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Review Article
Poor sleep as a potential causal factor in aggression and violence
Jeanine Kamphuis
a,
⇑
, Peter Meerlo
b
, Jaap M. Koolhaas
b
, Marike Lancel
a,c
a
Department of Forensic Psychiatry, Mental Health Services Drenthe, Assen, Netherlands
b
Department of Behavioral Physiology, University of Groningen, Haren, Netherlands
c
University Medical Center Groningen, Groningen, Netherlands
article info
Article history:
Received 18 August 2011
Received in revised form 21 October 2011
Accepted 15 December 2011
Available online 1 February 2012
Keywords:
Sleep
Sleep disturbances
Sleep deprivation
Aggression
Violence
Prefrontal cortex
Serotonin
Hypothalamic–pituitary–adrenal axis
abstract
Clinical observations suggest that sleep problems may be a causal factor in the development of reactive
aggression and violence. In this review we give an overview of existing literature on the relation between
poor sleep and aggression, irritability, and hostility. Correlational studies are supporting such a relation-
ship. Although limited in number, some studies suggest that treatment of sleep disturbances reduces
aggressiveness and problematic behavior. In line with this is the finding that sleep deprivation actually
increases aggressive behavior in animals and angriness, short-temperedness, and the outward expression
of aggressive impulses in humans. In most people poor sleep will not evoke actual physical aggression,
but certain individuals, such as forensic psychiatric patients, may be particularly vulnerable to the emo-
tional dysregulating effects of sleep disturbances. The relation between sleep problems and aggression
may be mediated by the negative effect of sleep loss on prefrontal cortical functioning. This most likely
contributes to loss of control over emotions, including loss of the regulation of aggressive impulses to
context-appropriate behavior. Other potential contributing mechanisms connecting sleep problems to
aggression and violence are most likely found within the central serotonergic and the hypothalamic–
pituitary–adrenal-axis. Individual variation within these neurobiological systems may be responsible
for amplified aggressive responses induced by sleep loss in certain individuals. It is of great importance
to identify the individuals at risk, since recognition and adequate treatment of their sleep problems may
reduce aggressive and violent incidents.
Ó2012 Elsevier B.V. All rights reserved.
1. Introduction
It is well known that sleep loss can have serious detrimental ef-
fects on cognitive performance. For example, sleep deprivation re-
duces sustained attention, executive functioning, and memory
[1,2]. Moreover, sleep loss may also affect emotional function [3],
and chronically disrupted sleep may even sensitize individuals to
mood disorders [4,5]. However, relatively little attention has been
paid to the effects of sleep loss on other areas of affective function-
ing. Yet, many people have the experience that disturbed sleep is
accompanied by emotional instability expressed, for instance, by
a greater irritability and short-temperedness. In most people this
will not result in physical outbursts of aggression. However, this
may be different in vulnerable individuals, such as psychiatric pa-
tients, who often experience serious sleep problems. For example,
about 80% of patients suffering from a depressive disorder experi-
ence sleep problems [6], and sleep disturbances are found in 30–
80% of schizophrenic patients [7]. Similar numbers may apply to
forensic psychiatric patients, but published data are lacking. Foren-
sic psychiatric hospitals treat patients who have committed (vio-
lent) offences but have diminished responsibility due to a mental
disorder. The most important treatment goal for this specific group
of patients is to reduce the risk of (violent) recidivism. Based on
clinical observations in forensic psychiatry, we have the impres-
sion that poor sleep may contribute to the loss of emotional con-
trol, including the regulation of aggression. If sleep problems are
a potential risk factor for hostile and (reactive) aggressive behavior,
treating sleep disturbances and promoting good sleep in mentally
disordered offenders may be beneficial in crime-preventing treat-
ment programs. Therefore, in this paper we present an overview
of existing literature concerning the relationship between sleep
and aggression/hostility.
2. Methods
A literature search was performed in PubMed and Ovid with the
following search terms: sleep, sleep disturbances, sleep problems,
sleep deprivation, sleep architecture, aggression and aggressive
behavior. The search terms were used in different combinations.
Cross-references were checked for relevant papers. We included
1389-9457/$ - see front matter Ó2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.sleep.2011.12.006
⇑
Corresponding author. Address: P.O. Box 30007, 9400 RA Assen, Netherlands.
Tel.: +31 592 334800; fax: +31 592 334402.
E-mail address: Jeanine.Kamphuis@ggzdrenthe.nl (J. Kamphuis).
Sleep Medicine 13 (2012) 327–334
Contents lists available at SciVerse ScienceDirect
Sleep Medicine
journal homepage: www.elsevier.com/locate/sleep
studies that covered sleep and its effects on aggression, anger, and
irritability.
