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Original Article
The relationship between nightmares, depression and suicide
Anna Karin Hedstr€
om
a
,
*
, Rino Bellocco
b
,
c
, Ola H€
ossjer
d
, Weimin Ye
b
,
Ylva Trolle Lagerros
e
,
f
, Torbj€
orn Åkerstedt
g
,
h
a
Department of Clinical Neuroscience and Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden
b
Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden
c
Department of Statistics and Quantitative Methods, University of Milano-Bicocca, Milan, Italy
d
Division of Mathematical Statistics, Stockholm University, Stockholm, Sweden
e
Clinical Epidemiology Division, Department of Medicine, Karolinska Institutet, Stockholm, Sweden
f
Center for Obesity, Academic Specialist Center, Stockholm Health Services, Stockholm, Sweden
g
Department of Clinical Neuroscience, Karolinska Institutet, Stockholm, Sweden
h
Stress Research, Stockholm University, Stockholm, Sweden
article info
Article history:
Available online 20 November 2020
Keywords:
Nightmares
Depression
Suicide
Prospective cohort study
Mediation analysis
abstract
Objective: Previous studies investigating the association between nightmares and suicide have yielded
different results. We aimed to investigate whether nightmares, directly or indirectly, influence the
incidence of suicide.
Methods: We used a prospective cohort study, based on 40,902 participants with a mean follow-up
duration of 19.0 years. Cox proportional hazards models with attained age as time-scale were fitted to
estimate hazard ratios (HR) of suicide with 95% confidence intervals (CI) as a function of the presence or
absence of depression and nightmares. Mediation analysis was used to asses to what extent the rela-
tionship between nightmares and the incidence rate of suicide could be mediated by depression.
Results: No association was observed between nightmares and the incidence of suicide among partici-
pants without depression. Compared with non-depressed participants without nightmares, the inci-
dence of suicide among participants with a diagnosis of depression was similar among those with and
without nightmares (HR 12.3, 95% CI 5.55e27.2 versus HR 13.2, 95% CI 7.25e24.1). The mediation analysis
revealed no significant effects of nightmares on suicide incidence. However, the incidence of depression
during follow-up was higher among those who suffered from nightmares than among those who did not
(p <0.001).
Conclusions: Our findings indicate that nightmares have no influence on the incidence rate of suicide, but
may reflect pre-existing depression. This is supported by a recent discovery of a strong genetic corre-
lation of nightmares with depressive disorders, with no evidence that nightmares would predispose to
psychiatric illness or psychological problems. Interventions targeting both depression and nightmares,
when these conditions co-occur, may provide additional therapeutic benefit.
©2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
1. Introduction
Suicide is a major public health concern with heterogeneous
etiology. On an individual level, the interplay between predis-
posing, mediating, and precipitating factors contribute to the risk
of developing suicidal behavior [1]. Improved recognition and
understanding of individual factors influencing suicidal behavior
may facilitate the detection of high-risk individuals.
Nightmares are terrifying or disturbing dreams, usually
involving threats to survival, safety or physical integrity, able to
awaken the sleeper [2]. Nightmares can be posttraumatic as part of
a posttraumatic stress reaction, idiopathic or drug induced [3].
Frequent nightmares have been related to both general psychopa-
thology [4] and psychiatric disorders, in particular post-traumatic
stress disorder [3e5], major depressive disorder [6], schizo-
phrenia [6], and borderline personality disorder [7]. Evidence
suggests that nightmares may persist over long periods of time
*Corresponding author. Department of Clinical Neuroscience and Institute of
Environmental Medicine, Karolinska Institutet, Nobels v€
ag 13, Stockholm, 171 77
Sweden.
E-mail address: anna.hedstrom@ki.se (A.K. Hedstr€
om).
Contents lists available at ScienceDirect
Sleep Medicine
journal homepage: www.elsevier.com/locate/sleep
https://doi.org/10.1016/j.sleep.2020.11.018
1389-9457/©2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Sleep Medicine 77 (2021) 1e6
[8e10]. As with other sleep disorders, nightmares have been
associated with increased risk of suicidal ideation, suicide attempts
and death by suicide [11 e20].
The association between depression and suicidal behavior is
well-documented, and most studies investigating the influence of
nightmares on suicidal behavior have reported that the prevalence
of nightmares increases with depressive symptoms [11e20].
