A study has revealed that outgoing people are affected the most by lack of sleep.
Researchers from the Walter Reed Army Institute in Maryland found that extroverts denied social contact did better at staying awake and in reaction tests, 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, divided into natural extroverts and introverts.
Results revealed that 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.
"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 BBC quoted the researchers as reporting.
One possibility, they said, was that intense social interactions might lead to fatigue in brain regions, which also played a role in alertness. Conversely, 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.