Por la noche, cierro el pestillo de la puerta para sentirme más tranquilo.

Por la noche is a very common Spanish expression meaning at night, in the evening/nighttime, or during the night, depending on context.

In this sentence, Por la noche means something like At night or At nighttime.

A few related time expressions are:

• por la mañana = in the morning
• por la tarde = in the afternoon / evening
• por la noche = at night

English speakers often want to translate word for word, but here it is best to learn the whole phrase as a set expression.

Cierro is the first person singular present tense of cerrar:

• yo cierro = I close / I shut
• tú cierras
• él / ella cierra
• nosotros cerramos
• vosotros cerráis
• ellos cierran

So cierro el pestillo means I close the latch/bolt or I slide the bolt shut.

It is not cerrar because cerrar is the infinitive, to close.
It is not cerrando because that means closing.

Also, cerrar is a stem-changing verb in the present tense: e → ie in most forms, which is why it becomes cierro.

El pestillo usually means a latch, bolt, or door bolt, depending on context.

In this sentence, it refers to the part of the door you fasten for extra security. So cierro el pestillo de la puerta suggests something like:

• I slide the door bolt shut
• I fasten the latch on the door
• I lock the bolt on the door

A learner should know that pestillo is not always the main lock with a key. It often suggests a smaller fastening mechanism, like a latch or bolt.

Because the sentence is specifically talking about the latch/bolt of the door, not simply closing the door.

Compare:

• Cierro la puerta = I close the door
• Cierro el pestillo de la puerta = I fasten/close the door latch or bolt

So de la puerta tells you which latch we mean: the one on the door.

In Spanish, when talking about a specific object belonging to or associated with another object, you normally use the article:

• el pestillo de la puerta = the latch of the door
• la ventana de la cocina = the kitchen window
• el color de la pared = the color of the wall

So de la puerta is the natural Spanish structure here.
Saying de puerta would sound incomplete or unnatural in this sentence.

Here para means in order to or so that.

So:

• para sentirme más tranquilo = in order to feel calmer / more at ease

This is a very common structure in Spanish:

• Estudio para aprender. = I study in order to learn.
• Cierro la ventana para no tener frío. = I close the window so as not to be cold.

When the subject stays the same, Spanish normally uses para + infinitive.

Because the verb here is sentirse, not just sentir.

• sentir = to feel, to regret, to sense something
• sentirse = to feel oneself, meaning to feel in a physical or emotional state

So:

• Siento frío. = I feel cold.
• Me siento tranquilo. = I feel calm.

In the sentence, sentirme combines:

• sentir
• plus the reflexive pronoun me

So para sentirme más tranquilo literally means to feel myself calmer, but in natural English: to feel calmer or to feel more at ease.

Because after a preposition like para, Spanish uses the infinitive, and reflexive pronouns can be attached directly to an infinitive.

So:

• para sentirme = to feel myself / to feel

This is completely normal. You could think of it as:

• me siento = I feel
• para sentirme = in order to feel

With infinitives, gerunds, and affirmative commands, pronouns are often attached:

• quiero levantarme = I want to get up
• está vistiéndose = he/she is getting dressed
• siéntate = sit down

Because tranquilo is describing how the speaker feels, not how they do the action.

With sentirse, Spanish normally uses an adjective:

• me siento tranquilo = I feel calm
• me siento cansado = I feel tired
• me siento seguro = I feel safe

An adverb like tranquilamente would describe the manner of an action, for example:

• duermo tranquilamente = I sleep peacefully/calmly

But here the meaning is about the speaker’s emotional state, so más tranquilo is correct.

Tranquilo agrees with the gender of the speaker.

• A male speaker would say me siento más tranquilo.
• A female speaker would say me siento más tranquila.

So the sentence as written suggests the speaker is male, or it is just being given in the masculine form as a default example.

Yes, and it would be very natural, but it is slightly different in nuance.

• para sentirme más tranquilo = in order to feel calmer / more at ease
• para estar más tranquilo = in order to be calmer / feel more at ease

In many everyday contexts, both work.
Sentirme focuses a little more on the speaker’s internal feeling.
Estar can sound a bit broader, referring to one’s state or condition.

A native speaker could use either one here.

Not necessarily.

Because it uses el pestillo, the sentence specifically refers to a latch or bolt, not automatically the main key lock.

If you wanted to clearly say I lock the door, you might hear:

• Cierro la puerta con llave = I lock the door with a key
• Echo el pestillo = I slide the bolt / fasten the latch

So this sentence suggests securing the door by fastening the latch/bolt, perhaps for extra peace of mind.

The comma is used because Por la noche is an introductory time expression at the beginning of the sentence.

So the sentence is structured like:

• Por la noche, = At night,
• cierro el pestillo de la puerta = I fasten the door bolt/latch
• para sentirme más tranquilo = in order to feel calmer

In Spanish, this comma is common and natural, especially in careful writing. Even without it, the sentence would still be understandable, but the comma helps separate the time phrase from the main clause.