If you want to avoid training your own Haar cascade to detect a single eye, you can attempt simpler techniques such as pupil detection. If you fail to detect a black circle, the eye is closed. If you have a smallish region of interest, this probably works very well. Another option would be color histograms of the eye region, which may look pretty different for the open and closed state.
If you cannot predict with reasonable accuracy where the eyes can be found in the image, these approaches are doomed and your best shot is training your own cascade I think.