With regards to the interpretation of relevant studies, it is impor-
tant to bear in mind that aggression is often defined and measured in
different ways, including self-report questionnaires, observed
behavior by family or experimenters, or endpoints measured in an
experimental task. These measures are not necessarily analogous.
In addition, in several studies it is not aggression that is measured
but anger, hostility, or irritability by means of mood questionnaires.
However, hostility, irritability, anger, and rage appear to play an
important role in violent behavior and individuals with a poor regu-
lation of these emotions are at greater risk for actual aggression. Fur-
thermore, aggressive acts do not occur as frequently as aggressive
feelings. Therefore, only studying aggressive behavioral actions re-
duces the ability to detect relationships.
We will first discuss studies investigating correlations between
sleep quality and aggression or angriness, separating for children/
adolescents and adults. Secondly, we will discuss studies per-
formed in aggressive individuals, including studies employing pol-
ysomnography. We will then review studies examining the effects
of treatment of sleep disorders on daytime problematic (aggres-
sive) behavior. Thereafter, we will focus on studies concerning
the influence of experimental sleep deprivation on emotional func-
tioning in animals as well as in human subjects. Finally, we will
theorize on potential physiological mechanisms for the putative
relation between sleep and aggression, such as poor prefrontal
functioning, serotonergic mechanisms, and Hypothalamic–Pitui-
tary–Adrenal (HPA)-axis mediation. We will end with speculations
on the possibility of individual vulnerability to the emotional ef-
fects of poor sleep.
3. Correlations between sleep and aggression
3.1. Childhood populations
In addition to the more obvious consequences – such as sleep-
iness, inattention, and poor cognitive performance and anxiety –
poor sleep in children and adolescents appears to be associated
with aggression and conduct problems [8,9]. In a large group of
2- to 3-year-old children the number of night time awakenings
was positively correlated with parent-rated aggressive behavior
[10]. Parent-reported sleep problems in 3- to 4-year-old twins cor-
related positively with conduct problems, anxiety, and hyperactiv-
ity [11]. In a sample of older children (mean age 7) suffering from
insomnia, parents were asked to rate daytime problematic behav-
ior with the Childhood Behavior Checklist (CBCL). Shorter total
sleep-time was associated with higher scores on delinquent behav-
ior [12]. A similar observation was made in a large group of 2- to 5-
year-old children in the US [13]. In contrast, in a large Canadian
population, those children who slept more than other children
were rated as more aggressive on a parent-rated scale [14]. How-
ever, the item used to assess sleep time asked the parents to in-
clude both night sleep and day sleep. More sleep during the day
could of course be a sign of inadequate night sleep quality, inde-
pendent of sleep duration. In fact, children who were assessed as
being overtired also had higher scores on aggressive behavior.
Comparable observations were made in adolescents: both total
sleep time and self-rated tiredness correlated positively with
self-rated aggression, also, after accounting for other risk factors
[15]. In a large 2- to 14-year-old population, those children at high
risk for sleep-disordered breathing-problems or periodic leg move-
ments during sleep (as measured by the Pediatric Sleep Question-
naire) had significantly more parent-reported aggression and
conduct problems [16]. In a large group of US schoolchildren symp-
toms of sleep-disordered breathing were significantly correlated
with parent- or teacher-rated conduct problems, bullying behavior,
and discipline referrals [17].
Interestingly, sleep problems at young age predicted, though
modestly, conduct behavior, anxiety, and hyperactivity several
years later [11]. In addition, parent-rated sleep problems in child-
hood are correlated with higher scores on an aggression behavior
scale later in life [18]. The authors state that sleep problems during
childhood may constitute risk indicators of behavioral difficulties
later in life.
Taken together, multiple studies in healthy and insomniac chil-
dren show correlations between inadequate sleep (in sleep dura-
tion as well as in sleep quality) and daytime problematic
behavior such as aggression and conduct problems.
3.2. Adult populations
In two groups of healthy young men, with one group habitually
sleeping 7–8 h per night and the other 9.5–10.5 h per night, the
group with the shorter sleep duration scored significantly higher
on anger and hostility on a mood scale [19]. Other studies failed
to show a significant positive correlation between sleep duration
and self-rated aggression, hostility, or angriness [20–22]. Yet,
Granö et al. found an association between shorter sleep duration
and higher hostility, which was partially related to psychiatric
problems [23]. In Israeli medical residents, work-induced sleep
deprivation and fragmentation, measured with actigraphy, ampli-
fied the negative emotional consequences of disruptive daytime
events while reducing the positive effect of goal-enhancing events
[24]. In a Polish study, the difference between desired sleep dura-
tion and actual self-reported sleep time correlated positively with
mood complaints (including irritability) in adolescents and stu-
dents, but not in young working adults [25]. To summarize, the
studies in adults correlating short sleep duration to daytime
aggression are not conclusive.
