I really don't get it why seroquel / quetiapine is prescribed so often as a sleep aid (besides the huge revenue it creates for AstraZeneca, of course). The same with other neuroleptics, and to a lesser extent also the sedative antidepressants. Hey, you're using antipsychotics just because of some of their side effects!
Yeah, they say seroquel is a low-potency one and an atypical. With 25-50mg it's probably not serious. Nevertheless it IS antidopaminergic with as less as 50mg, causing dysphoria and restlessness which is just overpowered by the sedation. And there is evidence that the ugly potential of inducing diabetes (!!) is not dosage dependent. Some people become adipose w/o eating much more, just from neuroleptics.
What you're looking for is a) a powerful / high-affinity, centrally acting H1 antihistamine with a short duration (so that it doesn't cause hangovers - sadly most of the old, sedating antihistaminergics have been designed to be long-acting afaik) and without anticholinergic / etc. properties.
And/or b) an anti-noradrenergic agent. This could either be an autoreceptor agonist like clonidine (a good sleep aid btw, non-addictive, not dysphoric and dirt cheap) or some adrenozeptor antagonist.
These are the mechanisms why things like mirtazapine, quetiapine etc. are sedative / hypnotic. Try a non-sedating neuroleptic once, and you'll know their true nature.
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If your sleep problems are due to disrupted circadian rhythm, then give melatonin a try (maybe combined with doxylamine - OTC antihistamine - for the first days).