It's understandable to have jetlag for a few days after you come back from a trip in a different time zone. Your body will take a few days to adjust -- when you normally would have lunch, you're thinking about hitting the hay for the night. But what if you are used to a certain work schedule (because you've worked it for a few years) and it takes months (even years) to adjust body's clock?
A few weeks ago, Matt took a promotion at work and started working a shift that was the exact opposite of what he worked before. Instead of waking up at 3am to be at work at 4:30, he goes in at noon and doesn't come home until 8:30pm. Sleeping in until 9 or 10 doesn't work for him. He still wakes up a few times around his previous wake-up time before deciding to get up for the day.
I remember how it took my body more than a year to get used to having two days off in a row. I had worked six or seven days straight for a couple of years before working the schedule I've worked for the past three years. The tired feeling would kick in when I wasn't wanting to feel tired.
In the industry Matt and I have worked, there are many reasons why people like the schedule. I find myself getting more out of the day by working in the mornings and taking a nap in the afternoon. The way I see it, no matter how late I get up on the weekends, my body still wants to take a nap sometime in the afternoon. Why not get more done in the mornings than sleep most of the morning away?