restait allongée à

Question

Any reason why we use restait allongées à as opposed to s'allongeait à ? 
I wrote : C'est là qu'elle aimait étendre sa couverture sur laquelle elle s'allongeait à rêvasser pendant des heures, but its not in the answers.

Accepted Answer

Bonjour Paris,
That's a really interesting question!
In French, s'allonger focuses (slightly) more on the initial act of lying down, while rester allongé places (slightly) more emphasis on being stretched out, luxuriating.
So in this case, since the narrator is talking about lying down for hours, rester allongé is the only option because s'allonger would make it sound like it took her hours to move from an upright position to lying down.
I hope this is helpful.
Bonne journée !