How safe are electrical outlets to sleep by?

Yes, it is perfectly safe. An outlet is just a place where electricity is available when you need it. It can do you no harm whatsoever by sleeping next to it any more than sleeping next to a electrical cord plugged into that outlet and then run up to a lamp or clock next to your bed. I assume you are worried about the magnetic field created. This field is so low that it is barely detectable at an outlet.