Prevalence of Insufficient Sleep and Its Associated Factors Among Working Adults in Malaysia

Abstract

Introduction
Given the scarcity of data on sleep insufficiency in developing countries and its emerging importance in public health, this study aims to provide the prevalence and factors associated with insufficient sleep among working Malaysian adults.

Materials and Methods
A total of 11,356 working adults (≥18 years) were recruited from a cross-sectional online health and work survey. Sleep duration was self-reported and was categorised as insufficient sleep (<7 h) and sufficient sleep (≥7 h) based on the National Sleep Guidelines’ daily sleep time recommendation for adults. The 10-item Kessler scale was used to assess psychological distress levels (cutoff score ≥20). Bivariate and multivariable regression analyses were performed to examine the factors associated with insufficient sleep.

Results
The mean age of the participants was 35.7 years (standard deviation, SD±9.2). There were more female (n=6613; 58.2%) respondents. The mean sleep duration was 6.49 h (SD±1.10). A total of 6212 individuals (54.7%) reported getting <7 h of sleep daily. After adjusting for gender, marital status and educational attainment, older age (odds ratio, OR=2.22, 95% confidence interval, 95% CI: 1.72–2.86), ever smoking (OR=1.38, 95% CI: 1.25–1.53), and high psychological distress (OR=1.33, 95% CI: 1.17–1.53) were found to be associated with insufficient sleep. Factors associated with sleep duration of <7 h per night included having children (OR=1.35, 95% CI: 1.21–1.50), lifestyle factors (OR=1.38, 95% CI: 1.25–1.53), poor sleeping conditions (OR=1.21, 95% CI: 1.07–1.36), and mental health issues (OR=1.32, 95% CI: 1.12–1.54).

Discussion and Conclusion
More than half of the participants reported sleep insufficiency. Older age, ever smoking, and high levels of psychological distress were significantly associated with insufficient sleep. Our findings recognise sleep as an emerging public health issue. Smoking cessation programmes and addressing high psychological distress may be beneficial to improve sleep.

Full text

ORIGINAL RESEARCH
Prevalence of Insufcient Sleep and Its Associated
Factors Among Working Adults in Malaysia
Caryn Mei Hsien Chan
1
Ching Sin Siau
1
Jyh Eiin Wong
1
Lei Hum Wee
1
Nor Aini Jamil
1
Victor Chee Wai Hoe
2
1
Centre for Community Health Studies,
Faculty of Health Sciences, Universiti
Kebangsaan Malaysia, Kuala Lumpur,
Malaysia;
2
Centre for Epidemiology and
Evidence-Based Practice, Department of
Social and Preventive Medicine, Faculty of
Medicine, University of Malaya, Kuala
Lumpur, Malaysia
Introduction: Given the scarcity of data on sleep insufciency in developing countries and
its emerging importance in public health, this study aims to provide the prevalence and
factors associated with insufcient sleep among working Malaysian adults.
Materials and Methods: A total of 11,356 working adults (≥18 years) were recruited from
a cross-sectional online health and work survey. Sleep duration was self-reported and was
categorised as insufcient sleep (<7 h) and sufcient sleep (≥7 h) based on the National
Sleep Guidelines’ daily sleep time recommendation for adults. The 10-item Kessler scale
was used to assess psychological distress levels (cutoff score ≥20). Bivariate and multi-
variable regression analyses were performed to examine the factors associated with insuf-
cient sleep.
Results: The mean age of the participants was 35.7 years (standard deviation, SD±9.2). There
were more female (n=6613; 58.2%) respondents. The mean sleep duration was 6.49 h (SD±1.10).
A total of 6212 individuals (54.7%) reported getting <7 h of sleep daily. After adjusting for gender,
marital status and educational attainment, older age (odds ratio, OR=2.22, 95% condence interval,
95% CI: 1.72–2.86), ever smoking (OR=1.38, 95% CI: 1.25–1.53), and high psychological distress
(OR=1.33, 95% CI: 1.17–1.53) were found to be associated with insufcient sleep. Factors
associated with sleep duration of <7 h per night included having children (OR=1.35, 95% CI:
1.21–1.50), lifestyle factors (OR=1.38, 95% CI: 1.25–1.53), poor sleeping conditions (OR=1.21,
95% CI: 1.07–1.36), and mental health issues (OR=1.32, 95% CI: 1.12–1.54).