Depressive symptoms may thus act as a confounder of the rela-
tionship between suicidal behavior and nightmares. However,
while some studies have reported that nightmares might signifi-
cantly increase suicidal behavior after controlling for depression
[14e16], other studies indicate that depressive symptoms could be
aggravated by low sleep quality and mediate the association be-
tween nightmares and suicide risk [19,20]. It is thus unclear
whether nightmares represent an independent risk factor for sui-
cidal behavior or whether the association between nightmares and
suicide risk is mediated by psychiatric conditions.
Most previous studies investigating the influence of nightmares
on suicidal behavior have used clinical samples that may not reflect
the general population. Using a large Swedish cohort with a mean
follow-up time of 19.0 years, we aimed to study the relationship
between nightmares, depression and the incidence of suicide, and
investigate whether nightmares, directly or indirectly, influence the
incidence rate of suicide.
2. Methods
In September 1997, the Swedish National March Cohort [21]was
established during a four-day nationwide fundraising event orga-
nized by the Swedish Cancer Society. Participants were invited to
fill out a 36-page questionnaire regarding demographic, lifestyle,
and medical information. All participants received written infor-
mation regarding the purpose of the study and provided written
informed consent to participate. They also provided their national
registration number, a unique identifier assigned to all Swedish
residents, which enabled us to follow the cohort by linkage to
multiple nation-wide, continuously updated and essentially com-
plete databases. The study was approved by the Regional Ethics
Committee in Stockholm.
Given the fundraising nature and nearly 3600 Swedish cities and
villages participating in the event, the number of individuals
offered a questionnaire could not be assessed. In total, 43,863
participants completed the questionnaire. Those with incorrect
national registration number were excluded (n ¼11) as were those
who were younger than 18 years (n ¼1732) or had emigrated or
died (n ¼55) before the start of follow-up. In the present study, we
also excluded participants with missing values on self-reported
depressive symptoms (n ¼721) or nightmares (n ¼442). After
applying the exclusion criteria, our final study population included
40,902 participants (64% women and 36% men) followed pro-
spectively for death by suicide until the end of April, 2018. Mean
age was 51.2 years (SD 16.0) with an age range between 18 and 94
years.
2.1. Exposure assessment
Information regarding diagnoses of psychiatric disorders was
obtained from the Swedish Patient Register. A diagnosis of
depression was defined as having received ICD-8 codes
296.0e296.3, 296.8e296.9, 298.0 or 300.4 or ICD-9 codes 296B-E,
298A or 300E before baseline or ICD-10 codes F31eF33 during
follow-up. Depressive symptoms were assessed by asking the
participants to estimate how often they felt sad, low-spirited or
depressed. The response alternatives were never/seldom, some-
times, often or always/almost always. The reference group was
represented by those who never or seldom experienced depressive
symptoms. The Karolinska Sleep Questionnaire [22] was used to
assess the prevalence and frequency of nightmares. Answer alter-
natives were never, seldom, sometimes, mostly or always. The
reference group was represented by those who never or seldom
experienced nightmares. In some of the analyses, we also dichot-
omized nightmares into yes (sometimes, mostly, or always expe-
riencing nightmares) or no (never or seldom experiencing
nightmares). Since few participants reported often or always hav-
ing nightmares, they were merged with those who sometimes re-
ported nightmares into the exposed group. In order to elucidate the
relationship between measures of depression, nightmares and
death by suicide, the participants were further categorized based
on both a diagnosis of depression, self-reported depressive symp-
toms and frequency of nightmares.
2.2. Follow-up and outcome
The cohort was followed from baseline on October 1, 1997.
Follow-up ended at the time of death, emigration or April 30, 2018,
whichever occurred first. Using the individually unique Swedish
national registration numbers, mortality data was obtained by
linkage to the Swedish Cause of Death Register held by the National
Board for Health and Welfare. A total of 8640 deaths occurred
during the follow-up period. Of these, 69 were suicides (ICD-10
codes X60-X84 and Y10eY34).
