hey hannah,
it's pretty common for CFers to need O2 at night as they get older. as skye said, we sometimes de-sat at night due to the deep relaxation of sleep -- actually, everyone de-sats to some degree, and it's "normal" to drop anywhere from 2-7 percentage points overnight. so for a CFer, who might be satting in the low-mid 90s even during the day, it makes sense that even a "normal" drop could mean dipping below 90.
i've been on nighttime O2 for a few years. it's tough to get used to, in a way, but it does help you sleep better and wake up feeling more rested -- plus it will help get rid of those killer morning headaches! it does NOT mean that your lungs are shutting down or some other disaster. i actually know several people with pretty high lung function who still use O2 at night -- people who are active and healthy.
honestly, what really helped me deal with needing oxygen was the advice of a friend of mine. i was upset about it and he basically told me that i should look at it like reading glasses: not as a sign of my sickness but rather as a TOOL to help allow me (and my body) to do everything that i wanted to do. he just shrugged and was like "look, some people need glasses, some people need help walking, and some people need oxygen. do whatever you have to do to help your body feel good enough to do the things you WANT to be able to do." it was like a light went on when he said that -- suddenly this big, scary concept became just this little extra tool in my toolbox so that i could live a full and healthy life, just like all the other little things we do for our CF.
hope the O2 brings you many nights of great rest and pleasant dreams!
piper