Differences

According to Meyers, introverts actually require a certain amount of alone time in order to go about their normal day. Its a need to reflect, perhaps contemplate.

Extraverts on the other hand often gain energy by being in the company of others. Often times they find alone time to be hard.

Yahoo: Brains of Lonely People Work Differently

The ventral striatum - a region of the brain known from other research to light up in association with rewards such as food and money - was much less activated in the lonely group.

By 2010, 31 million Americans - roughly 10 percent of the population - will live alone...

...those who scored highest on measures of loneliness also had higher blood pressure, a major risk factor for heart disease.