2.3. Statistical analysis
Differences in baseline variables across categories of nightmare
frequency were assessed using one-way analysis of variance
(ANOVA) for continuous variables and the KruskaleWallis test for
categorical variables. Cox proportional hazards models with
attained age as time-scale were used to estimate hazard ratios of
suicide (HRs) with 95% confidence intervals (CI) for participants
categorized by depressive symptoms and nightmares. A trend test
for a dose response relationship regarding the frequency of night-
mares and suicide incidence rate was performed by using a cate-
gorical variable for nightmare frequency (never/seldom,
sometimes, often, or always) in a Cox proportional hazards model.
Residual analyses were conducted to study the proportionality
hazard assumption, based on the Schoenfeld residual plots and
statistical tests.
Among participants without a diagnosis of depression at base-
line, we used a logistic regression model to assess the risk of
receiving a diagnosis of depression during follow-up among those
who suffered from nightmares at baseline, compared to those who
never or seldom had nightmares. Mediation analysis using Cox
models under the rare outcome assumption was carried out to
asses to what extent the relationship between nightmares and the
incidence of suicide was mediated by depression [23,24]. The ef-
fects were estimated on the HR scale and the CI's were calculated
using the delta method. Participants with a diagnosis of depression
at baseline were excluded in the mediation analyses.
All analyses were adjusted for a number of potential con-
founding variables. In presence of confounding, the adjusted esti-
mates of the beta's coefficient will change compared to the crude
estimate. The final analyses were adjusted for sex, educational
level, occupation, smoking, sleep duration, hypnotic use, and
presence of cardiovascular disease at baseline When appropriate,
adjustments were also made for depressive disorders, anxiety
disorders, and psychotic disorders, or self-reported depressive
symptoms and self-reported anxiety symptoms.
Educational level was summarized into a binary variable, based
on having reached a university degree. Occupation was categorized
A.K. Hedstr€
om, R. Bellocco, O. H€
ossjer et al. Sleep Medicine 77 (2021) 1e6
2
into working, retired, student, unemployed, long-term sick-leave or
other. Smoking wascategorizedinto never, past or current smokers.
Habitual sleep duration was represented by a continuous variable
for sleep duration (number or hours per weekday night). Hypnotic
use was dichotomized into yes or no. Information regarding di-
agnoses of cardiovascular disease (ICD-10 codes I00eI99), anxiety
disorders (F40eF48), and psychotic disorders (F20eF29) at baseline
was obtained from the Swedish Patient Register and the variables
were dichotomized into those who had a diagnosis and those who
had not. Anxiety symptoms were assessed by asking the partici-
pants to estimate how often they felt worried, tensed or anxious.
The response alternatives were never/seldom, sometimes, often or
always/almost always. The reference group was represented by
those who never or seldom experienced anxiety symptoms.
Adjustments were also made for the following potential con-
founding variables; body mass index (BMI), physical activity,
alcohol consumption, coffee consumption, insomnia, and cancer.
However, these factors only had minor influence on the results and
were therefore not kept in the final analyses. BMI was calculated by
dividing weight in kilograms by height in meters squared, and
categorized into underweight (<18.5 kg/m
2
), normal weight
(18.5e24.99 kg/m
2
), overweight (25e30 kg/m
2
) or obese (>30 kg/
m
2
). Physical activity was based on reported responses on weekly
exercise levels ranging from none or easy physical activity to hard
physical activity and dichotomized into those active (more than
120 min) or inactive (120 min or less). Alcohol consumption was
categorized into drinkers, non-drinkers or unknown. We further
adjusted for alcohol as a continuous variable (gram per months).
Coffee consumption was categorized into 0, 1e4, 5e7or>7 cups of
coffee per day. Insomnia symptoms were assessed by asking par-
ticipants to estimate how often they experienced difficulties initi-
ating sleep, difficulties maintaining sleep, early-morning
awakenings, not rested at awakening and daytime sleepiness. The
response alternatives were never, seldom, sometimes, mostly or
always. Insomnia was defined as mostly or always experiencing any
of the nocturnal insomnia symptoms (difficulties initiating sleep,
difficulties maintaining sleep and early-morning awakenings), as
well as mostly or always experiencing symptoms of non-restorative
sleep (not rested at awakening and daytime sleepiness). Informa-
tion regarding diagnoses of cancer (ICD-10 codes C00eC97) was
obtained from the Cancer Register and was dichotomized into those
who had a diagnosis and those who had not.
