There's a wonderful poem called Dog in Bed by Joyce Sidman in which she describes how her dog hogs the bed at night, forcing her to reposition herself to accommodate her pet. At the end she writes:
This is how it is with love.
it steps in gently,
and takes up as much space
as you will give it.
This is how it is with teaching, too. Our contract day may be seven-and-a-half hours, but the job is impossible to do in that time span. Planning and grading alone will push your day to nine hours or more, never mind any clubs or study halls you sponsor. It's easy to see how each additional hour you spend will benefit your students, and isn't that why so many of us are here? But if you're not careful, it might start to seem like time you take for yourself is time you take from your students, and that's not good for anyone.