Had sleep apnea before leaving the states. Used to sleep 14 hour days, still sleepy during the day. Didn't figure it out until I got depression too and started examining the sleep issues as potential depression symptoms (they weren't, and in fact sleep apnea sometimes causes depression).

Note my example is extreme, and I've seen many complaints about other issues (like waking up in the middle of the night, I never did), so ymmv