A common complaint of grammarians is that people use the word "lay" when they ought to use the word "lie." People will say something like "I need to go lay down." but that is incorrect. It should be, "I need to go lie down." You don't lay down, you lay things down. "I spent the day laying down the carpet."
Where did this confusion come from? I suspect a common childhood prayer contributes to the problem. This one:
Now I lay me down to sleep,
I pray the Lord my soul to keep.
If I should die before I wake,
I pray the Lord my soul to take.
It reinforces, nightly, that when one rests, one lays down rather than lies down. Yet the first line is correct, if worded strangely. Something is being laid down, the 'me' of the line. "Now I lay me down to sleep." Without the 'me' it would rightly be "Now I lie down to sleep."
Why isn't the shorter, less apt to cause later confusion, version of the first line used? I can think of a couple reasons. Who would wants a prayer, of all things, where it is said, "I lie" somewhere in it? Especially as it is taught to children who will almost certainly point out that they aren't lying and lying is a sin, isn't it? Also, 'lay' in the first line rhymes with 'pray' in the second and fourth lines and that can make the prayer a bit easier to remember. As this is taught to children, ease of learning is advantageous.