We have some experience in different splicing in some RNA depending of the time that the blood is in the bench waiting for the RNA extraction. We are wondering if that can occurs also with circRNA.
Are you sure you are observing splicing and not degradation?
RNA in human brain tissue (postmortem) takes 3 hours to degrade at room temperature (anecdotal observation). I suspect that you need to collect blood in PAX tubes if you want stable RNA at room temperature.
We use several times PAX gene adn it works pretty well (although they are expensive and the backnoise in the RNA studies is higher). Of course tehre are degradation (not so fast as 3 hours in our experience) but in this case I am talking about diferent transcripts (checked by qpCR) that are time-dependent.
If you have differences in splicing patterns depending on the collection time of the samples, be sure that you will also see differences in the circRNA expression pattern (quality and quantity), since these species are depending on non-canonical splicing.