dormir vs dormirse
Dormir is the state of sleeping. Dormirse is the moment of falling asleep.
dormir
To sleep — the ongoing state.
Duermo ocho horas.
I sleep eight hours.
How much you sleep. Stem change: duermo, duermes.
¿Has dormido bien?
Did you sleep well?
The standard morning question.
Duerme como un lirón.
He sleeps like a log.
Literally 'like a dormouse'. A fixed idiom.
dormirse
To fall asleep, to oversleep, or for a limb to go numb.
Me dormí en el sofá.
I fell asleep on the sofa.
The transition from awake to asleep.
Me quedé dormido y llegué tarde.
I overslept and was late.
Quedarse dormido = to oversleep or doze off.
Se me ha dormido el pie.
My foot has gone to sleep.
Numb limbs. Accidental structure with 'se me'.
Tricky bit
'Dormí en el sofá' = I slept on the sofa (spent the night). 'Me dormí en el sofá' = I dozed off on the sofa. One reflexive pronoun, one big difference.