One complication in many of the studies that rely on indices of
sleep duration is that sleep quality is not taken into account. In fact,
sleep quality may be even more important than actual sleep time
with regard to the relationship between sleep problems and nega-
tive emotions, such as angriness and hostility. In a large group of
Finnish hospital employees a hostility scale was used to assess
proneness for angriness, irritability, and argumentativeness [23].
Sleep problems during the past four weeks were assessed with a
four-item scale. The authors found that sleep disturbances, sleep-
onset and sleep-maintenance insomnia, and unrefreshing sleep cor-
related independently with self-reported hostility. This relationship
remained after adjusting for several potential confounders, includ-
ing psychiatric disorders. A large adult Korean population was asked
whether they were suffering from sleep problems. In addition, trait-
anger was measured with the Spielberger Trait Anger Scale, which
measures the general disposition toward angry feelings (referred
to by the authors as temperament) and the tendency to express an-
ger (referred to as reaction). Difficulties maintaining sleep, excessive
daytime sleepiness, a bad condition in the morning after awakening,
and awakening late in the morning were all significantly associated
with trait-anger [22]. Unfortunately, the authors did not differenti-
ate between the angry temperament and angry reaction.
A small group of US students with insomnia complaints were
asked, using the Novaco Provocative Inventory [26], with how much
anger they would react to a number of potential provocative situa-
tions. As the authors expected, the insomniacs scored higher than
the controls. The subjects also underwent a stress elicitation test
with the simultaneous recording of electrophysiological measures:
sleep problems, such as difficulty falling asleep and overall dimin-
ished sleep quality, were correlated with higher skin conductance
and heart rate, both signs indicative of higher arousal. In two studies,
US college students were asked how often they experienced mood
328 J. Kamphuis et al. / Sleep Medicine 13 (2012) 327–334
changes during the last weeks using the Profile of Mood States
(POMS) [21,27]. The POMS is a questionnaire which assesses the
severity of active fluctuating mood states such as depression, ten-
sion, fatigue, confusion, vigor, and anger. Those students with poor
subjective sleep quality, as measured by the Pittsburgh Sleep Quality
Index (PSQI; used cut of >8), experienced significantly more anger
during the last weeks.
All together, poor sleep quality in both healthy and insomniac
adults indeed correlates with increased frequency and intensity
of self-reported anger, hostility, and aggression.
4. Aggressive individuals
4.1. Correlations
The correlation between disturbed or short sleep and disturbed
control of aggression could be even stronger in subjects with a high
anger-trait as part of their character. A condition in which aggres-
sion and angriness are core symptoms and can be considered part
of the subject’s character is antisocial personality disorder (ASPD).
Individuals with ASPD probably suffer from sleep problems compa-
rable to other psychiatric patients. In fact, in a group of Turkish
male military subjects diagnosed with ASPD, approximately 80%
scored >5 on the PSQI, indicating poor sleep quality [28]. As ex-
pected, these subjects had significantly higher levels of aggression
compared to the control group, but also more subjective sleep
complaints. Aggression was assessed with the Aggression Ques-
tionnaire, in which subjects have to judge how applicable state-
ments on aggression and angriness are to themselves, measuring
trait aggression. Ireland and Culpin studied an adolescent prison
population [29]. Since 50–80% of incarcerated offenders have diag-
nosable ASPD [30], it is reasonable to assume that a majority of
these prisoners met the criteria for ASPD. In the adolescent offend-
ers sleep disturbances and short sleep duration were associated
with increased hostility. Female victims of domestic violence re-
ported significantly more sleep problems in their abusive partners
as compared to controls [31]. Intriguingly, significantly more of the
abused women reported increased aggression from their partners
after a poor night’s sleep (58% vs 7% in controls) and half of them
remembered being battered on those days. The authors suggested
that sleep deprivation could be an important causal factor for
aggression in domestic violence. To our knowledge, there are no
studies on aggressive populations, such as ASPD patients or a pris-
on population, using objective measures of aggression or employ-
ing more provocative inventories to measure reactive aggression.
Also, no correlational studies were found in which objective mea-
sures of aggression were employed.
Taken together, aggressive populations seem to have serious
sleep problems which correlate positively with increased self-re-
ported aggression/hostility and partner-reported aggressive
outbursts.
