Yeah they sure do sleep with their heads down. Mine huddle into a long grass plant that I have!! They look great.
If they are doing this during the day it could mean a few things:
1: Water conditions are not 100% right
2: They didn't get a good nights rest and are just catching up!!
3: They have been bullied/chased/tormented by a dominant male/female and are just chilling out to regain some energy
4: They are backing out of a fight and tend to point their heads downwards to show to the victor that they are giving up.
Just a short list. This info is from books I have read and on the behaviour of my own fish.
If you find that they are doing it during feeding time and are not showing any interest in their food then keep them under close observation and consider separating them from all the other fish in the tank, they could be sick. My female did this at one point but I knew that she spent the day before laying eggs so it was obvious that she was shattered!!