A la nit, sempre apago la llum abans de dormir.

No. A la nit is an idiomatic time expression meaning at night or in the nighttime.

Even though a often means to, in Catalan it is also used in many expressions of time:

• al matí = in the morning
• a la tarda = in the afternoon
• a la nit = at night

So here, A la nit is just the natural Catalan way to say at night.

Catalan usually does not need subject pronouns unless you want emphasis, contrast, or clarity.

The verb form apago already tells you the subject is I:

• apago = I turn off
• apagues = you turn off
• apaga = he/she/it turns off

So sempre apago naturally means I always turn off.
You could say Jo sempre apago la llum..., but it would sound more emphatic, like I always turn off the light...

Apago is the 1st person singular present tense of apagar, which means to turn off or to put out.

So:

• apagar = to turn off
• apago = I turn off

In this sentence, the present tense is used for a habitual action:

• sempre apago la llum = I always turn off the light

That is very natural in Catalan, just like in English.

Yes. Llum is a feminine noun in Catalan, so it takes la:

• la llum = the light

That is something you just have to learn with the noun.
Also, in this sentence la llum usually refers to the light in a general everyday sense, not necessarily a specific lamp mentioned earlier.

Yes. Apagar la llum is the normal Catalan expression for to turn off the light.

A few useful comparisons:

• encendre la llum = to turn on the light
• apagar la llum = to turn off the light

So this is a very common everyday phrase.

Sempre means always, and in Catalan adverbs like this often go before the conjugated verb:

• sempre apago
• sempre menjo aquí
• sempre arribo d’hora

That placement is very natural.
Catalan word order is a bit flexible, but sempre apago la llum is the standard, neutral order here.

Because after abans, Catalan normally uses de before an infinitive.

So:

• abans de dormir = before sleeping / before going to sleep
• abans de menjar = before eating
• abans de sortir = before leaving

So de is required here.

Because after abans de, Catalan uses the infinitive, not a conjugated verb.

Compare:

• dormir = to sleep
• dormo = I sleep

So:

• abans de dormir = before sleeping not
• abans de dormo

This is similar to English using before sleeping or before going to sleep, rather than before I sleep in this structure.

Yes, absolutely.

Both are natural:

• abans de dormir = before sleeping
• abans d’anar a dormir = before going to sleep

The second version is a little fuller and more explicit.
The original sentence is shorter and very natural in everyday Catalan.

It is natural because A la nit is a time expression placed at the beginning of the sentence.

The comma helps separate that introductory phrase:

• A la nit, sempre apago la llum abans de dormir.

You may sometimes see Catalan sentences without such a comma in informal writing, but with this kind of fronted time phrase, the comma is very common and helpful.

Llum can be tricky for English speakers because of ll.

In standard Catalan, ll is traditionally a palatal sound, similar to the lli sound in some careful pronunciations of Spanish ll, though in many modern accents it may sound closer to a y sound.

A simple learner-friendly approximation is:

• llum ≈ lyoom or sometimes almost yum, depending on accent

Also:

• u here sounds like oo in food
• the final m is pronounced

So a rough pronunciation guide is lyoom.