4.2. Measuring sleep in aggressive individuals
Though limited in number, there are some studies suggesting
that polysomnographically recorded night sleep is different in
aggressive individuals. In healthy adults certain sleep variables
appeared to correlate with specific personality factors: aggressive
tension and impulsiveness, including a lack of affect control and
frustration tolerance and a need to dominate and criticize others,
correlate positively with the amount of superficial sleep during
the night [20]. No correlation was found with deep, slow wave
sleep (SWS). In another study, 19 male psychiatric patients with
ASPD who committed violent crimes exhibited significantly more
night time awakenings and, consequently, lower sleep efficiency
than control subjects [32]. These results remained after correc-
tion for alcoholism, sleep deprivation, and head injuries. On
the other hand, the ASPD patients also had more SWS, which
is supposed to be the most refreshing part of sleep; despite this,
they had a lower self-reported sleep quality. No differences in
rapid eye movement (REM) sleep parameters were found. A
higher amount of SWS was also observed in three female psychi-
atric patients with ASPD charged for violent crimes [33] and in
preadolescent boys with conduct disorder [34], a childhood psy-
chiatric disorder which may develop into ASPD after the age of
18 years.
Measurements of autonomic physiological functions suggest
that hyperarousal might mediate the relationship between sleep
problems and aggression. In a small study, male subjects with
so-called type A behavior – characterized by impatience, hostility,
aggression, and ambition – and high scores on a trait-anger-scale
were compared with males with the exact opposite, type B
behavior and low scores on a trait-anger-scale [35]. Compared to
the latter group, the angry type A scorers showed cardiovascular
hyper-reactivity even when asleep.
All together, sleep and physiological recordings suggest that
sleep patterns are different in aggressive individuals compared to
controls. The causes and implications of these observations remain
to be elucidated.
5. Effects of treatment of sleep problems
An interesting case report describes two boys (six and eight
years old), both admitted to a psychiatric unit for increasingly
aggressive and violent behavior towards peers and family mem-
bers [36]. During admission, they were diagnosed with obstructive
sleep apnea syndrome (OSAS), which in children is most often due
to enlarged tonsils. After adenotonsillectomy, both children had, as
expected, a significant reduction in their apneic episodes. More
importantly, prominent reductions both in the number and sever-
ity of violent outbursts were observed. After several months this
effect was still present and improvements had taken place in other
areas as well, including school performance and social interactions.
In unreferred schoolchildren nighttime breathing problems corre-
lated positively with aggression and oppositional behavior [37].
In addition, children with OSAS receiving adenotonsillectomy
showed long-term improvement in parent-reported aggression
[38,39]. Furthermore, a small sample of sex offenders suffering
from OSAS showed significantly lower scores on an aggression
questionnaire after OSAS treatment with continuous positive air-
way pressure (CPAP) [40].
Treatment of sleep disturbances other than OSAS may also lead
to a reduction of aggression. This is suggested by a study in adoles-
cents recently treated for substance abuse who also experienced
sleep problems [41]. These adolescents received weekly sessions
of behavioral sleep therapy over the course of six weeks. Adoles-
cents with greater improvements in total sleep time reported sig-
nificantly less aggressive ideations and fewer aggressive actions, as
compared to those with less change in total sleep time. A case re-
port on an aggressive and severely mentally retarded adult male
described that he showed increased aggression during demand
conditions, especially after having slept for less than 5 h [42]. After
implementation of a support plan, explicitly including rest periods,
his aggressive behavior diminished.
J. Kamphuis et al. /Sleep Medicine 13 (2012) 327–334 329
Taken together, although limited in number and not always
properly controlled, these studies suggest that treatment of sleep
problems can reduce daytime aggression and problematic
behavior.
6. Laboratory findings
6.1. Animal studies
Various experimental studies in rats suggest that sleep depriva-
tion may increase aggression: rats subjected to enforced wakeful-
ness by placing them together on top of a rotating drum
surrounded by water, died after 3–14 days, not directly from sleep
loss per se, but from fighting [43]. These rats became so hyper-
reactive that even slight physical contacts precipitated vicious,
aggressive behavior. This behavior was not always directed against
the actual offender, and occasionally several innocent rats would
become involved in a fight, while the original aggressor stood by
and watched. Webb kept six rats continuously awake by individu-
ally forced locomotion for 27 days [44]. After 16 days the animals
began to exhibit aggressive behavior, when they were paired in
an observation cage during 5 min. Treadmill-induced total and par-
tial sleep deprivation in rats increases exploratory behavior in an
open field test of anxiety [45]. According to the authors this can
be interpreted as a reduction in anxiety, loss of fear, and increase
in locomotor activity, but can possibly also be interpreted as irrita-
bility. Whether changes in aggression in these studies were a direct
result of sleep loss or, instead, physical activity or other confound-
ing factors involved in the procedures remains a question.
