Я волновался не столько из‑за давления, сколько из‑за бессонницы.

Because the speaker is Я, and in the past tense Russian verbs agree with the speaker’s gender and number.

• волновался = masculine singular
• волновалась = feminine singular
• волновалось = neuter singular
• волновались = plural

So a man would say Я волновался, and a woman would say Я волновалась.

Волноваться means to worry, to be anxious, or to get worked up.

In this sentence, it suggests emotional anxiety or concern, not physical motion. Even though the root is connected historically with wave (волна), in modern Russian волноваться is a very common verb for to worry.

This is a common Russian comparison pattern meaning:

• not so much ..., as ...
• not столько X, сколько Y = less because of X than because of Y

So:

• не столько из-за давления, сколько из-за бессонницы

means something like:

• not so much because of the pressure as because of the insomnia
• less because of the pressure than because of the insomnia

It does not mean the first thing is completely false. It usually means the second cause is the more important one.

Because each part of the comparison keeps the same structure:

• из-за давления
• из-за бессонницы

Russian often repeats the preposition in this pattern for clarity and balance. That is the most natural full version here.

In some contexts, repetition can sometimes be reduced, but with не столько ..., сколько ..., repeating the preposition is very standard and natural.

Из-за takes the genitive case.

So here we get:

• давление → давления
• бессонница → бессонницы

That is why both nouns have genitive endings.

Here из-за means because of.

It can express cause, often with a slightly negative flavor, especially when talking about problems, difficulties, or unpleasant causes:

• из-за дождя = because of the rain
• из-за ошибки = because of the mistake
• из-за бессонницы = because of insomnia

So in this sentence it introduces the reason for the worrying.

It could mean either, depending on context.

Давление can mean:

• pressure in a general or emotional sense
• pressure from work, life, other people
• blood pressure

Without more context, the Russian is ambiguous in the same way English can be. Since it is contrasted with бессонница (insomnia), a health-related reading such as blood pressure may feel especially possible, but the exact intended meaning depends on the broader context.

The dictionary form is бессонница (insomnia). After из-за, it must go into the genitive singular:

• бессонница → бессонницы

This is a regular feminine noun pattern:

• nominative: бессонница
• genitive: бессонницы

Yes, Russian word order is flexible, but this version is very natural and clear:

• Я волновался не столько из-за давления, сколько из-за бессонницы.

The pattern не столько ..., сколько ... is usually kept together, because it functions as a set expression. If you change the order too much, the sentence may sound less natural or harder to process.

It is not especially formal, but it is a bit more structured and polished than very casual speech. You can hear it in everyday Russian, writing, journalism, and conversation.

It is a useful pattern when you want to compare two causes, reasons, or qualities in a precise way.

Yes, that is a very good close translation.

A more natural English version might be:

• I was worried less because of the pressure than because of the insomnia.
• I wasn’t as worried about the pressure as I was about the insomnia.

But the literal version captures the Russian structure well.

Волноваться is imperfective.

Here that matters because the sentence describes a state or ongoing feeling of worry, not a single completed event. Russian commonly uses the imperfective for emotions, states, and processes like this.

The perfective partner is often разволноваться, which means something more like to become upset / to get agitated, focusing on the change of state rather than the ongoing worrying.

Из-за is written with a hyphen because it is a fixed compound preposition.

Pronunciation is roughly:

• iz-ZA

The stress is on the second part: за.

In connected speech, it sounds like one unit rather than two separate words.