can't sleep with one's feet pointing toward someone; feel deeply indebted
A set phrase expressing profound gratitude: one feels so indebted that it would be disrespectful to point one's feet toward the benefactor while sleeping. Often used hyperbolically.
He helped me so much that I can never thank him enough.
I'm filled with such gratitude toward that teacher that I feel I can never repay him.
Both convey a sense of being unable to hold one's head high due to indebtedness or respect, but 足を向けて寝られない specifically uses the imagery of sleeping posture.
A fixed expression from Japanese cultural etiquette where pointing one's feet toward someone is considered disrespectful. The phrase literally means 'cannot sleep with one's feet pointing toward (someone)', figuratively expressing deep gratitude.