I am most definitely an introvert. I have a fairly demanding job with long hours and some travel mixed in. At work, I participate in conversations, give presentations and lead a group, so I feel as though I am always "on." Probably because I am much more social than I would be inclined to be naturally. Therefore, it is important that I have down time.
When in the office, I have a 35-40 minute commute each way, unless traffic is horrible (I go to work later and stay late on days I go into the office to avoid the worst traffic) my drive time to listen to the radio and some 80's music serves as my down time and some "thinking" time. We do not have children, so I do not have activities or social events related to that. However, when my husband is in town, we always have dinner at home together. After dinner and before bed, I need some time to read on the computer or a book. This is very important to me. Between waking up and going to the gym in the morning, I get up at 4:15 just so I can sit with a cup of coffee and read or talk to my husband before the day begins. When I do not get this "relax time" at either end of my day for an extended period of time, it really does impact me.