20mgs seems like a fairly high dose. Olanzapine does have some antidepressant properties, but it is also very sedative. I take 5mg at night if I can't sleep and it knocks me out. You may ask your Doc. if you can lower your dose. Your Doc. should also be monitoring your blood sugar levels, esp. if there is a history of diabetes in your family.
Olanzapine can increase your blood sugar levels. Hope you feel better. ~ Sindi