That's a very high dosage if you're using the Seroquel just for sleep. Seroquel works only as a (sleep-inducing) antihistamine at doses up to around 150mg. After that its use as an antipsychotic kicks in. The antipsychotic effects may in fact negate the sleep-inducing antihistamine effects. Doses of 450mg or even more are used to prevent manic and depressive symptoms.