The proportion of missing data in the potential confounding
variables was 6.7% for smoking habits, 4.6% for sleep duration, 4.3%
for BMI, 1.7% for coffee consumption, 1.2% for insomnia status, and
less than 1% for occupational level, educational level, physical ac-
tivity, and alcohol consumption. We therefore conducted supple-
mentary analyses after imputing missing data using the multiple
imputation chained equation procedure.
Since low incidence rate affects statistical power, we performed
a power analysis, described in detail in Supplementary Table 1. All
analyses were performed using Statistical Analysis System 9.4. All
statistical tests were two-sided, and p values less than 0.05 were
considered statistically significant.
3. Results
Characteristics of participants at baseline, overall and by
category of nightmare frequency, are presented in Table 1.The
occurrence of nightmares was highly correlated to other sleep-
related difficulties and measures of depression. A larger propor-
tion of women reported having nightmares. Generally, nightmare
sufferers had a lower educational level than those who reported
never or seldom having nightmares. They reported lower phys-
ical activity, higher BMI, and were more often smokers. They
were more likely to have a diagnosis of cardiovascular disease or
cancer, compared to those who reported never or seldom having
nightmares.
There was a significant correlation between a diagnosis of
depression and self-reported depressive symptoms (correlation
coefficient ¼0.38, p <0.001). Baseline characteristics among par-
ticipants with different frequencies of depressive symptoms are
presented in Table 2. Participants who reported depressive symp-
toms had longer sleep duration on average, compared to those who
never or seldom experienced depressive symptoms, and they
considerably more often suffered from insomnia symptoms. They
were more often smokers and reported a lower level of physical
activity. Women were overrepresented among those with self-
reported depressive symptoms.
During a mean follow-up time of 19.0 years (SD 4.0), 69 deaths
by suicide occurred (64% men and 36% women). The mean time
from baseline to suicide was 9.0 years (SD 5.0). Often or always
having nightmares was associated with a significantly increased
incidence of suicide. However, after adjustment, statistical signifi-
cance was lost (Table 3).
When participants were categorized based on a diagnosis of
depression and nightmares, no association was observed between
nightmares and death by suicide among those without a diagnosis
of depression (HR 1.00, 95% CI 0.44e2.28) (Table 4). Compared
with participants without a diagnosis of depression or night-
mares, there was a more than 12-fold increased incidence of
suicide among depressed participants, whereas nightmares did
not further increase the risk. There were no significant gender
differences (Table 4). Similar results were obtained when partic-
ipants instead were categorized based on subjective depressive
and anxiety symptoms without considering psychiatric diagnoses
(Table 5).
3.1. Incidence of depression by nightmare frequency
Among participants without a diagnosis of depression at base-
line, the odds of depression during follow-up was higher among
those who suffered from nightmares than among those who did not
(OR 1.35, 95% CI 1.19e1.53). When the analysis was stratified by
gender, similar results were obtained for women (OR 1.37, 95% CI
1.19e1.57) and men (OR 1.30, 95% CI 1.02e1.66). There was also a
trend showing increasing incidence of receiving a diagnosis of
depression during follow-up with increasing frequency of night-
mares at baseline (p value for trend <0.001).
3.2. Mediation analysis
The total effect of nightmares on the incidence of suicide, esti-
mated on the HR scale, was 1.07 (95% CI 0.49e2.37). The direct
effect was 1.02 (95% CI 0.57e1.83) and the indirect effect, mediated
by depression, was 1.05 (95% CI 0.62e1.80). Estimates were similar
when women and men were analyzed separately. There was no
interaction between a diagnosis of depression and nightmares with
regard to suicide risk.
Our results remained almost identical after carrying out the
analyses on the multiple imputed data (data not shown).
4. Discussion
In the present cohort, comprising 40,902 participants with a
mean follow-up of 19.0 years, we found no evidence suggesting
that nightmares influence the incidence of suicide.
Nightmare frequency was highly correlated with self-reported
depressive and anxiety symptoms, sleep duration, and insomnia.
These results are in accordance with previous research showing an
A.K. Hedstr€
om, R. Bellocco, O. H€
ossjer et al. Sleep Medicine 77 (2021) 1e6
3
Table 1
Baseline characteristics, overall and by frequency of nightmares.
