I... can't believe so many of your (plural) doctors are prescribing seroquel anymore. If you're psychotic, then by all means, atypical anti-psychotics are not a bad idea. If you don't actually suffer psychotic disorders, stay the hell away from them.
The risk benefit ratio of the atypical antipsychotics is better than the typical antipsychotics for tardive dyskinesia, extra-pyramidal symptoms, but worse for weight gain, diabetes, increased cholesterol/triglycerides, and various effects on the heart. Atypical antipsychotics increase the risk for sudden-death, serotonin syndrome and neuroleptic malignant syndrome.
Some people do have psychosis as part of their PTSD, but this is not the usual. The atypical antipsychotics have also been used off-label for resistant depression, but depression that is part of PTSD isn't pure Major Depressive Disorder. Research hasn't supported the use of atypical antipsychotics in randomized trials and their use in most patients should be questioned - especially if sleep is the relevant issue.
Antiseizure medications like Lamictal, Neurontin, Topomax as well as some tricyclic antidepressants are safer to obtain sleep, less likely to cause untoward side-effects and typically are effective at low doses. For instance, 5-10mg of amitryptiline can induce sleep as can 100-300mg of gabapentin used 30 minutes or so before bedtime.
While some people may require Seroquel for mood stabilization - particularly if they have severe anger issues or have Bipolar with psychotic features in addition to PTSD, patients should question why they are being started on a drug and have a real discussion about the risks involved.