It is a constant struggle, not just in science but in life in general. I think ring-fencing your time helps, as in making work time just for work and completely switching off in personal time (difficult if your writing a paper and ideas are coming to you!). Overall, imo in science there is greater pressure than average to be creative and analytical, so it is important to make sure that you are able to do the things you need to do to keep you healthy – eating properly, exercising, sleep, time for friends and family and hobbies and breaks. I definitely don’t succeed all the time in this regards!
I find it difficult to balance the stress of doing a PhD and having a personal life. Its really easy to shut yourself off from everyone and concentrate on studying but this can lead to burning out. I think its really important to find a balance and always do something for yourself 🙂