Variable Total Nightmares P-value for difference between groups
Often, always Sometimes Never, seldom
N 40,902 407 7496 32,999
Mean age (SD) 51.2 (16.0) 50.0 (19.1) 50.5 (16.7) 51.4 (15.7) 0.003
Women, n (%) 26,301 (64) 291 (72) 5489 (73) 20,521 (62) <0.001
University degree, n (%) 11,534 (28) 88 (22) 1815 (24) 9631 (29) <0.001
Working, n (%) 20,274 (50) 138 (34) 3388 (45) 16,748 (51) <0.001
Retired, n (%) 10,603 (26) 128 (31) 1928 (26) 8547 (26) 0.036
Student, n (%) 1604 (3.9) 37 (9.1) 415 (5.5) 1152 (3.5) <0.001
Unemployed, n (%) 821 (2.0) 15 (3.7) 184 (2.5) 622 (1.9) <0.001
Long-term sick-leave, n (%) 670 (1.6) 21 (5.2) 197 (2.6) 452 (1.4) <0.001
Other, n (%) 1850 (4.5) 30 (7.4) 359 (4.8) 1461 (4.4) 0.038
Difficulty falling asleep, n (%) 2243 (5.5) 131 (32) 701 (9.4) 1411 (4.3) <0.001
Difficulty maintaining sleep, n (%) 2922 (7.1) 136 (33) 897 (12) 1889 (5.7) <0.001
Early morning awakening, n (%) 3550 (8.7) 117 (29) 950 (13) 2483 (7.5) <0.001
Tired at awakening, n (%) 5934 (15) 142 (35) 1526 (20) 4266 (13) <0.001
Daytime sleepiness, n (%) 2955 (7.2) 129 (32) 884 (12) 1942 (5.9) <0.001
Insomnia, n (%) 2350 (5.8) 127 (31) 748 (10) 1475 (4.5) <0.001
Mean sleep duration, hours/night (SD) 6.8 (1.0) 6.4 (1.4) 6.8 (1.1) 6.9 (1.0) <0.001
Often/always depressive symptoms, n (%) 2565 (6.3) 142 (35) 942 (13) 1481 (4.5) <0.001
Often/always anxiety symptoms, n (%) 3785 (9.4) 156 (39) 1367 (19) 2262 (7.0) <0.001
Current smokers, n (%) 2957 (7.2) 50 (12) 742 (9.9) 2165 (6.6) <0.001
Past smokers, n (%) 10,506 (26) 92 (23) 1917 (26) 8497 (26) 0.340
BMI, kg/m
2
(SD) 24.6 (3.5) 25.1 (4.1) 24.7 (3.8) 24.6 (3.5) 0.025
Low physical activity, n (%) 6492 (16) 101 (25) 1227 (16) 5164 (16) <0.001
Coffee, no of cups/daily (SD) 2.9 (1.8) 2.6 (1.9) 2.8 (1.8) 2.9 (1.8) <0.001
Alcohol drinkers, n (%) 36,011 (88) 351 (86) 6595 (88) 29,065 (88) 0.516
Standard glasses of alcohol per week (SD) 6.3 (4.3) 6.0 (4.5) 6.1 (4.3) 6.3 (4.3) <0.001
Cancer, n (%) 2558 (6.3) 35 (8.6) 538 (7.2) 1985 (6.0) <0.001
Cardiovascular disease, n (%) 4458 (11) 81 (20) 939 (13) 3438 (11) <0.001
Differences in baseline variables across categories of nightmare frequency were assessed using one-way analysis of variance (ANOVA) for continuous variables and the
KruskaleWallis test for categorical variables.
Table 2
Baseline characteristics among subjects with different frequencies of self-reported depressive symptoms.