Several studies report increased aggressive behavior following
selective REM-sleep deprivation in rats [46–50] and mice [51].
Although these findings suggest that manipulative changes in sleep
patterns can produce aggression, the results are hard to interpret,
since only REM-sleep is deprived. In addition to this, the flower
pot method, by which REM-sleep is selectively deprived, is often
criticized because it induces high levels of stress.
The animal studies discussed so far differ in the type of aggressive
outcome measured (proactive vs reactive, fighting with conspecifics
vs rats that display mouse killing, pain-induced aggression, irritabil-
ity towards experimenters, etc.). It is important to realise that this in
itself is another source of variability, thereby further complicating
comparability and extrapolation to human aggressiveness.
Another view on the direction of the relationship between sleep
and aggression is offered by studies using a model of social stress
in which animals are placed in the territory of an older, bigger,
and more aggressive male animal and are attacked and defeated.
Immediately following defeat, rats [52,53] and mice [54,55] show
increased amounts of NREM sleep or increased NREM sleep inten-
sity as reflected in elevated slow wave activity in the electroen-
cephalography. This was not seen after a sexual interaction, a
non-aversive social interaction [55].
So, results of animal studies support a potential causal relation-
ship between sleep disturbances and aggressive discharges, but fu-
ture studies are needed to elucidate the exact effect and type of
behavior observed after total sleep deprivation.
6.2. Human studies
In 1964 a 17-year-old male in the US stayed awake for 264 h,
thereby setting the record for prolonged wakefulness. Psychiatric
and neurological consequences were investigated during the entire
sleep deprivation period. In addition to becoming intermittently
delusional and paranoid after several days, he became irritable
and uncooperative [56]. This course of symptom development
has been observed in more cases of long-lasting total sleep
deprivation in humans [57]. Understandably, most of these exper-
iments used shorter sleep deprivations. In fact, after one night of
sleep deprivation healthy young men already scored higher on
the aggression scale of a mood check list [58]. But, the subjects also
showed higher scores on the friendly subscale, which makes the
results difficult to interpret.
In a small British sample subjects who were sleep deprived for
30 h scored higher on the Profile of Mood subscales of depression
and fatigue, but not on the subscales anger and tension [59]. Another
study investigated the effect of chronic insufficient sleep on mood
[60]. Both the experimental and control group slept for 16 days in
the laboratory. The experimental group was allowed 4 h of sleep
per night and the control group was allowed 8 h. Four factors of
mood and physical symptoms, namely optimism-sociability, tired-
ness-fatigue, anger-aggression, and bodily discomfort, showed sig-
nificant effects. Anger-aggression was moderately but significantly
elevated in the 4-h sleep group compared to the 8-h sleep group.
Unfortunately, the authors do not discuss this finding.
In a US healthy adult sample, 56 h of total sleep deprivation in-
duced significant changes in psychopathological symptoms mea-
sured with the Personality Assessment Inventory [61]. Scores on
the antisocial subscale changed significantly in 8.3% of the sample.
Changes on other subscales were seen in more subjects, especially
paranoia, which represented a subtle elevation of interpersonal
mistrust and hostility toward others.
The studies discussed so far use self-report questionnaires to as-
sess the mood state of aggression and anger. To our knowledge
only two studies tried to measure aggression objectively. Kahn-
Greene et al. asked their 55-h sleep-deprived subjects to write a re-
sponse for a cartoon character that is confronted with a frustrating
situation [62], e.g., he gets splashed from a water puddle by a pass-
ing car. The responses after sleep deprivation showed a signifi-
cantly greater tendency to assign blame to others. Sleep
deprivation was also associated with increased outward expres-
sion of aggressive responses and a reduced willingness to take
the blame or offer amends to the other party. The authors suggest
that sleep deprivation weakens the inhibition of aggression. Con-
trary to this Vohs et al. did not measure higher aggression after
sleep deprivation [63]. Participants played an aggression-game
where level of noise blasted at the opponent (the computer) was
the measure of aggression. A part of the study group was also de-
prived of emotional regulation abilities, the so-called ego deple-
tion, by watching a disgusting video while they were not allowed
to show any emotion on their face. Ego-depleted subjects behaved
more aggressively in the game, but there was no additional effect
of sleep deprivation. Unfortunately, the authors only deprived
sleep for 24 h. Possibly, longer total sleep deprivation or a longer
period of partial sleep deprivation is needed in humans to elicit
the effect of a failing inhibition of aggressive urges.
