Eu durmo bem quando não bebo café à noite.

Not necessarily. Portuguese often leaves out subject pronouns because the verb ending already shows who is doing the action.

• durmo = I sleep
• bebo = I drink

So Durmo bem quando não bebo café à noite is perfectly natural too.
Eu is often added for emphasis, contrast, or clarity.

Both are in the present indicative.

Here, the present tense expresses a general habit or regular truth:

• Eu durmo bem = I sleep well
• quando não bebo café à noite = when I don’t drink coffee at night

So the sentence is not about one specific night; it is about what usually happens.

After quando, Portuguese uses different verb forms depending on the meaning:

• present indicative for something habitual or generally true
• future subjunctive for a future event

In this sentence, the meaning is habitual, so quando não bebo is correct.

Compare:

• Quando não bebo café à noite, durmo bem.
  = whenever I don’t drink coffee at night, I sleep well

• Quando não beber café à noite, dormirei bem.
  = when I don’t drink coffee at night (in the future), I’ll sleep well

Bem is an adverb, and here it means well.

It modifies the verb durmo:

• durmo bem = sleep well

This is the normal way to say that you sleep with good quality or comfortably.

In simple sentences like this, não normally goes right before the verb:

• não bebo = I do not drink

So:

• quando não bebo café = when I don’t drink coffee

That placement is very regular in Portuguese and is one of the first negation patterns learners meet.

Because café is being used in a general sense, not as a specific coffee.

In Portuguese, with food and drink after verbs like beber, the article is often omitted:

• beber água
• beber vinho
• beber café

You would use an article if you meant a specific one:

• Não bebo o café que me trouxeste.
  = I’m not drinking the coffee you brought me.

À noite means at night or sometimes in the evening/nighttime, depending on context.

The à has a grave accent because it represents a contraction:

• a + a = à

This is a very common fixed expression of time, so it is best learned as a chunk:

• de manhã = in the morning
• à tarde = in the afternoon
• à noite = at night / in the evening

Yes, but à noite is the more natural choice here.

Very roughly:

• à noite = at night / in the evening as a time period
• de noite = by night / during the night, often contrasted with de dia

So in this sentence, à noite sounds more standard and idiomatic.

Yes. Both are possible.

• beber café = to drink coffee
• tomar café = to have/drink coffee

In Portugal, tomar café is very common and can also suggest having a coffee as an everyday action.
So both sentences are natural:

• Eu não bebo café à noite.
• Eu não tomo café à noite.

Yes, absolutely. That version is also correct and very natural.

Portuguese allows both orders:

• Eu durmo bem quando não bebo café à noite.
• Quando não bebo café à noite, durmo bem.

If the quando clause comes first, a comma is normally used.
In the original order, a comma is usually not needed.

Because durmo bem describes a general habit or tendency, not something happening right now.

• Durmo bem quando não bebo café à noite.
  = I sleep well when I don’t drink coffee at night.
  (general truth / habit)

• Estou a dormir bem.
  = I’m sleeping well.
  (current situation, these days / at the moment)

This is a common difference between English and Portuguese: English often uses the progressive more, but Portuguese uses the simple present for habits very naturally.