Eu tenho sono quando durmo pouco.

In Brazilian Portuguese, ter sono is a very common idiomatic way to express sleepiness. Literally, it looks like have sleep, but the natural meaning is be sleepy or feel sleepy.

Two very common options are:

• tenho sono
• estou com sono

Both are natural in everyday Brazilian Portuguese.

By contrast:

• estou sonolento = I am drowsy; possible, but less common in casual speech
• sou sonolento = I am a sleepy/drowsy person by nature; this sounds like a characteristic, not just a temporary feeling

Here, sono means sleepiness or drowsiness.

It is a masculine noun: o sono. But in the expression ter sono, Portuguese normally does not use the article, so the natural form is:

• tenho sono

not

• tenho o sono

In other contexts, sono can also refer more generally to sleep, depending on the sentence.

The sentence is expressing a general habit or repeated situation, not just something happening right now.

So the present tense in Portuguese can mean something like:

• I get sleepy when I sleep too little
• Whenever I sleep too little, I feel sleepy

This is very normal. Portuguese often uses the simple present for general truths, habits, and repeated cause-and-effect situations.

Because dormir is not fully regular in the present tense. In the eu form, the stem vowel changes:

• eu durmo
• você dorme
• ele/ela dorme

So even though the infinitive is dormir, the first-person singular present form is durmo.

This kind of vowel change happens with some other Portuguese verbs too.

Yes. In fact, that is very natural.

Portuguese often drops subject pronouns because the verb ending already shows the subject:

• tenho = clearly I have
• durmo = clearly I sleep

So both are correct:

• Eu tenho sono quando durmo pouco
• Tenho sono quando durmo pouco

Using eu can add emphasis, contrast, or clarity, but it is not required here.

Here pouco means little or not much, and it works as an adverb, modifying the verb durmo.

So:

• durmo pouco = I sleep little / I do not sleep much

Because it is an adverb here, pouco does not change form.

Compare:

• pouco as an adverb: durmo pouco
• pouco / pouca / poucos / poucas as an adjective: it agrees with a noun

Example:

• tenho pouca energia = I have little energy

In this sentence, quando can be understood as when or whenever, because the sentence describes a repeated situation.

So the idea is not one specific time, but a pattern:

• Whenever I sleep little, I get sleepy

This is a very common use of quando with the present tense.

Yes, absolutely.

Both are natural:

• Eu tenho sono quando durmo pouco
• Quando durmo pouco, eu tenho sono

The difference is mainly one of focus and style:

• Eu tenho sono quando durmo pouco starts with the result
• Quando durmo pouco, eu tenho sono starts with the condition/situation

Also, if the quando clause comes first, Portuguese normally uses a comma:

• Quando durmo pouco, tenho sono.

If it comes second, a comma is usually not needed:

• Tenho sono quando durmo pouco.