You could try Dalmane or Temazepam which are benzodiazepine classed sleeping tablets, they can be addictive though so be careful.
Seroquel does in fact cause weight gain, I gained weight from it (among other meds) as well. There are other medicines in the anti psychotic range that cause drowsiness and aid sleeping easier such as zyprexa. I am on it and it usually works wonders but it does give you an appetite.
You could try promethazine (phenergan) which someone suggested above, it is an anti histamine but quite sedating. Another similar one would be piriton.
Benzodiazepines such as xanax, valium, ativan, bromazepam, clonazepam etc.. could relax you enough into a comfortable sleep as well but again be cautious, they are addictive.
There are plenty of options but I would suggest you talk to your doctor before taking literally anything
Wishing you the best of luck.