Zyprexa and Seroquel are at best 3rd line sleep aids, so If (you don't have a medical history of drug abuse) and you've already tried :
1. Z-Drugs. These are the new first line sleeping drugs. Zolpidiem, Zolpiclone, etc. (Ambien, Lunesta...)
2. Benzodiazapines. (as myself and someone else mentioned Tamazepam is at the highest end of this scale. If you are only on valium or clonzapam right now, try going to a more effective benzo.
3. Chloral Hydrate. This is old, and has some abuse liability but few side effects, is extremely effective, and has almost no long term side effects.
4. Barbituates. high abuse liability, but work well and don't have long term side effects.
5. Anti-cholrigenics/ Alergy pills. Benadryl, Hydroxizine (that should probably be at like #2 since it works well and has no dependence liability) Meclazine, Cyclizine, Dihydramine, etc.
6. and then if nothing else worked : Zyprexa or Seroquel which are atypical antispychotics an aren't really indicated for sleep although they do work well for it because psychiatrits have been convinced by EliLilly and Co. that they can be used off-label which was the subject of huge lawsuit regarding olanzapine a couple of years ago, so if you are currently receiving olanzapine for sleep (and not manic bi-polar disorder or schizophrenia or sundry other psychiatric conditions more sever than typical depression or anxiety) then your psychiatrist is doing something wrong.
The logic of prescribing Seroquel or Zyprexa, which have serious long-term side effects (not to piss off anyone that HAS to take them for the conditions they were designed for) is the same as prescribing someone an Opiate or barbituates(including alcohol) or that shit they use in anesthesia for sleep. they all have side effects (dependence and abuse, and some long-term effects as well) and were NOT designed for sleep, it is a side-effect.