Not long ago on a Monday, a colleague said to a group of his graduate students something like "I was thinking about this [research project] over the weekend, and had an idea about that problem we were discussing last week." A first year PhD student said, amazed "You work on weekends?".
Aside from the issue of whether it was wise of the PhD student to admit to his advisor that he doesn't even think about his research on weekends (the advisor's interpretation of the question, owing to the emphasis on the word work) or to imply that he is surprised that his advisor thinks about research on weekends (another possible interpretation, if the word you was the intended emphasis): Is thinking working?
Of course thinking is an important part of research, and research is our work, so thinking is working in that respect. I could be very wrong, but I think most people who work by choice in a research environment think about their research on weekends. Even if I spent an entire weekend (or week) doing nothing but recreational activities with my family and cats, it is not possible for me to not think about my research at all.
That doesn't mean I don't know how to 'leave work at the office' in some respects. I have no interest in discussing office politics at the dinner table, for example, and I certainly don't spend every waking hour thinking about work. But I can't imagine not thinking at all about my work (research, teaching, some of the more interesting aspects of professional service) outside of normal working hours, and I can't imagine not wanting to think about these things (i.e. I can't imagine wanting to not think about work).
If you're curious about something and are trying to figure something out, you think about it. That doesn't (necessarily) mean that you are an obsessed monomaniac workaholic, nor is your only other option to be a work-brain-turned-off-when-not-at-work person. I think that being so interested in your work that you want to think about it even when you don't 'have' to is simply a characteristic of someone who enjoys their work.
My response to the amazed student's question would have been similar to my colleague's: Even if I wanted to stop thinking about my work, I couldn't. And even if I could stop, I wouldn't want to.
Someone can get a PhD in Science without working (and thinking about) research 24/7, and you don't even have to feel that your PhD research is absolutely the most fascinating thing in the universe, but I would hope that there would be something about the subject that was interesting enough to think about now and then on the weekend.
3 days ago