Image caption The study could go some way to explaining the performances of people who work shifts

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.

Perception

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."

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."