Discussion and Conclusion: More than half of the participants reported sleep insuf-
ciency. Older age, ever smoking, and high levels of psychological distress were signicantly
associated with insufcient sleep. Our ndings recognise sleep as an emerging public health
issue. Smoking cessation programmes and addressing high psychological distress may be
benecial to improve sleep.
Keywords: sleep, sleep deprivation, adults, ageing, psychological distress, smoking,
Malaysia
Introduction
Insufcient sleep is gaining recognition as a public health concern, given emerging
evidence on the consequential and modiable nature of sleep.
1
Chronic insufcient
sleep is linked to poorer health outcomes,
1,2
greater economic burden
3,4
and as
a contributor to health disparities.
5
Most healthy adults aged 18 to 64 years require between seven and nine hours of
sleep per night to function at their best. This recommended sleep time duration for adult
individuals is backed by guidelines established by the American Academy of Sleep
Medicine,
6
the Sleep Research Society,
6
and the National Sleep Foundation.
7
Correspondence: Lei Hum Wee
Faculty of Health Sciences, Universiti
Kebangsaan Malaysia, Jalan Raja Muda
Abdul Aziz, Kuala Lumpur, 50300,
Malaysia
Tel +601 6309 0771
Fax +603 2693 8717
Email psychooncology.ukm@gmail.com
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Received: 1 March 2021
Accepted: 12 May 2021
Published: 13 July 2021

It is unclear to what extent adults adhere to this recom-
mendation. Regularly, even a single hour decit in opti-
mum sleep duration is associated with 60–80% higher
odds of depression, hopelessness, nervousness, and
restlessness.
8
Current evidence indicate that weekday
sleep debt cannot be overcome by compensatory measures
such as sleeping in on weekends.
9
Large-scale studies that have examined insufcient sleep
among adults in the general population are mostly limited to
those in developed countries.
9–13
Thus, there is a need for
evidence comprising samples from developing countries.
While there is research on nationally representative samples
in developed countries, sleep remains largely unsystemati-
cally explored in developing countries.
9,10,14
Current studies on sleep from developing countries are
limited to smaller samples targeting specic demographic
groups. Specically, sleep is often examined in older
adults,
15
individuals with comorbid illnesses,
2
and differ-
ent occupational sectors (eg, nurses, doctors, military, shift
workers),
16
as well as vulnerable subgroups such as those
exposed to trauma.
Much of the available literature is also focused on
clinical samples examining sleep disorders such as insom-
nia. However, the prevalence of non-disordered sleep is
much less known. However, there are increasing evidence
indicating that insufcient sleep syndrome should be
recognised as a clinical entity.
1,17
Even studies that had
looked at insufcient sleep tend to focus on its effects and
consequences.
1,14
Currently, there is still a lack of evi-
dence on the sociodemographic and modiable risk factors
that are associated with insufcient sleep among working
adults in Malaysia.
There is a need to establish baseline data and greater
epidemiological insight into factors associated with insuf-
cient sleep in the Asia region. Data on sleep among
working adults in Malaysia will allow us to better under-
stand the extent of this public health burden. Identifying
modiable factors associated with insufcient sleep can
help inform the development of appropriate interventions.
Accordingly, the study aimed to determine the prevalence
of insufcient sleep among adults and identify the factors
associated with sleep insufciency.
Methods
A cross-sectional survey was conducted in 2018 among
11,356 workers from 117 organisations in Malaysia.
Participants were recruited through emails sent to their
work email addresses. Participants were required to
complete an anonymous online survey questionnaire.
Their participation was entirely voluntary. This study
adopted the same methodology as the rst wave of the
study conducted in 2017 and 2018, which has been
described in full.
18,19
Informed consent was obtained from participants
before study commencement. Participants were informed
beforehand that the data analysis would be based on a de-
identied aggregated dataset. The de-identied aggregated
data are available for organisational and academic
research. It will not be available to the employer or for
marketing purposes. The management of the data is com-
pliant with the Malaysian Personal Data Protection Act
2010 and is regulated by the European Union’s General
Data Protection Regulation (GDPR) and UK Data
Protection Act 2018. Ethical approval was obtained (JEP-
2019-692). This study was conducted in accordance with
the Declaration of Helsinki.
