4 To our knowledge, the current study is the first population-based prospective study to investigate the association of insomnia symptoms with risk for fatal motor vehicle injuries.

In a prospective Swedish study,3 166 fatal occupational accidents occurred among a Swedish national sample of 47,860 persons during 20 y of follow-up. The authors reported that sleep disturbances in the past 2 w were associated with increased risk of fatal occupational accidents (HR: 1.89, 95% CI: 1.22–2.94). However, sleep disturbance was evaluated by a single question only: “Have you had difficulties in sleeping during the past two weeks?” (Yes/No). Therefore, the authors could not assess the association of individual insomnia symptoms and risk of fatal injuries.

In a recent Finnish study,4 978 workers were recorded with an occupational injury among a total of 48,598 employees in the public sector over 1 y of follow-up. They found that having difficulties initiating sleep, maintaining sleep, and having a feeling of nonrestorative sleep predicted occupational injury in both men and women after adjustment for age, socioeconomic status, marital status and shift work. Similar to our findings, difficulties initiating sleep appeared to be the insomnia symptom that displayed the strongest and most robust association with injuries in that study.

Potential Mechanisms for the Observed Associations

A plausible mechanism behind the association of insomnia and risk of unintentional injuries involves a lack of sufficient daytime alertness, compromised cognitive function, and impaired psychomotor performance capabilities.33–35

One way of treating insomnia involves prescription of hypnotic medications, such as benzodiazepine derivatives, with the addition of psychopharmacological drugs when the origin of the insomnia is a mood disorder, such as depression or anxiety.36 However, these types of medication may substantially impair driving abilities.37,38 Our research suggests that insomnia is a major contributor to the risk of unintentional fatal injuries, although the extent to which such a risk is influenced by the concomitant use of hypnotic drugs is not clear. In our study, the effects of sleep medications/ sedatives and insomnia are not fully separable because nearly all participants who reported to have all three insomnia symptoms at baseline were also using sleep medication/sedatives. We adjusted for a large set of potential confounders, and the association between cumulative insomnia symptoms and fatal injuries, which showed that the association that we report was largely independent of socioeconomic status, alcohol consumption, psychological distress, and use of sleep medication.

Insomnia affects all age groups, but it increases from age 45 y and among individuals age 65 y or older the prevalence may be as high as 50%.39 Thus, insomnia symptoms could have different implications for young people compared with older people. We found a more prominent association of difficulties initiating sleep and risk of fatal accidents before age 50 y. Because insomnia symptoms at older ages may be part of the aging process, it may also have less effect on daily functioning, whereas at younger ages, insomnia symptoms may have stronger negative effect on daily functioning and signal vulnerability to unintentional injuries.

Limitations

Apart from the clear strengths reflected by the large sample size, the population-based design and the extensive ability to adjust for potentially confounding factors such as socioeconomic status, alcohol consumption, psychological distress, use of sleep medication, and common chronic disorders, the current work has some important limitations.

Insomnia symptoms were self-reported and therefore subject to misclassification. Similar to other prospective studies, we did not rely on a formal diagnosis of insomnia, but analyzed the severity of symptoms separately and in combination in relation to fatal accidents. Moreover, no validation was performed for the information on insomnia. Similar to previous prospective studies, there was a lack of specificity for symptoms of having difficulties maintaining sleep and feeling of nonrestorative sleep in our study. Difficulties maintaining sleep is often divided into middle of the night awakenings and early morning awakenings, and they are also often conceptualized as distinct insomnia symptoms.40 Also, nonrestorative sleep is highly prevalent in other sleep pathologies, especially sleep apnea, and recent research has suggested that nonrestorative sleep might be considered as a primary diagnosis distinct from other insomnia symptoms.41,42 Nevertheless, our evaluation of insomnia symptoms largely reflected the current diagnostic criteria used in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders.43

Furthermore, we did not assess sleep objectively, for example, by performing polysomnography. Nevertheless, insomnia is defined as a subjective feeling of having difficulties falling asleep, remaining asleep, or receiving restorative sleep and thus, such problems are not routinely evaluated by polysomnography.44 Because insomnia was only evaluated once before follow-up started, we could not examine the possibility of time-dependent effects of insomnia.

The identification and ascertainment of fatal injuries as outcomes could also be prone to misclassification. There was no validation of nonintentional fatal injuries in our study, and we had no data available on the time of the injuries or whether the injuries were occupational. However, forensic autopsies are routinely conducted in cases of sudden unexpected deaths in Norway. Moreover, the overall quality and reliability of fatal injuries as causes of death is high in Nordic countries.45 To further increase specificity of the fatal injuries we only included unintentional deaths and excluded suicides and homicides, although the true number of hidden suicides among fatal injuries is difficult to assess. Also, we did adjust for symptoms of anxiety and depression, which is suggested to largely explain associations between sleeping problems and suicide risk.46 Thus, it seems unlikely that low reliability of fatal injuries could explain the higher risk for fatal injuries among people with insomnia symptoms.