6.3. Failing behavioral inhibition: sleep deprivation and impulsivity
The failing inhibition of aggressive urges possibly reflects en-
hanced impulsivity. There are different forms of impulsive behav-
ior, such as delay discounting, risk taking, and sensation seeking,
but there is also a lack of behavioral response inhibition. The latter
is, in our view, most important in discussing the relationship be-
tween sleep problems and aggressive behavior. Although the detri-
mental effects of sleep deprivation on decision-making [64] and
risk-taking [65,66] are quite clear, the effect of sleep deprivation
on behavioral response inhibition has not been studied exten-
sively. In healthy 24-h sleep-deprived subjects no effect was found
on a behavioral response inhibition task, namely the Stop Task
[67]. This task uses neutral signals, such as a tone or a letter on a
computer screen, to measure a response. Anderson and Platten
sleep deprived healthy subjects for 36 h and had them perform a
330 J. Kamphuis et al. / Sleep Medicine 13 (2012) 327–334
behavioral inhibition task using positive and negative emotional
stimuli in addition to neutral stimuli [68]. They also did not find
a significant difference in ability to suppress the response to neu-
tral stimuli after sleep deprivation. But, interestingly and more
importantly, sleep-deprived subjects were less able to inhibit their
response to negative emotional stimuli.
Speculatively, loss of behavioral inhibition to negative emo-
tional circumstances mediates the relationship between sleep
deprivation and unwanted or context-inappropriate aggressive
responses.
7. Potential mechanisms
7.1. Prefrontal cortical functioning
One hypothesis on the relationship between poor sleep and
aggression is that sleep deprivation results in poor prefrontal cor-
tical (PFC) functioning. When PFC functioning is reduced, the abil-
ity to anticipate, delay, and initiate behavioral responses based on
cognitive and social context declines [69]. That sleep deprivation
affects the PFC is supported by the finding that sleep deprivation
as short as 24 h leads to significant declines in PFC metabolic activ-
ity [70]. In addition, after sleep deprivation, behavior comparable
to PFC neuropsychological anomalies is observed, which is re-
versed after recovery sleep [69]. Among these anomalies are the
weakening or diminution of goal directed behaviors and instability
in emotional responses [71]. The instability in emotional responses
is reflected by a study showing that healthy volunteers had de-
creased emotional intelligence after 50 h of sleep deprivation
[72]. Emotional intelligence includes the ability to understand
one’s own and others’ emotions, effectiveness in dealing with
interpersonal relationships, and capacity to cope with environ-
mental needs [62], and can be considered a reflection of the level
of PFC functioning. In a group of US healthy subjects, 30 h of sleep
deprivation impaired accurate recognition of human facial emo-
tions [73]. Extreme emotional faces were still correctly recognized,
but the ability to recognize moderately angry and happy faces was
reduced after sleep deprivation. The blunted recognition could re-
flect the susceptibility of the prefrontal lobe to sleep deprivation. A
specific region of the prefrontal lobe, the anterior cingulate cortex,
is implicated in the emotions anger and happiness [73,74].
PFC dysfunctioning is also associated with the loss of inhibition
of context-inappropriate responses, which is also seen after sleep
deprivation [71]. A potential mechanism for this loss of inhibition
and these changed emotional reactions after sleep deprivation is
provided by an fMRI study of Yoo et al. [75]. During scanning, sub-
jects performed an emotional viewing task: 35-h sleep deprivation
significantly weakened the connectivity between the amygdala
and the medial PFC and elevated amygdala activation in response
to negative pictures. The authors suggest that, as the medial PFC
is proposed to exert top-down control of amygdala activity, result-
ing in context-appropriate emotional responses, sleep deprivation
leads to a failure of top-down prefrontal control [75].
Although aggression may in some situations be considered
appropriate and meaningful behavior, most often it is not compat-
ible with the rules in modern society. Raine and Yang hypothesize
that rule-breaking behavior is in part due to impairments in brain
structures belonging to the PFC [76]. In this regard the PFC can be
considered as the cortical region where context-inappropriate
aggression is kept under control. Speculating further, when PFC
control is lost or diminished as a consequence of sleep deprivation,
this could potentially lead to uncontrolled, impulsive aggressive
responses (primitive reactions).
