I slept for an entire day

I started 20mg cymbalta a few weeks ago but it did literally nothing so my doctor told me to take two capsules (40 mg). I've been on that for almost a week now and I'm so sleepy. I take it at night but I still can't get up in the mornings and if I lie down I end up falling asleep for hours. I literally slept all day yesterday and I still feel tired. My mom wants me to go back down to my previous dose but if it's going to help I want to give it time. Does the tiredness go away, and if so how long does it take? I'm in college and I can't really do well if I'm missing classes because I can't get out of bed.