My SO and I are both introverts. My SO has mostly extrovert friends, and my sister is very extrovert as well.

We've been talking about how it appears that extroverts lack some internal feedback system or something, causing them to seek constant external stimuli. Whereas with us, we've got more than enough with our own internal stimuli.

In the study, the test period was a mere 15 minutes. To me that's an exceptionally short time to be alone, and not a problem at all.