Sleep
Participants self-reported their sleep duration based on the
single-item question “On average, how many hours of
sleep do you get in a 24-hour period?” Insufcient sleep
was dened as rest/sleep of <7 h per night on average over
the past 30 days. Mean daily sleep time (in hours) for adult
individuals was reported and categorised as inadequate
sleep (<7 h of sleep) and adequate sleep (≥7 h of sleep).
The categories were based on the American Academy of
Sleep Medicine, the Sleep Research Society, and the
National Sleep Foundation.
6,7
Psychological Distress
The Kessler screening tool (K10)
20
was used to measure
the presence and severity of non-specic psychological
distress. The K10 scale consists of ten items assessing
the frequency with which respondents experienced symp-
toms of psychological distress during the past 30 days. The
response options range from 0 (none of the time) to 4 (all
the time). The internal consistency was found to be accep-
table. The scale has been translated and validated for use
in this population. Participants’ scores of ≥20 were classi-
ed as high psychological distress.
21
Smoking
Smoking was evaluated using a single-item question: “Do
you smoke cigarettes?” Responses were as follows: No,
I have never smoked cigarettes, No, but I used to smoke
cigarettes, and I currently smoke cigarettes. Responses
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were classied according to ever smoking or never smok-
ing status. Participants who indicated never having
smoked cigarettes were considered never smokers,
whereas individuals who reported previous or current use
of cigarettes were designated as ever smokers.
Lifestyle Factors
Respondents were also asked about the factors that have
affected their sleep quality, to which “yes/ no” responses
were solicited. These factors were as follows: work stress,
child/children, neighbours or housemates, lifestyle, nan-
cial concerns, health issues, relationship problems, poor
sleeping conditions, and other mental health concerns (eg,
depression).
Statistical Analyses
Data were assessed with IBM SPSS Statistics for
Windows, Version 26.0 (IBM Corp., Armonk, N.Y.,
USA). We compared population characteristics, psycholo-
gical distress, and smoking status by sleep group (suf-
cient vs insufcient sleep).
A multivariable logistic regression model was con-
ducted to examine associations between insufcient sleep
(dependent variable) and associated factors. The results are
reported as crude and adjusted odds ratios (ORs) with 95%
condence interval (95% CI). To account for possible
confounding, analyses were adjusted for age, gender, mar-
ital status, and educational attainment.
Results
Of 11,551 individuals who completed the survey, 195
(1.7%) respondents who were non-Malaysians were
excluded. Non-Malaysians were excluded due to the fact
that their inclusion would likely inuence statistical pat-
terns and conclusions. This is because non-Malaysians
may have different cultural backgrounds and face adjust-
ment issues in Malaysia.
22
These may serve as confound-
ing variables which are not addressed in this study. A total
of 11,356 adults were retained for analysis. Demographic,
sleep, and mental health characteristics of the sample are
shown in Table 1. The mean age of the participants was
35.7 (Standard Deviation, SD±9.2) years, there were more
females (n=6613; 58.2%) and the majority were married
(n=6606; 58.2%). The mean sleep duration was 6.49 h (SD
±1.10). A total of 6212 individuals (54.7%) reported get-
ting less than 7 h of sleep regularly. Long sleep duration (9
+ hours) was excluded from analysis as the very low
prevalence of long sleep (n = 72; 0.6%) did not allow for
meaningful analysis. Using the K10, 1121 (9.9%) had high
levels of psychological distress (K10 scores ≥20). In terms
of smoking status, 8880 (78.2%) were never smokers,
whereas 2476 (21.8%) were ever smokers (ex-smokers
n = 1220; 10.7% and current smokers n = 1256; 11.1%).
A sleep duration of <7 h per night was associated with
older age (OR=2.22, 95% CI: 1.72–2.86), ever smoking
(OR=1.38, 95% CI: 1.25–1.53), and high psychological
distress (OR=1.33, 95% CI: 1.17–1.53) (Table 2). Other
self-reported factors deemed by respondents to affect their
sleep duration of <7 h per night included having children
(OR=1.35, 95% CI: 1.21–1.50), lifestyle factors
(OR=1.38, 95% CI: 1.25–1.53), poor sleeping conditions
(OR=1.21, 95% CI: 1.07–1.36), and other mental health
concerns (OR=1.32, 95% CI: 1.12–1.54).
