Cuando la bebé se duerme temprano, la madre se siente más tranquila.

Because dormirse and dormir are not exactly the same.

• dormir = to sleep
• dormirse = to fall asleep

So:

• La bebé duerme temprano suggests the baby sleeps early / is sleeping early
• La bebé se duerme temprano means the baby falls asleep early

In this sentence, the idea is that the mother feels calmer when the baby gets to sleep early, so se duerme is the natural choice.

The verb here is sentirse, which means to feel in the sense of to feel emotionally or physically in a certain state.

• sentir = to feel something, to notice, or to regret
• sentirse = to feel oneself, to feel a certain way

So:

• La madre se siente más tranquila = The mother feels calmer
• not The mother feels something calmer

That is why Spanish uses se here.

Because adjectives in Spanish usually agree with the noun they describe.

Here, tranquila describes la madre, which is feminine singular.

• la madre → tranquila
• el padre → tranquilo

Also notice that más does not change:

• más tranquila
• más tranquilo

Only the adjective changes to match the person.

Bebé is a common-gender noun, which means the article tells you whether the baby is male or female.

• el bebé = a male baby
• la bebé = a female baby

So la bebé is correct here because the sentence is talking about a girl.

You may sometimes hear el bebé used more generally, but la bebé is perfectly correct and very clear.

The accent mark shows where the stress goes: be-BÉ.

Without the accent, Spanish spelling rules would make you stress it differently. Since bebé ends in a vowel, the normal stress would fall on the second-to-last syllable, but the real stress is on the last syllable, so it needs the written accent.

So the accent mark tells you to say:

• be-BÉ

not

• BÉ-be

Because here cuando is a conjunction meaning when.

Spanish uses cuando without an accent in normal connecting clauses:

• Cuando la bebé se duerme temprano... = When/Whenever the baby falls asleep early...

You use cuándo with an accent in questions or indirect questions:

• ¿Cuándo se duerme la bebé? = When does the baby fall asleep?
• No sé cuándo se duerme. = I don't know when she falls asleep.

So:

• cuando = conjunction
• cuándo = question word

In this sentence, it most naturally means whenever or when(ever) this happens.

The sentence is in the present tense and sounds like a general or habitual situation:

• Cuando la bebé se duerme temprano, la madre se siente más tranquila.
• Whenever the baby falls asleep early, the mother feels calmer.

If you wanted to talk about one specific future occasion, Spanish would usually change the verb form:

• Cuando la bebé se duerma temprano, la madre se sentirá más tranquila.

So in your sentence, the meaning is more general/habitual.

Spanish often uses the present tense for general truths, habits, and repeated situations.

That is exactly what is happening here:

• when this happens, this is the result

So the sentence does not necessarily describe one single event happening right now. It describes a pattern:

• When the baby falls asleep early, the mother feels calmer

This use of the present is very common in both Spanish and English.

Because the sentence is written in a general, neutral way.

• la madre = the mother
• su madre = her mother / his mother / your mother / their mother, depending on context

Using la madre makes the sentence sound more like a general statement or description. It is not focusing on possession; it is just naming the person involved.

If the context already made the relationship important, su madre could also work, but it would shift the focus a little.

It is standard and natural to use a comma when a cuando clause comes first.

So this is good punctuation:

• Cuando la bebé se duerme temprano, la madre se siente más tranquila.

If you reverse the order, the comma is often omitted:

• La madre se siente más tranquila cuando la bebé se duerme temprano.

So the comma here is normal because the time clause is placed at the beginning.

Both are possible, but they sound a little different.

• la madre = more neutral, descriptive, slightly more formal
• mamá = more personal, everyday, family-style

So this sentence sounds like a neutral statement, maybe from a textbook, article, or explanation. In everyday speech, many people might naturally say:

• Cuando la bebé se duerme temprano, mamá se siente más tranquila.

But la madre is completely correct.

Here más tranquila means something like:

• calmer
• more relaxed
• less worried
• more at ease

It does not mean quieter in the literal noise sense. It describes the mother’s emotional state.

So the idea is that when the baby falls asleep early, the mother feels more calm and less stressed.