7.2. Serotonin
Evidence from both animal [77] and human [78,79] studies
strongly support an important role for serotonin in aggressive
behavior. Emphasizing the importance of serotonin is the finding
that selective serotonin reuptake inhibitors decrease irritability,
angriness, impulsivity, and assault in violent offenders [80]. An eti-
ological hypothesis states that lower brain serotonin (5-HT) neuro-
transmission in the orbitofrontal cortex contributes to reduced
cortical top-down control, which is implicated in the development
of aggression and violence [81]. The serotonin deficiency hypothe-
sis of aggression is currently debated [77]. Booij et al. showed that
male adult individuals with a history of childhood physical aggres-
sion had currently low 5-HT brain synthesis, measured with posi-
tron emission tomography, but, despite this, no differences in
measures of aggression, emotional intelligence, or impulsivity
compared to control subjects [82]. The authors suggest that low
5-HT does not mediate current behavior, but should be considered
a vulnerability factor for impulsive-aggressive behavior that may
or may not be expressed depending on other variables. For exam-
ple, trait aggression has been shown to be significantly higher in
men with decreased 5-HT availability in combination with high
testosterone levels, but also in men with increased 5-HT availabil-
ity and low testosterone, reflecting the interaction of 5-HT func-
tioning with other variables, in this case testosterone [83]. Thus,
although serotonin seems to have an important role in aggression,
it is almost certainly not the only causal factor for the development
of aggression.
Sleep and wakefulness are associated with clear changes in
serotonergic activity [84]. Particularly, wakefulness and sleep
deprivation are associated with higher serotonergic activity than
during sleep, as established by numerous microdialysis studies
[85]. Total sleep deprivation in rats seems to increase 5-HT turn-
over in the frontal cortex, hippocampus, hypothalamus, and brain
stem [86]. Whether this is also true in humans remains to be elu-
cidated. Moreover, experimental studies in rats have shown that
chronically restricted and disrupted sleep leads to gradual changes
in serotonin-receptor sensitivity [87,88]. To our knowledge, there
are no studies directly investigating the relationship between sleep
problems, serotonergic dysfunctioning, and aggression. However,
indirect evidence supporting such a relationship comes from stud-
ies on suicidal behavior. Both sleep loss and decreased serotonergic
activity are associated with suicide and suicidal behavior [89–91].
The association between low cerebrospinal fluid 5-HIAA (the prin-
cipal metabolite of 5-HT) and suicide is most prominent in or may
even be confined to patients who use particularly violent methods
in their suicide attempts [89]. In addition, lifetime externally direc-
ted aggression is more frequent in suicide attempters, and criminal
offenders also have increased suicidal behavior. Singareddy and
Balon suggest that decreased serotonergic function in suicidal pa-
tients and criminal offenders may predispose them to reduced
inhibitive capabilities or to increased impulsive behavior [89]. This
can be self-directed (suicidal behavior) or externally directed
(impulsive and aggressive behavior toward property or other
persons).
In conclusion, both preclinical and clinical evidence supports
the view that serotonin may be an important causal or modulatory
link between sleep problems and aggression.
7.3. The Hypothalamic–Pituitary–Adrenal (HPA) axis system
HPA axis dysfunctioning contributes to aggressive behavior in
antisocial and conduct-disordered individuals [92]. However, the
generally accepted hypothesis is that this is explained by the
J. Kamphuis et al. /Sleep Medicine 13 (2012) 327–334 331
hypo-arousal driven aggression model. This is not compatible with
the hyperarousal seen in insomniacs: Waters et al. observed that
insomniac subjects exhibited signs indicative of hyperarousal,
namely higher skin conductance and heart rate, and also scored
higher on an anger scale compared to non-insomniac controls
[26]. Animal studies focusing on the involvement of the HPA sys-
tem in aggressive behavior produce contradicting results [93].
Sleep, particularly deep sleep, has an inhibitory influence on the
HPA axis, whereas HPA axis activation produces arousal and sleep-
lessness [5,94,95]. Insomnia is associated with higher levels of
ACTH and cortisol, especially during the evening and first part of
the night, suggesting central nervous system hyperarousal [95].A
rat study which showed increased exploration in an open field test
of anxiety after sleep deprivation – possibly reflecting irritability –
did not reveal differences in plasma corticosterone levels [45]. This
suggests that sleep deprivation-induced explorative behavior is not
mediated by HPA activation.
In conclusion, sleep disturbance may affect the HPA axis, but
whether this contributes to the relation between sleep problems and
aggression remains to be elucidated. To our knowledge, no study exists
specifically addressing the role of the HPA-system in this relation.