Sleep apnea may be a confounder because sleep apnea syndrome has been strongly associated with increased risk for accidents15 and insomnia.16 We had no available information on sleep apnea, and could not account for this potential confounder in the analysis. However, in a large polysomnographic study, only 6% of those with insomnia symptoms had sleep apnea syndrome,47 and it seems possible that the association between sleep apnea and insomnia, at least in part, could be explained by confounding by age or by depression.16,48 In this study, we adjusted for age and psychological distress as well as for blood pressure and BMI, which are strong correlates of sleep apnea syndrome, and we think it is reasonable to conclude that sleep apnea is not a likely explanation for the higher risk of fatal injuries among people with insomnia symptoms.

Length of sleep was not evaluated in the current work. Both short and long duration of sleep have been associated with increased risk of fatal injuries.49 However, insomnia is not synonymous with short sleep, and people with an average or long duration of sleep may also have insomnia symptoms.50 Similarly, many individuals with short sleep duration do not have insomnia,50 probably because there is a large interindividual variation in the length of sleep that is required for physiological and psychological restoration.51 It is also important to recognize that insomnia also covers the quality of sleep, and not only its duration.

As in any observational study, unevenly distributed study characteristics could have confounding effects that may not be accounted for in the analyses. Although we adjusted for several potential confounders in our multivariable analyses, we cannot exclude the possibility of uncontrolled confounding behind the observed associations. Nevertheless, any remaining confounder potentially able to influence our results considerably would need to be strongly associated with both insomnia and with risk of unintentional fatal injuries and generally be unrelated to the factors included in our models.

Finally, our findings cannot be directly generalized to other countries at different latitudes, with different underlying risk of injuries, or with different sleeping patterns or circadian habits. Moreover, the question related to nonrestorative sleep was only asked of participants younger than 70 y. Our findings concerning this variable and the cumulative insomnia variable cannot be generalized to elderly populations. Also, these latter analyses had less statistical power than the analyses on difficulties initiating and maintaining sleep.

CONCLUSION

In summary, we identified insomnia as a major contributor to unintentional fatal injuries overall and to fatal motor-vehicle injuries. Insomnia is a frequent, easily recognizable and potentially manageable condition. Increasing public health awareness about insomnia and the identification of people with insomnia seem to be important for the prevention of fatal injuries.

DISCLOSURE STATEMENT

This was not an industry supported study. Dr. Laugsand received a research fellowship grant from Department of Public Health, Faculty of Medicine, Norwegian University of Science and Technology. Dr. Strand received a research fellowship grant from the Liaison Committee between the Central Norway Regional Health Authority and the Norwegian University of Science and Technology. Dr. Janszky was supported by the Liaison Committee of the Regional Health Authority and the Norwegian University of Science and Technology, by the Swedish Council of Working Life and Social Research and by the Swedish Research Council. The authors have indicated no financial conflicts of interest.

ACKNOWLEDGMENTS

The Nord-Trøndelag Health Study (The HUNT study) is a collaboration between the Faculty of Medicine, Norwegian University of Science and Technology, Nord-Trøndelag County Council, and the Norwegian Institute of Public Health. The authors of this manuscript have certified that they comply with the principles of ethical publishing. Dr. Laugsand analyzed the data, interpreted the findings, and wrote the paper; Dr. Strand interpreted the data and reviewed the paper for important intellectual content; Dr. Vatten interpreted the data and reviewed the paper for important intellectual content; Dr. Janszky interpreted the data and reviewed the paper for important intellectual content; and Dr. Bjørngaard had the original idea for this study, interpreted the findings, and wrote the paper. Dr. Laugsand had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

SUPPLEMENTAL MATERIAL

Table S1

Hazard ratios (95% confidence intervals) for unintentional fatal injuries according to use of psychopharmacological drugs at baseline.

Table S2

Hazard ratios (95% confidence intervals) for unintentional fatal injuries according to insomnia symptoms including occupational status as covariate in models 2-4.

aasm.37.11.1777.tS2.tif (569.3KB, tif)
Table S3

Hazard ratios (95% confidence intervals) for unintentional fatal injuries according to the number of insomnia symptoms including occupational status as covariate in models 2-4.

aasm.37.11.1777.tS3.tif (276.2KB, tif)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Table S1

Hazard ratios (95% confidence intervals) for unintentional fatal injuries according to use of psychopharmacological drugs at baseline.

aasm.37.11.1777.tS1.tif (525KB, tif)
Table S2

Hazard ratios (95% confidence intervals) for unintentional fatal injuries according to insomnia symptoms including occupational status as covariate in models 2-4.

aasm.37.11.1777.tS2.tif (569.3KB, tif)
Table S3

Hazard ratios (95% confidence intervals) for unintentional fatal injuries according to the number of insomnia symptoms including occupational status as covariate in models 2-4.

aasm.37.11.1777.tS3.tif (276.2KB, tif)

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