Third shift (also known as graveyard shift), is the work shift that runs overnight into the morning. The exact hours will vary, but in general a third shift worker will go into work in the very late evening, and get off sometime around sunrise.
I work third shift myself (and have for years). It changes your life in so many ways. I sit behind my quiet desk at work, answering the occasional phone call, and dealing with the few customers who may show up in my hotel in the middle of the night (not many, since I work at a rather fancy hotel). Most of that time I am alone. I have found that my eyes have become much less accustomed to changes in light and dark. You would probably think they would be more used to the dark, right? Wrong. I am rarely ever in darkness at all. I have to sleep during the daytime, and I spend my nights under artificial light. This has made me very sensitive to bright lights, and low levels of light (I am no longer used to either one).
After working third shift for a long period of time you will begin to sleep less and less. I myself am down to about four hours a day. My reasoning for why goes something like this. If you wake up at 4 am, you will go back to sleep right, of course you will. But I sleep during the daytime. If I wake up at noon (which I often do), I think to myself, "Wow, I still have almost the entire day left". This may not be the true culprit, but many people do seem to develop reduced sleep patterns when they work on the third shift.
The third shift is best for people who are single, since you will not be coming home to sleep in your lovers bed every night, and even when you can, you will find yourself unable to sleep (because you should be awake, and you have gotten used to sleeping alone). On the other side of the same coin, the third shift is great for people who want to have active social lives. You can literally party all night long on your days off, with out disrupting your sleep pattern, or even getting tired. You can't stay out as late on your work days, but it sure beats the social life killing 3PM-11PM shift (also known as second shift).
Many people eventually tire of working this shift, and seek employment during regular hours. Actually, more like most people tire of this shift, as third shift almost always has the highest turnover at any place of business (even though it often pays more). But it is difficult to search for a new job while working a third shift job, after all, you have to look for jobs during the day (which is when you should be sleeping). This sometimes makes it difficult to get out of the third shift lifestyle (at least it has been hard for me).