My biggest concern generally is in-ear comfort. Short of molded customs, every in-ear headphone I've tried causes canal soreness within an hour or two, including the "sound delivery straw" designs with delicate foam or rubber surrounds. Secondarily, it looks like the current model sticks out, causing pressure if one's head is resting on its side.

That said, my ear canal soreness is largely due to canal deformation relating to jaw movement. Perhaps this is not an issue for sleeping. Also, foam earplugs (compared to earphones) are not uncomfortable for me.