A new study finds that vulnerability to sleep deprivation is influenced by the interaction between waking social activity and individual personality traits. Results show that extroverts who were exposed to 12 hours of social interaction were more vulnerable to subsequent sleep deprivation than those who were exposed to an identical period of isolated activity. Speed on the Psychomotor Vigilance Task (PVT) for extroverts in the socially enriched group was significantly slower at 4 a.m., 6 a.m. and noon compared with speed for extroverts in the socially impoverished condition. Introverts' speed on the PVT was relatively unaffected by prior social exposure.