I'm a woman and after sex there's only three things I ever want to do - smoke, eat, and/or sleep.
But a lot of women stay awake afterward because they feel emotionally vulnerable and might want to talk or cuddle. The solution? Naked spooning as you fall asleep.
Its probably because they are relaxed and they get sleepy. They are probably programmed this way so they cant get up and leave and hurt the girls feelings.