Variable Total Self-reported depressive symptoms P-value for difference between groups
Often or always Sometimes Never or seldom
N 40,902 2565 20,792 17,545
Mean age (SD) 51.2 (16.0) 52.6 (15.6) 50.6 (16.1) 46.1 (16.0) <0.001
Women, n (%) 26,301 (64) 1988 (78) 14,573 (70) 9740 (56) <0.001
University degree, n (%) 11,534 (28) 755 (29) 5765 (28) 5014 (29) 0.065
Working, n (%) 20,274 (50) 1220 (48) 10,073 (48) 8981 (51) <0.001
Retired, n (%) 10,603 (26) 395 (15) 5179 (25) 5029 (29) <0.001
Student, n (%) 1604 (3.9) 191 (7.5) 943 (4.5) 470 (2.7) <0.001
Unemployed, n (%) 821 (2.0) 87 (3.4) 464 (2.2) 270 (1.5) <0.001
Long-term sick-leave, n (%) 670 (1.6) 104 (4.1) 382 (1.8) 184 (1.1) <0.001
Other, n (%) 1850 (4.5) 113 (4.4) 726 (3.5) 597 (3.4) 0.035
Difficulty falling asleep, n (%) 2243 (5.5) 511 (20) 1295 (6.2) 437 (2.5) <0.001
Difficulty maintaining sleep, n (%) 2922 (7.1) 533 (21) 1759 (8.5) 630 (3.6) <0.001
Early morning awakening, n (%) 3550 (8.7) 542 (21) 2039 (10) 969 (5.5) <0.001
Tired at awakening, n (%) 5934 (15) 985 (38) 3365 (16) 1584 (9.0) <0.001
Daytime sleepiness, n (%) 2955 (7.2) 719 (28) 1685 (8.1) 551 (3.1) <0.001
Insomnia, n (%) 2350 (5.8) 591 (23) 1353 (6.5) 406 (2.3) <0.001
Mean sleep duration, hours/night (SD) 6.8 (1.0) 6.9 (0.8) 6.8 (1.0) 6.6 (1.2) <0.001
Often/always nightmares, n (%) 407 (1.0) 142 (5.5) 206 (1.0) 59 (0.3) <0.001
Sometimes nightmares, n (%) 7496 (18) 942 (37) 4784 (23) 1770 (10) <0.001
Current smokers, n (%) 2957 (7.2) 317 (12) 1594 (7.7) 1046 (6.0) <0.001
Past smokers, n (%) 10,506 (26) 640 (25) 5319 (26) 4547 (26) 0.514
BMI, kg/m
2
(SD) 24.6 (3.5) 24.6 (3.3) 24.6 (3.6) 24.7 (4.1) 0.462
Low physical activity, n (%) 6492 (16) 568 (22) 3326 (16) 2598 (15) <0.001
Coffee, no of cups/daily (SD) 2.9 (1.8) 2.9 (1.8) 2.8 (1.8) 2.8 (2.0) <0.001
Alcohol drinkers, n (%) 36,011 (88) 2254 (88) 18,360 (88) 15,397 (88) 0.251
Standard glasses of alcohol per week (SD) 6.3 (4.3) 6.5 (4.4) 6.2 (4.2) 5.9 (4.2) <0.001
Cancer, n (%) 2558 (6.3) 164 (6.4) 1355 (6.5) 1039 (5.9) 0.054
Cardiovascular disease, n (%) 4458 (11) 274 (11) 2307 (11) 1877 (11) 0.432
Differences in baseline variables across categories of nightmare frequency were assessed using one-way analysis of variance (ANOVA) for continuous variables and the
KruskaleWallis test for categorical variables.
A.K. Hedstr€
om, R. Bellocco, O. H€
ossjer et al. Sleep Medicine 77 (2021) 1e6
4
association between nightmare frequency and the general level of
psychopathology, mood and anxiety disorders, and other sleep
disorders [3e7].
Nightmares were not associated with an increased incidence of
suicide among non-depressed participants, and did not further
increase the incidence of suicide associated with depression.
Mediation analysis showed no evidence suggesting that night-
mares directly influence suicide risk. However, among participants
without a diagnosis of depression at baseline, nightmare sufferers
had an increased probability of receiving a diagnosis of depression
during follow-up. Our findings indicate that nightmares may reflect
pre-existing depression.
In accordance with previous studies suggesting a gender dif-
ference in nightmare frequency [25], women tended to report
nightmares more often than men. We also observed a gender dif-
ference in the prevalence of self-reported depressive symptoms
and depressive disorders, which has also previously been well-
documented [26]. Our study was also in line with the consistent
finding that men have higher suicide mortality rates than women
[27]. Although there are gender differences in nightmare frequency,
in the prevalence of depressive symptoms and depressive disor-
ders, as well as in suicide mortality rates, our finding that night-
mares have no direct influence on suicide incidence applied to both
women and men.
