Sleep loss 'worse for extroverts'

Outgoing people appear to suffer worse from the effects of lack of sleep, US army researchers suggest.

They kept 48 volunteers awake for 36 hours, with some allowed to mix with others, the journal Sleep reports.

Those defined as introverts did better at staying awake and in reaction tests.

And those extroverts who were denied social contact also did well, suggesting it is "social stimulation" that tires out the parts of the extroverts' brains linked to alertness.

The study involved 48 people aged 18 to 39, who were divided into two roughly equal groups following personality type screening, which defined whether they were natural extroverts or introverts.

After a good night's sleep, they remained awake for a day and a half, with various tests each hour to measure the effects of lack of sleep.

Some of the test subjects - both introverts and extroverts - were allowed to take part in group discussions, and play board games and puzzles for 12 hours of the 36. The others were not allowed any such social interaction.

First of all, the test subjects who were "socially enriched" in this way were tested to see if there was any difference between the natural extroverts and introverts.

While there was little difference in one of the tests, in which volunteers had to push a button as soon as possible in response to a light, introverts fared better in a "maintenance of wakefulness test", which checks whether sleep-deprived people are able to stay awake over a set period of time.

The extroverts in that group did badly in the test, but the extroverts in the second group - those denied social contact - performed markedly better.

The researchers, from the Walter Reed Army Institute in Maryland, said the results suggested that personality type might not only have a bearing on ability to cope with military tasks which required being awake for long period, but also with shift work.

They reported: "Overall, the present results might also be interpreted more generally to suggest that waking experiences, along with their interaction with individual characteristics, influence vulnerability to subsequent sleep loss."
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“The extrovert is more likely to be influenced by a perception of what is going on in the group”

Professor Mark Blagrove, University of Swansea

One possibility, they said, was that intense social interactions might lead to fatigue in brain regions which also played a role in alertness.

Conversely, they said, it was possible that introverts might always have a relatively high level of activity in parts of the brain affected by social situations.

On a day-to-day basis, it is suggested this could mean that social contact leads to "over-stimulation", explaining why introverts would withdraw or shy away.

However, the constant activity might also make their brains better placed to fight the effects of sleep deprivation, they said.

One UK academic said that there might be a simpler explanation for the different impact of sleep deprivation.

Professor Mark Blagrove, a neuroscientist from the University of Swansea, has published similar research into effects of sleep deprivation on the mood of introverts and extroverts.

Again, he found extroverts more vulnerable to mood changes driven by lack of sleep.

He said: "We suggested that extrovert people might be more heavily influenced by the sleep-deprived appearance of people in the group around them.

"They found no differences in the objective test of alertness they used, but did find differences in the wakefulness test, which is a slightly less objective measure of how someone is feeling.

"This supports a slightly simpler argument - that the extrovert is more likely to be influenced by a perception of what is going on in the group."

Sleep Loss Hits Extroverts Harder
By Charles Bankhead

Explain to interested patients that extroverts seem to be more affected by sleep deprivation than do introverts.

Sleep deprivation takes a heavier toll on the performance and alertness of people who are extroverts than it does on their introverted counterparts, according to results of a randomized clinical study.

Extroverts had lower scores on tests of alertness and wakefulness during 36 consecutive hours awake, including a 12-hour period of social interaction, Tracy L. Rupp, PhD, of Walter Reed Army Institute of Research in Silver Spring, Md., and co-authors reported in the November issue of Sleep.

In contrast, 36 hours of sleep deprivation that included no social interaction had minimal effect on performance or alertness of extroverts or introverts.

The findings provide insights into interindividual differences in vulnerability to sleep deprivation, particularly interaction between personality traits and social conditions. However, the effect of the interaction was the opposite of what the investigators had hypothesized.

"Although the hypothesized main effects of social condition and personality were not confirmed, the significant interaction between these two factors tends to confirm our primary hypothesis that social experience does significantly affect the ability to resist subsequent sleep deprivation," they wrote in the discussion of their findings.

"But this effect is mediated by individual differences in introversion-extroversion, a personality trait that is believed to reflect general cerebral arousal level."

The findings reflect the investigators' ongoing investigation of Eysenck's theory, which revolves around the concept of cortical arousal as a determinant of introverted or extroverted personality.

According to the theory, social gregariousness and sensation-seeking behaviors arise, at least in part, from lower levels of tonic arousal. Because of their presumed lower level of cortical arousal, extroverts seek out social contact and stimulation to increase brain arousal to optimum levels, the authors noted.

In contrast, introverts are thought to have relatively higher levels of cortical arousal and avoid socially active environments that would lead to more cortical arousal. Consistent with the theory, Rupp and colleagues previously reported that higher scores on a test of introversion were associated with greater resistance to sleep deprivation (J Sleep Res 2007; 16: 354-363).

To continue their investigation of the theory, the authors recruited 48 volunteers for a study of the interaction among personality traits, social exposures, and vulnerability to sleep deprivation. All participants completed a personality inventory assessment that led to categorization of 23 participants as extroverts and 25 as introverts.

The participants were randomly assigned to one of two social experiences, which followed eight hours in bed and two hours to eat breakfast and get ready for the evaluation.

Participants assigned to the socially enriched condition could watch television, play games, read, or eat, and remained with laboratory technicians, who were instructed to keep the participants engaged socially throughout the exposure.

Participants assigned to the socially impoverished condition had access to the same activities as in the socially enriched condition, but had no interaction with each other or with technicians.

Both social conditions lasted 12 hours (10 a.m. to 10 p.m.), and was followed by 22 hours of sleep deprivation, during which time participants were tested hourly for alertness and performance. Technicians monitored participants continuously to ensure that participants did not fall asleep. All told, participants remained awake for 36 consecutive hours.

During sleep deprivation, scores for speed on the Psychomotor Vigilance Test (PVT) deteriorated in all groups but was more pronounced in extroverts assigned to the socially enriched exposure compared with extroverts assigned to the socially impoverished condition at 4 a.m., 6 a.m., and noon.

The socially impoverished condition had minimal impact on test performance or subjective sleepiness in any of the groups.

"The ability of introverts to resist sleep loss, on the other hand, was relatively unaffected by the social environment," the authors noted.