Swedeguy is having the same problem, only worse, it sounds like. I haven't used an ROP yet, so I can only guess what's happening. One possibility could be that your ROP is too tight. From the description, it sounds as if lymphatic drainage from the tissues distal to the ROP might be impeded due to the external compression. If you have other symptoms besides the swelling - e.g., redness or itching - it is possible that you could be experiencing an allergic reaction. However, since this is evidently occurring only when you sleep, the allergy theory is highly improbable. Perhaps you should consider making a second ROP for night time use that is a bit looser. Also, if you're not doing so already, consider positioning the metal rods to the top & bottom, rather than to the sides. Supra reported major problems with scrotal swelling and discomfort after sleeping while wearing the ROP until he made this change. If none of this helps, then consider switching to silicone rubber tubing - it is non-allergenic, unlike latex. One final, remote possibility: some people are actually allergic to certain types of metals. I have encountered people who are allergic to the nickel that is often found in the snaps that are used on jeans and other clothing. If a metal allergy is the source of your problem, I'd suggest that you visit Tom Hubbard's Penis Workshop and study the informative contribution from "Gunther." There are other combinations of galvanic elements that you could try besides copper and zinc.
I could be mistaken, but I also think that jellyroll in the morning would be way better than a donut.