My first full-time job was as the overnight cashier at a 24-hour grocery store. 10pm to 6am. At first it was kind of cool. We'd get a lot of regulars in that I got to know pretty well, and apart from the rush we'd have early on in my shift and then between 2 and 3 when the bars let out, it was a pretty cushy job. Lots of down time - and as I can't stand just sitting around doing nothing, I was able to help out the stockers most of the night too.
Eventually I moved to a full-time stocking position, and over time ended up as the Night Crew Leader for a couple of stores. Partly it's because that's just a really lowsy job that's pretty much guaranteed to be too much responsibility for the pay grade; and I was always expected to accomplish much more than I had the manpower to realistically make do with.
But after half a dozen years working those hours, it does begin to takes it's toll. After I left that job, I swore I'd never work a shift like that again.
I had no problem in them early years either, but it may have something to do with the fact I was 18-21 as well. It became more difficult over the years and even harder when you get a partner.
Yeah, when I was younger, it wasn't any big deal to be up all night long, and then either sleep until 2 in the afternoon, or even stay up until lunchtime and then sleep in the afternoon. (I preferred staying up after work - I've always preferred being able to wake up and go to work, and then have the rest of the "day" open after I got out.) Over time it did really begin to wear me down, though.
For me - a lot of it was just how much it complicated things. Banks hours always seemed to exactly match my sleep schedule, and most stores weren't open at the times when I actually felt like going out for the day. And especially when I started dating my wife - it wasn't easy getting our schedules to synch up so that we could actually spend time together.
Plus, I remember having a lot of trouble getting friends and family to understand why I wasn't always available at the times they wanted. The absolute worst for me were the times I got talked into going on camping trips. When it gets dark out and everyone is going to sleep at the time you're usually waking up - it can get kind of boring...