Seroquel makes you sleepy because it acts only as an antihistamine through a dose of about 150mg. After that other effects also kick in. Some people think that those other effects work as an antipsychotic, true, but that they also negate the effects of the antihistamine (which can include weight gain as well). That would be why a higher dose of Seroquel might not work as well as a sleep aid and why you might not have the same other side effects at a higher dose either.

Also you know that you are using a highly potent antipsychotic just as a sleep aid. I'm going to assume that nothing safer works for you...?