I don't know at what dose you started out on so this may be useless, but Seroquel at low doses (25-100mg) acts only as an antihistamine which is why it makes you sleepy. At that dose it doesn't work as a mood stabilizer. Anyway I've read more than once that when Seroquel's other properties kick in (200mg?) they actually counteract the antihistamine effects--meaning they work less well for sleep at high doses. But of course if you started out very low and each dosage increase made the Seroquel more effective for sleep, then that explanation would seem not to apply.

I take 50mg of Seroquel and 3mg of Lunesta each night and sometimes 1mg Xanax. Could you cut down the Seroquel and maybe add something else? Or are you trying to get off sleeping medications altogether?

I've quit Seroquel abruptly more than once with no problems, but I was never as high as 500mg, and of course it's always safer to taper.