Discussion
Main Findings of This Study
This is the rst large-scale study to estimate the prevalence
of sleep insufciency and its psychosocial determinants
among working adults in Malaysia. We identied a high
prevalence of insufcient sleep in this sample of working
adults, with 54.7% of our sample reporting an average of
<7 h of sleep daily.
We found that insufcient sleep was associated with
psychological distress, although it should be noted that
only a small percentage (9.87%) of respondents in this
study reported psychological distress. Individuals with
mental health issues had 1.32 times higher odds of insuf-
cient sleep compared to those without psychiatric illness.
Our ndings may be explained by numerous factors,
particularly lifestyle, which can be adjusted. Many indivi-
duals have habits that make it difcult to get a good
night’s sleep.
There was an association between insufcient sleep
and smoking, with workers who reported ever smoking
having 1.38 times higher odds of insufcient sleep. In this
sample, individuals who reported insufcient sleep also
attributed it in part to the presence of children.
While we expected to see an age-related decrease in
sleep duration, we were surprised to nd that sleep insuf-
ciency was an issue across most age brackets, starting
from individuals ≥24 years old. These odds increased with
older age, with those in the 24–34 age bracket having 1.2
times higher odds of reporting sleep insufciency, which
increased to 1.5 for those 35–44 years, 1.9 for those 45–54
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years, and 2.2 for those aged ≥55 years as compared to
those in the 18–24 age bracket.
What is Already Known
The rate of sleep insufciency in this sample is much
higher in comparison with the United States and
Australia, with the prevalence rates of 34.8%
2
and
24.0%,
23
respectively. Previous data from Malaysia tend
to focus on specic subpopulations
24–26
and clinically
diagnosed sleep disorders,
27
albeit we do not discount
the fact that individuals who report insufcient sleep are
much more likely to have a sleeping disorder. This study,
therefore, addresses a gap in the literature on the extent of
insufcient sleep among working adults in Malaysia.
Table 1 Characteristics of Malaysian Working Adults by Sleep Length (N = 11,356)
Sleep Duration Overall Combined p-value
<7 Hours n (%) ≥7 Hours n (%) n
Age categories 0.001
a
18–24 years 386 (6.2) 444 (8.6) 830
25–34 years 2633 (42.4) 2414 (46.9) 5047
35–44 years 1929 (31.1) 1469 (28.6) 3398
45–54 years 991 (16.0) 654 (12.7) 1645
55 years and above 273 (4.4) 163 (3.2) 436
Gender 0.010
b
Male 2662 (42.9) 2081 (40.5) 4743
Female 3550 (57.1) 3063 (59.5) 6613
Marital status 0.001
a
Cohabitating/Separated/Divorced/Widowed 319 (5.1) 201 (3.9) 520
Married 3724 (59.9) 2882 (56.0) 6606
Single 2169 (34.9) 2061 (40.1) 4230
Educational attainment 0.733
a
No formal education, primary, lower & upper
secondary
866 (13.9) 732 (14.2) 1598
A-levels or equivalent 1411 (22.7) 1133 (22.0) 2544
Undergraduate degree 3067 (49.4) 2534 (49.3) 5601
Postgraduate degree 868 (14.0) 745 (14.5) 1613
Smoking status 0.001
b
Never smokers 4703 (75.7) 4177 (81.2) 8880
Ever smokers 1509 (24.3) 967 (18.8) 2476
Psychological distress
c
K10 score <20 5526 (89.0) 4709 (91.5) 10,235 0.001
b
K10 score ≥20 686 (11.0) 435 (8.5) 1121
Factors affecting sleep quality:
Work stress 2558 (41.2) 1995 (38.8) 4553 (40.1) 0.100
a
Child/ children 1395 (22.5) 940 (18.3) 2335 (20.6) 0.001
a
Neighbours/ housing 229 (3.7) 206 (4.0) 435 (3.8) 0.379
a
Lifestyle 1272 (20.5) 828 (16.1) 2100 (18.5) 0.001
a
Financial concerns 1162 (18.7) 809 (15.7) 1971 (17.4) 0.001
a
Health issues 892 (14.4) 700 (13.6) 1592 (14.0) 0.251
a
Relationship problems 761 (12.3) 559 (10.9) 1320 (11.6) 0.022
a
Poor sleeping conditions 830 (13.4) 568 (11.0) 1398 (12.3) 0.001
a
Mental health issues 547 (8.8) 321 (6.2) 868 (7.6) 0.001
a
Notes: Percentages are based on column. For factors affecting sleep quality, participants selected all drop-down options that applied, with these numbers and percentages,
including the total for this section, based on items endorsed.
a
Pearson’s chi square.
b
Fisher’s exact test.
c
Psychological distress scores based on the Kessler 10 (K10): a score
of ≥20 indicates the presence of psychological distress.
