Passenger fatigue

I would really like to know why a day spent sitting in a car as a passenger makes a person so much more tired at the end of the day than a day spent actually working at manual labor. After a nine hour car ride, we had a few brief moments to go to our room to freshen up before tonight's conference meetings started. That bed looked mighty tempting. I was hoping to have some spare moments to spend with my nephew and niece who live in Lubbock, but spare moments are at a premium. Tomorrow's schedule starts at 7AM and lasts until around 4PM when we will be off in the car again to go to Nazareth, TX for a dinner meeting. An overnight stay in a B&B there and we will be off again the next morning headed to Rocky Ford, CO. Thursday night we have a Board Meeting for High Plains Food Coop and Friday morning a producer workshop is scheduled in Rocky Ford. I will be back home again Friday night. I don't think I'll be leaving the farm for a while after that! Home Sweet Home.