Nightmare distress, the extent to which nightmares compro-
mise daytime functioning and well-being, has been associated with
mental complaints such as anxiety and depression rather than with
nightmare frequency [28,29]. Since nightmares are potentially
modifiable, and the clinical efficacy of psychological treatments is
well-documented [2], interventions in order to reduce clinically
significant nightmare symptoms are important, particularly in the
context of psychiatric comorbidity.
There are substantial genetic effects on the disposition to
nightmares [8]. A recent genome-wide association study,
Table 3
HR with 95% CI of death by suicide among subjects who suffer from nightmares, compared to those who never or seldom have nightmares.
Nightmares N Person years Deaths (%) HR (95% CI)
a
HR (95% CI)
b
HR (95% CI)
c
HR (95% CI)
d
Never/seldom 32,999 628,389 52 (0.16) 1.0 (reference) 1.0 (reference) 1.0 (reference) 1.0 (reference)
Sometimes 7496 141,425 14 (0.19) 1.41 (0.78e2.56) 1.10 (0.60e2.01) 1.03 (0.56e1.90) 1.04 (0.56e1.92)
Often/always 407 7342 3 (0.74) 5.78 (1.80e18.6) 2.90 (0.86e9.75) 2.45 (0.70e8.53) 2.27 (0.65e7.89)
P value for trend 0.04 0.36 0.53 0.55
a
Adjusted for gender.
b
Adjusted for gender, occupational status, educational status, smoking, sleep duration, hypnotic use, and cardiovascular disease.
c
Adjusted for gender, occupational status, educational status, smoking, sleep duration, hypnotic use, cardiovascular disease, depressive disorders, anxiety disorders, and
psychotic disorders.
d
Adjusted for gender, occupational status, educational status, smoking, sleep duration, hypnotic use, cardiovascular disease, self-reported depressive symptoms, and self-
reported anxiety symptoms. Significant HRs are in bold.
Table 4
HR with 95% CI of death by suicide among subjects with different combinations of depression and nightmares, compared to subjects without a diagnosis of depressionwho
never or seldom experience nightmares, overall and stratified by gender.
Depression diagnosis Nightmares N Person years Deaths (%) P value for mortality rate difference HR (95% CI)
a
HR (95% CI)
b
No Never/seldom 31,643 603,205 32 (0.1) 0.94 1.0 (reference) 1.0 (reference)
No Sometimes/often/always 7243 136,370 7 (0.1) 1.16 (0.51e2.64) 1.00 (0.44e2.28)
Yes Never/seldom 1356 25,184 20 (1.5) 0.97 16.5 (9.39e28.9) 12.3 (5.55e27.2)
Yes Sometimes/often/always 660 12,398 10 (1.5) 20.2 (9.83e41.3) 13.2 (7.25e24.1)
Women
No Never/seldom 19,585 381,273 9 (0.05) 0.78 1.0 (reference) 1.0 (reference)
No Sometimes/often/always 5263 102,010 3 (0.06) 1.27 (0.34e4.70) 1.08 (0.29e4.04)
Yes Never/seldom 936 17,819 8 (0.85) 0.94 19.1 (7.36e49.7) 15.2 (5.67e40.5)
Yes Sometimes/often/always 517 9910 5 (0.97) 22.0 (7.35e66.1) 13.3 (4.05e43.5)
Men
No Never/seldom 12,058 221,932 23 (0.19) 0.85 1.0 (reference) 1.0 (reference)
No Sometimes/often/always 1980 34,360 4 (0.20) 1.11 (0.38e3.20) 0.96 (0.33e2.79)
Yes Never/seldom 420 7365 12 (2.8) 0.73 15.3 (7.58e30.7) 12.6 (6.08e26.0)
Yes Sometimes/often/always 143 2488 5 (3.5) 19.4 (7.35e51.0) 12.6 (4.45e35.9)
We used the test-based method to calculate p values.
a
Adjusted for gender when appropriate.
b
Adjusted for gender when appropriate, occupational status, educational status, smoking, sleep duration, hypnotic use, cardiovascular disease, anxiety disorders, and
psychotic disorders. Significant HRs are in bold.
Table 5
HR with 95% CI of death by suicide among subjects with different combinations of self-reported depressive symptoms and nightmares, compared to those who never or seldom
experience depressive symptoms or nightmares.
