Dormir vs. dormirse
The Spanish verbs dormir and dormirse both relate to sleep, but they are not interchangeable. The key difference lies in whether you are talking about the state of being asleep or the action of falling asleep.
Dormir
A1Dormir is an intransitive verb that means to sleep. It describes the general act or state of being asleep, focusing on the duration, quality, or habit of sleeping.
Necesito dormir al menos siete horas cada noche.
(I need to sleep at least seven hours every night.)
El gato duerme todo el día en el sofá.
(The cat sleeps all day on the sofa.)
Anoche no dormí bien por el calor.
(Last night I didn't sleep well because of the heat.)
¿A qué hora duermes normalmente?
(What time do you normally sleep?)
Los osos duermen durante todo el invierno.
(Bears sleep throughout the entire winter.)
Dormirse
A2Dormirse is a pronominal verb that means to fall asleep. It describes the transition from being awake to being asleep. It can also mean to oversleep or for a part of the body to go numb or fall asleep.
Me dormí mientras leía el libro.
(I fell asleep while I was reading the book.)
El conductor se durmió al volante y tuvo un accidente.
(The driver fell asleep at the wheel and had an accident.)
Lo siento, llegué tarde porque me dormí esta mañana.
(I'm sorry, I arrived late because I overslept this morning.)
Después de estar sentado mucho tiempo, se me durmió la pierna.
(After sitting for a long time, my leg fell asleep.)
El niño se duerme en cuanto lo acuestas en la cuna.
(The child falls asleep as soon as you put him in the crib.)
No te duermas en los laureles, todavía queda mucho trabajo.
(Don't rest on your laurels, there's still a lot of work to do.)
Summary
In summary, use dormir to talk about the general act or state of sleeping (to sleep). Use dormirse to describe the process of entering sleep (to fall asleep). Remember that dormirse also uniquely means to oversleep or for a body part to go numb.