Origin of phrase "I slept at"
It doesn't sound at all odd to me.
At seven I ate dinner, and at nine I slept.
While laughably untrue, this is grammatically correct, and the sense is clear. If you look at the native language of a non-English speaker who says it this way, you'll probably find that the typical way of expressing the sense is I slept rather than I fell asleep or I went to bed, that's all. (I know this to be the case in Chinese, for example.)