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At least one previous study has found mildly insufcient
sleep to be associated with symptoms of anxiety and
depression.
8
Our ndings concerning smoking and insuf-
cient sleep are consistent with ndings from other studies
which reported that sleep-deprived individuals tend to smoke
and have caffeinated drinks two hours before bedtime.
28,29
Our ndings differ from past evidence that young adults
are more vulnerable to chronic sleep deciency and recurrent
circadian disruption than older adults,
30
and are at odds with
reports which indicate that older adults are less likely to
experience issues related to sleep.
22
The presence of children has been found to be a risk
factor for insufcient sleep among adults.
31
A study in
Singapore found that adults who slept with children, stu-
died, read leisurely, drank caffeinated beverages or
smoked were more likely to experience inadequate
sleep.
29
Individuals who reported insufcient sleep were
also far more likely to tend to use mobile devices in bed or
Table 2 Multivariable Logistic Regression of Factors Associated with Insufcient Sleep Among Working Adults in Malaysia
(N = 11,356)
Adjusted
Odds Ratio 95% CI p-value
Age categories
18–24 years (ref) – – –
25–34 years 1.21 1.03–1.41 0.017
35–44 years 1.50 1.26–1.77 0.001
45–54 years 1.88 1.56–2.27 0.001
55 years and above 2.22 1.72–2.86 0.001
Gender
Male (ref) – – –
Female 1.05 0.96–1.14 0.298
Marital status
Cohabitating/Separated/Divorced/Widowed (ref) – – –
Married 0.82 0.68–0.99 0.039
Single 0.80 0.65–0.97 0.024
Educational attainment 0.132
No formal education, primary, lower & upper secondary – – –
A-levels or equivalent 1.10 0.97–1.25 0.156
Undergraduate degree 1.13 1.01–1.28 0.042
Postgraduate degree 1.03 0.89–1.19 0.678
Smoking status
Never smokers (ref) – – –
Ever smokers 1.38 1.25–1.53 0.001
Psychological distress
K10 score less than 20 (ref) – – –
K10 psychologically distressed score ≥20 1.33 1.17–1.53 0.001
Factors affecting sleep quality:
Work stress 1.02 0.94–1.10 0.701
Child/ children 1.35 1.21–1.50 0.001
Neighbours/ housing 0.89 0.71–1.06 0.163
Lifestyle 1.38 1.25–1.53 0.001
Financial concerns 1.12 1.01–1.25 0.040
Health issues 0.92 0.82–1.03 0.149
Relationship problems 1.03 0.91–1.17 0.632
Poor sleeping conditions 1.21 1.07–1.36 0.002
Mental health issues 1.32 1.12–1.54 0.001
Note: DV: Sleep, less than 7 hours of sleep.
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in the bedroom.
29
At least one past study has listed neigh-
bours as a reason for insufcient sleep, as poorer neigh-
bourhoods may be less conducive environmentally for
sleep.
32
In tandem with the lack of evidence in this regard,
neighbourhood factors were found not signicant factors
for insufcient sleep in our study.
Existing research suggests that older age, female gen-
der, comorbidities, and high distress are risk factors for
insufcient sleep.
1
Consistent with previous studies, age,
high psychological distress, and smoking were signi-
cantly associated with insufcient sleep in this study.
Unfortunately, we cannot clarify the order of precedence
for either outcome or predictor, given the cross-sectional
nature of this study. Insufcient sleep may be a precursor
to mental health issues, although this potential bi-
directionality cannot be established in this study. It is
also possible that the numerous risk factors known to be
associated with insufcient sleep are buffered by other
protective factors not considered in this study.
What This Study Adds
These ndings are important as they will allow us to iden-
tify sleep as a potential public health priority. Based on
these ndings, the researchers plan to further explore life-
style factors for inadequate sleep and develop a checklist for
working adults who experience sleep-related problems.