Depressive symptoms Nightmares N Person years Deaths (%) P value for mortality rate difference HR (95% CI)
a
HR (95% CI)
b
Never, seldom Never/seldom 15,716 298,394 10 (0.06) 0.91 1.0 (reference) 1.0 (reference)
Never, seldom Sometimes/often/always 1829 33,663 1 (0.05) 0.98 (0.12e7.48) 0.94 (0.11e6.69)
Sometimes Never/seldom 15,802 301,377 36 (0.23) 0.95 4.47 (2.21e9.04) 3.90 (1.92e7.92)
Sometimes Sometimes/often/always 4990 94,167 11 (0.22) 4.95 (2.09e11.7) 3.61 (1.49e8.71)
Often, always Never/seldom 1481 28,617 6 (0.41) 0.83 9.23 (3.32e25.6) 6.01 (2.13e17.4)
Often, always Sometimes/often/always 1084 20,937 5 (0.46) 12.4 (4.16e36.8) 5.66 (1.78e18.0)
We used the test-based method to calculate p values.
a
Adjusted for gender.
b
Adjusted for gender, occupational status, educational status, smoking, sleep duration, hypnotic use, cardiovascular disease, and self-reported anxiety symptoms. Sig-
nificant HRs are in bold.
A.K. Hedstr€
om, R. Bellocco, O. H€
ossjer et al. Sleep Medicine 77 (2021) 1e6
5
examining the genetics of nightmares, indicate that shared genetics
may account for the observed association between nightmares and
depression, and thus contribute to the comorbidity of these con-
ditions [30]. Furthermore, analysis of directionality showed that
psychiatric traits were predictors for nightmares, whereas no evi-
dence was observed suggesting that nightmares would predispose
to psychiatric illness or psychological problems [30]. More research
in this area is warranted since knowledge of the role of genetics in
nightmares and depression might be applied in developing pre-
ventative strategies in the future.
The strengths of this prospective cohort study are the large
sample size, the long follow-up duration, and the almost complete
follow-up ascertained by linking baseline information with
nationwide, continuously updated registers. The study provides
detailed information of high quality regarding exposure informa-
tion [21].
Weaknesses are that all self-reported information was only
measured at baseline. Potential changes in the occurrence of night-
mares during the follow-up period would go undetected. However,
evidence suggests that nightmares may persist over long periods of
time [8e10]. Another limitation is that the participants were not
provided with a definition of nightmares and we were unable to
differentiate between nightmares with intense emotional impact
that awakens the sleeper and bad dreams with no temporal rela-
tionship between the content of the dream and waking up. However,
the gender difference in nightmare frequency is not affected by
nightmare definition [25], indicating that there is a continuum from
bad dreams to nightmares with comparable etiological factors.
When the participants were categorized by measures of depression
and nightmares, relatively few deaths by suicide occurred in each
category, and our results should therefore be interpreted with
caution. Furthermore, apart from self-reported use of hypnotics, we
did not have the opportunity to adjust the analyses for medications
that have beenassociated with the occurrence of nightmares, such as
selective serotonin reuptake inhibitors [31].
Since subjects were recruited during a fund-raising event in
order to support cancer research, the cohort may be prone to a
potential healthy volunteer bias. However, while poor response
rates and incomplete follow-up is a problem in many population-
based studies, the shortcomings of a non-representative sample
must be weighed against the fact that choosing a restricted sample
can increase the feasibility of the study, the prevalence of the
exposure and completeness of the follow-up. These factors all in-
crease the validity and precision of the study. For example, the level
of missing data was very low in our study.
In conclusion, our findings indicate that nightmares have no
influence on the incidence rate of suicide, but may reflect pre-
existing depression. This is supported by a recent discovery of a
strong genetic correlation of nightmares with depressive disorders,
with no evidence that nightmares would predispose to psychiatric
illness or psychological problems. Interventions targeting both
depression and nightmares, when these conditions co-occur, may
provide additional therapeutic benefit.
Source of funding
The study was supported by funding from the Swedish research
council for health, working life and welfare.
Conflict of interest
The authors report no conflict of interest.
The ICMJE Uniform Disclosure Form for Potential Conflicts of
Interest associated with this article can be viewed byclicking on the
following link: https://doi.org/10.1016/j.sleep.2020.11.018.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.sleep.2020.11.018.
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