When I was in graduate school, I had a law student as a neighbor for a while. We were both pretty quiet, and got along fine in our respective dinky efficiency apartments, but we couldn't help but notice some differences between our studies. The biggest one became clear around this time of the year: he left, and I stayed. I still remember the look of surprise on his face when I told him that we didn't have any time off.
Well, I know that law students don't generally go off and laze around on the ol' hammock during the summer, but they at least get to go somewhere else for a while. But grad students just keep banging away, and if they're in the sciences, they're up there nights, weekends, and holidays.
Ah, those holidays. I still have in my files a memo from my old chemistry department, reminding everyone that the university (undergraduate) vacation schedule most definitely did not apply to us. Do not attempt to take these holidays was the very pointed message, because we will notice if you do. I sure didn't. I did take off some time at Thanksgiving and Christmas, and I didn't work every single July 4th, but otherwise it was a rare, rare day when I wasn't in the lab.
I've written before about my physical surroundings during that time, but things like this memo didn't add to the festive atmosphere very much, either. On the university level, it became clear pretty quickly that we weren't students, and we weren't staff - well, not all the time, anyway. We were whichever caused the least expense and inconvenience to the school at the time you asked the question.
But the biggest factor was the work. It was a strain. I like variety up here in my head, and this was the first time I'd ever had to do the same thing, think about the same thing, day after day (and night after night). It brought on, eventually, the mental equivalent of a leg cramp - I know for sure that I was in a much crabbier mood during my grad school years than I was afterwards, and I'm sure that it was largely because I was venting off some of the pressure. My project had the usual twists and turns, which during one point just about had me tearing my hair in frustration, but the real problem was that there was no escape from it.
Every hour I spend doing something else, besides necessities like eating and laundry, I found myself thinking about how I'd just added another hour to my graduate studies. When I could have, you know, been back in the lab trying to find a way out of the place. That is to say, doing something useful with my time. In the end, anything that didn't directly involve getting out was classified as a luxury, and I tried to ration such things. I remember going past a TV set at one point that had a golf tournament on, and I found myself amazed at these people - not the golfers, the spectators, these people who felt free enough to just wander off and spend the whole day in a sunny park watching a sporting event, without having to worry the whole time about what it was taking them away from.
All of which is why I reiterate my advice to my grad-school readers, who may be watching the summer weather out the windows of their labs (if they have windows, that is): get out. As far as it's compatible with taking a reasonably honorable and complete degree and not leaving any bad blood behind you, get out. The whole point of graduate school is proving that you can make it out of there.