8. Individual vulnerability
It is possible that certain individuals are more susceptible to the
emotional consequences of poor sleep. For example, individual dif-
ferences in emotional intelligence predict the influence of sleep
deprivation on written responses to cartoons displaying frustrating
situations [62]. This could indicate that individuals with low emo-
tional intelligence are more vulnerable to the negative effects of
sleep deprivation on mental functioning. Speculatively, individuals
with poor prefrontal functioning, such as certain psychiatric pa-
tients, are potentially more vulnerable to the effects of sleep depri-
vation. This may imply that individuals with an aggressive trait,
such as many forensic psychiatric patients, may represent a group
with poor prefrontal inhibition of aggressive impulses. In fact,
researchers have found lower volumes of blood flow in the frontal
lobes of people with various kinds of impulse-control disorders
[96]. A meta-analysis of brain-imaging studies confirmed prefron-
tal structural and functional impairments in antisocial populations
[97]. Possibly, poor sleep exacerbates the loss of their already low
impulse control, leading to verbal or even physical acting out of
aggressive impulses. This makes the relationship between sleep
and aggression of special importance in these subgroups.
In addition to individual differences in prefrontal cortical func-
tioning, interindividual differences in serotonergic function may
also contribute to the variability in effects of sleep loss on aggres-
sion. Variability in central serotonin function is associated with indi-
vidual differences in effect, temperament, and personality: for
example, high-aggressive male rats respond in a more sensitive
way to a 5-HT
1A
-receptor agonist compared to low-aggressive ani-
mals [93]. As discussed earlier, low central 5-HT may be a vulnera-
bility factor for impulsive-aggressive behavior [82]. Sleep
deprivation most certainly has an effect on serotonergic activity in
the brain, but to what degree this effect differs between individuals
is not clear. Thus, although we know that individual variation in the
serotonergic system is associated with differences in aggressive-
ness, the potential role of sleep in this relation remains to be
elucidated.
Similar speculations can be made for individual variation in the
HPA axis. Differences in behavioral coping style between high and
low aggressive mice have been associated to differences in the HPA
stress system [98]. Overall, the general picture of the relationship
between aspects of variation in coping style and HPA-axis activity
is rather complicated and results are not conclusive [93]; yet, the
possibility that certain variations in the HPA-axis make individuals
vulnerable to the effect of sleep problems on behavioral problems
cannot be excluded.
9. Conclusions
Clinical and anecdotal observations in forensic psychiatric pa-
tients suggest that sleep loss is a potential risk factor for impulsive,
reactive aggression. The larger part of the reviewed literature con-
cerning the relationship between poor sleep and emotional regula-
tion supports this hypothesis. Yet, most studies measured
correlations and, thus, did not provide information on causality:
poor sleep may affect aggressiveness or aggressiveness may produce
sleep problems. Another option is that both sleep problems and
aggressive tensions/behavior are due to a shared risk factor, such
as a psychiatric disorder. However, the limited number of studies
showing that adequate treatment of sleep problems reduces day-
time aggression support the hypothesis that sleep problems are, in
fact, a risk factor for aggression. Unfortunately, relevant animal re-
search on sleep deprivation and aggression is largely confined to
the influence of REM-sleep deprivation. Moreover, many of these
studies have not taken into account various confounding factors
associated with the sleep deprivation procedures. Sleep deprivation
in humans appears to exert profound effects on mood, including in-
creased irritability and altered reactions to frustrations. Whether or
not these effects actually result in physically acting out aggressive
urges is still obscure.
Sleep problems may impair prefrontal cortical functioning,
thereby weakening the top-down inhibition of aggressive impulses.
This offers a neurobiological mechanism explaining how sleep prob-
lems may be a risk factor for aggression and violent behavior. Possi-
bly, some individuals may be more vulnerable to the emotional
consequences of poor sleep. If so, it is highly important to identify
these individuals, because promoting good sleep in these people
may reduce the frequency or severity of aggressive outbursts. People
known to be aggressive, such as violent delinquents and some ASPD
patients, are most likely to belong to this risk group.
In view of the fact that most existing literature supports an
association between sleep loss and aggression, it seems worth-
while to investigate in more detail the impact of sleep problems
and experimental sleep deprivation on aggression and hostility.
Studies exploring individual vulnerability for the negative conse-
quences of poor sleep on the regulation of emotions are also of par-
ticular interest. Moreover, there is a need for studies elucidating
the neurobiological mechanisms that may mediate the influence
of poor sleep on aggressive feelings and behavior. Speculatively,
when evidence in support of the relationship between sleep and
aggression is substantiated, it is of utmost importance to pay atten-
tion to sleep complaints and correctly diagnose and adequately
treat sleep disturbances in individuals at risk. The treatment of
sleep problems may be a valuable, innovative element in aggres-
sion-reducing and crime-preventing treatment programmes.
Conflict of Interest
The ICMJE Uniform Disclosure Form for Potential Conflicts of
Interest associated with this article can be viewed by clicking on
the following link: doi:10.1016/j.sleep.2011.12.006.
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