Diagnostic and Statistical Manual of Mental Disorders, Fifth edition (DSM-V) defines nightmares as extremely dysphoric and well-remembered dreams that usually involve efforts to avoid threats to survival, security, or physical integrity1. While occasional nightmares are common and usually do not lead to long-term negative consequences, around 5% of adults suffer from frequent nightmares2,3,4. Frequent nightmares have been linked to other sleep problems, especially insomnia, and mental health problems, especially symptoms of depression, as well as to lowered quality of life in general3,5,6,7,8,9.

Nightmares are also a defining symptom of post-traumatic stress disorder (PTSD)10. PTSD is a condition that may develop after exposure to traumatic event, for example, death, violence, serious injury or sexual violence. The person developing PTSD may be subject of the traumatic event, witness it or encounter it in a professional setting. Symptoms of PTSD include repeated re-experiencing of the trauma which typically take the form of intrusive thoughts, nightmares and flashbacks, but also avoidance of trauma related stimuli, and depressive and anxiety symptoms1. The prevalence of PTSD is elevated among populations that have high risk of encountering traumatic events, for example war veterans, and these populations also have higher prevalence of nightmares than the general population2.

In 2001, Tanskanen et al.11 published a study on nightmares as predictors of suicide in Finland, where suicide rate has been high (25–45 per 100 000 person years for men) during the last four decades12. They used a series of population level health surveys, FINRISK, to obtain information on self-estimated nightmare frequency and linked this data to the Finnish National Causes of Death Register which contains information on suicides. They found that men who had frequent nightmares had a significantly higher risk of suicide (Hazard Ratio (HR) 1.66 [1.15–2.39]) than those who did not have nightmares. Among women, similar trend (HR 1.22 [0.57–2.60]) emerged, but it lacked statistical significance due to the small number of suicides (n = 30) by women in the dataset.

In addition to the study of Tanskanen et al.11, several other studies have investigated the association between nightmares and suicidality. These have been summarized in Table 1. The methods and study populations of these studies vary: Many studies have used clinical samples and therefore their results may not reflect the general population. However, most studies report a significant link between nightmares and either suicidal thoughts as measured by questionnaires or suicide attempts. In addition to nightmares, disturbed sleep in general and insomnia in particular increase the risk for suicidality13,14,15,16,17,18,19,20.

Table 1 Studies investigating the association between nightmares and suicide. Full size table

As nightmares are strongly associated with depression and PTSD, we agree with Tanskanen et al.11 that “The common denominator between the frequency of nightmares and an increased risk for suicide may be a history of trauma”. In our earlier study2 in which we utilized the same FINRISK data used by Tanskanen et al., we noticed a sizeable and identifiable population of veterans of the Second World War (WWII) in these data. Compared to the general population with nightmare prevalence of 3.5% among men, the war veterans had elevated nightmare frequency of 7–11%, even 30 or more years after the war had ended. We hypothesise that this elevated nightmare frequency among the veterans is related to post-traumatic nightmares caused by war experiences. There is also evidence that war veterans, especially those with PTSD, have increased risk for suicidality, although the evidence is mostly based on studies from the Vietnam era or later21,22,23,24.

The sample of the original study by Tanskanen et al.11 included veterans of the Second Word War (WWII), but they were not identified and analysed separately from the general population. Thus, the possible effect that war veterans may have had on the association between nightmares and suicide was not explored - a state of affairs we set out to remedy.

The aim of the current study was thus to perform an extension of the original study by Tanskanen et al.11. We analysed partially overlapping data from the FINRISK series of health surveys linked to the National Causes of Death Register, as in the original study, with three significant improvements:

First, we identify war veterans in the data and investigate whether war experiences are a more significant predictor of suicide than nightmare frequency by controlling for war experiences in the analyses between nightmares and suicide. We also explore this association among the veteran population separately.

Second, we study risk factors for nightmares among war veterans and compare them with the risk factors for nightmares in the general population.

Third, our sample size is 91% larger and the follow-up period 9 years longer than in the study of 2001, equalling 191% more person-years and resulting in increased statistical power.

We test the following two hypotheses: First, we predict that the association between nightmares and suicide becomes markedly less significant in the whole sample when the war veterans are excluded from the study population. Second, we expect that among war veterans nightmares are a stronger predictor of suicide than among the general population, as post-war nightmares often stem from serious mental health issues, including PTSD and major depression.