Been on day shift for nearly 3 years now, which is the longest I've gone shift-work wise. Just in the last month or so though I've been having trouble staying up in the 1500 to 2000 range, no matter whether I got decent sleep the day before or not. Then around 8PM I get a second wind and I'm up til 1AM or so. It's like I'm sleepy from 3 to 8 but then after 8 I'm tired after that, but not sleepy at all...if that makes sense. Have tried forcing myself down earlier but just end up laying there til I get pissed off and just get back up for a while. Waking up at 0630 dragging ass is getting old, and I like my 8 hours

Yeah

Power through that sleepy time (no caffeine), then go to sleep.  You'll sleep like a baby and get reset.

Personally I think this can also be a matter of whatever you train your body to do.  I used to stay up all night long, and/or work at all hours of the day and night.  After awhile I was able to deliberately fall asleep, anytime, anywhere, and in any position - Even while I was standing on my feet!  

On occasion I watched some really good men get into trouble by allowing themselves to fall asleep when they shouldn't have.  The way I avoided getting myself into the same kind of trouble is by knowing when to take a break, and catnapping whenever time and circumstance allowed.  

For whatever it's worth, meditation (or prayer) and other physical relaxation techniques can work to help you overcome sleep anomalies.  Watch your diet and food intake, as well.  Believe it, or not, caffeine in any form (either hot or cold) can be a real killer!  First it'll pick you up; and, then, if you don't get another, 'fix' it'll just drop you on your head.  (Which is, 'Why' I always tended to drink a lot of water.)  

That's why you should always use condoms.

I forcefully change mine to work hours each spring, and it can be a pain.

Naturally my body tends to want to stay up until midnight or so, then sleep until about 8, but for work I need to be up as early as 4am fairly often. After a few weeks I can naturally wake up around 530 on my own, but within a week of winter time I am right back to staying up late again.

I work swing shift, do it all the time.

Here's my schedule, 12-hour shift from 5:15 to 5:15.
Monday Tuesday, day shift
Wednesday Thursday, off
Friday Saturday Sunday, night shift

Monday Tuesday, off
Wednesday Thursday , day shift
Friday Saturday Sunday, off

Monday Tuesday, night shift
Wednesday Thursday, off
Friday Saturday Sunday, day shift

Rinse repeat, rinse repeat...  Been doing it 17 years, never REALLY get used to it.
Throw a substantial amount of overtime on top of that schedule, and you NEVER catch up.

Diabetes.

Don't think I am kidding, it might be.  Has to do with your blood sugar at the very least.  I get the same thing and the DR threatened me that I have pre-diabetes and need to cut out the sugar.

old age