Another SWF question

I have class in the morning. I was wondering if I could go to class without eating anything, and then do the SWF (Salt-Water-Flush) when I get back? (since I would have still not eaten anything?) Or does this have to be done RIGHT upon wakening?? I am just worried b/c I have never done it and would rather do it when I don't have anywhere I have to be for the rest of the day.