The following information is posted as (we hope) a useful clarification for students whose advisers and/or committee members go on sabbatical leave.
Sabbaticals are time given to faculty without teaching or service obligations, on the idea that this is the only way faculty have a hope of getting any extensive research done. (At UCLA, faculty earn sabbaticals by teaching for some number of quarters.) This time is so precious that many faculty take a pay cut to get it sooner than they otherwise would. Therefore, in principle, students are supposed to leave faculty on leave alone.
Indeed, it's in your best long-term interests to do so - not only to let the faculty get rejuvenated, but also: the department's stature, the value of your future degree, the influence of your adviser in helping you to get jobs, the ability of faculty to get grants that will support students - lots of things like these are enhanced if faculty get more research done.
However, we do realize that life doesn't stop for you when we go on leave, and most faculty are willing to meet with their own students - especially students working towards one of the hurdles in the program - at least occasionally, and/or keep in touch by email. Nonetheless, you should NEVER take this for granted, and you should understand that this is done at the faculty member's choice and convenience. A faculty member may even choose to meet regularly with a student who is working on research related to his/her own (and especially, one working as an RA), yet decline to meet with you to discuss something that only you are interested in - and that's fair under these circumstances!
Basically the same situation obtains during the summers for all faculty. Either we aren't paid at all, or we are paid by some agency outside UCLA to accomplish some piece of research.