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If you have a family, you will have to balance your priorities even more carefully. Graduate school isn't worth risking your personal relationships over; be sure that you save time and energy to focus on the people who matter to you.
One of the keys to balancing your life is to develop a schedule that's more or less consistent. You may decide that you will only work during the days, and that evenings are for your hobbies. Or you might decide that afternoons are for socializing and exercising, and work late at night. I decided very early on in graduate school that weekends were for me, not for my thesis, and I think it helped me to stay sane.
Many graduate students hit the doldrums around the end of the second or beginning of the third year, when they're finishing up their coursework and trying to focus in on a thesis topic. Sometimes this process can take quite a while. Try to find useful, enjoyable activities that can take your mind off of the thesis. Sing in a choir, learn a foreign language, study the history of ancient Greece, garden, or knit. If you schedule regular activities (rehearsals, tennis lessons), you will probably find it easier to avoid drifting aimlessly from day to day.
In the final push to finish your thesis, though, you will almost certainly have less time for social activities than you used to. Your friends may start to make you feel guilty, whether they intend to or not. Warn them in advance that you expect to turn down lots of invitations, and it's nothing personal -- but you need to focus on your thesis for a while. Then you'll be all done and free as a bird! (Until the next phase of your life starts...)
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