Appetite fluctuations in second trimester?

I've had a pretty unproblematic pregnancy so far (I'm 16 weeks) and one of the most reliable symptoms since I entered the second trimester has been a ravenous appetite. For the last few weeks I've been so hungry, but this morning I woke up and just felt ... full. Is this normal?

I know that pregnancy symptoms can come and go (sore boobs, nausea, headaches, etc), but does this apply to appetite too? I haven't been able to find any mention in any good threads, so I thought I'd ask... Since it's my only 'real' symptom, I want to understand how it works!