This will likely prove benecial in identifying modiable
factors which can be targeted for intervention among indi-
viduals with insufcient sleep, eg, eliminating consumption
of caffeinated beverages before bedtime. Napping could be
implemented in the workplace to counter short sleep dura-
tion, as it has been found to reduce coronary mortality,
improve cognition, and improve immune functions (see
Faraut et al
33
for a clinical review).
Future research needs to be devoted to continued
research with samples from developing countries, and
should include more diverse samples concerning partici-
pant age, health status (ie, presence of comorbid illness),
gender, and ethnicity with increased cross-cultural
research. Working to explore the psychological media-
tors of sleep is also essential. Future research is needed
to examine longitudinal relationships between sleep and
mental health prospectively. Respite interventions have
been found to help employees replenish and build energy
resources at work.
34
Several meta-analyses have found
strong evidence of a benecial effect of respite interven-
tions on sleep and mental health.
35–36
This study offers
evidence of the actual rate of sleep issues in the working
population in Malaysia and suggests a need for further
study.
Our ndings contribute to a greater understanding of
the impact that psychosocial factors have on health and
psychological well-being, and carry implications for men-
tal and physical well-being, as sleep inuences both men-
tal and physical health. In terms of policy, ndings in this
study can help researchers, clinicians, and public health
policymakers gain a better understanding of insufcient
sleep, especially its risk and protective factors in Malaysia.
More systematic and empirically stringent methodologies
and research frameworks need to be used, however.
A holistic approach for the planning of preventative stra-
tegies and public health policies should be made according
to these risk factors.
Limitations
Among the limitations of this study is that a validated
measure of sleep was not used. It is also unclear to what
extent sleep insufciency represents overall psychological
well-being. Furthermore, we focused on insufcient sleep
duration and not on sleep quality. Past studies using mod-
erator analyses indicate that relationships between sleep
quality and quantity may be affected by measurement
method and the number of self-reported items used,
while there is little evidence of the effect of measurement
time frame.
34
No causality can be determined due to the
cross-sectional nature of our study. It is unclear to what
extent the various factors tested here are bidirectionally
associated with sleep. The sample is not nationally repre-
sentative due to the use of non-probability sampling
approach in this study. The exact participation rate for
the study is impossible to estimate because we cannot
determine the number of employees who actually received
or read the invitation email as only de-identied aggre-
gated data were available for analysis. We did not check
for the possible occurrence of multiple entries from the
same individual (eg, through cookies used, IP check and
log le analysis). We were unable to provide useful infor-
mation that helps interpret the working population or the
industries involved in this study. Also, because the sam-
ples were self-selected, these results have to be interpreted
with caution.
Conclusion
The good news is that sleep is a malleable behaviour that
can be improved by individual and organisational changes.
Findings underscore the need for further study to
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determine the complex links and factors that can be iden-
tied for intervention via programs to inform comprehen-
sive policies taking sleep into account among public health
challenges in this setting, as a public health priority.
Data Sharing Statement
Due to condentiality agreements with research collabora-
tors, the data that support the ndings of this study cannot
be made publicly available online. Restrictions apply to
the availability of these data, which were used by research-
ers subject to a non-disclosure agreement for this study.
Requests to access the data may be sought from RAND
Europe with the permission of AIA Malaysia.
Ethics
Electronic informed consent was completed by partici-
pants before study commencement. Ethical approval was
obtained from Universiti Kebangsaan Malaysia (JEP-
2019-692). This study was conducted in accordance with
the Declaration of Helsinki.
Acknowledgments
This work was supported by the Universiti Kebangsaan
Malaysia Grants No. NN-2018-168 and NN-2018-148.
Author Contributions
CMHC, CSS, WJE, LHW, NAJ participated in the study
conception, design, and coordination of the manuscript.
CMHC performed statistical analysis and drafted the ear-
liest iterations of the manuscript. VCWH revised multiple
versions of the manuscript and contributed signicant
intellectual content. All authors contributed to data analy-
sis, drafting or revising the article, have agreed on the
journal to which the article will be submitted, gave nal
approval of the version to be published, and agree to be
accountable for all aspects of the work.
Funding
This work was supported by the Universiti Kebangsaan
Malaysia, Grants No. NN-2018-168 and NN-2018-148,
which played no role or involvement in any of the stages
from study design to submission of the paper for
publication.
Disclosure
The authors report no conicts of interest in this work.
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