Learn Hardcore Croatian: Što manje spavam, to sam nervozniji i teže se koncentriram. - The less I sleep, the more nervous I am and the harder it is to concentrate.

Što manje spavam – the less I sleep
to sam nervozniji i teže se koncentriram – the more nervous I am and the harder it is for me to concentrate.

You can use the same pattern with many adjectives/adverbs:

Što više radim, to sam umorniji. – The more I work, the more tired I am.
Što prije dođeš, to bolje. – The earlier you come, the better.

Why is što used here? I thought što meant what.

Yes, što usually means what, but in this fixed pattern Što X-er, to Y-er it has a different, idiomatic function: it works like “the” in English “the more…, the less…”.

So:

Što manje spavam ≈ The less I sleep
Što više učim ≈ The more I study

Outside this construction, što keeps its usual meanings (what, that which, why in some dialects), but here it is just part of the correlative structure and is not translated as what.

What does to mean in to sam nervozniji? Is it the pronoun that/it?

In this context, to is also part of the fixed pattern Što X-er, to Y-er and does not really mean that/it in the usual sense.

You can think of:

Što manje spavam, to sam nervozniji…
as a whole equivalent of
The less I sleep, the more nervous I am…

So:

što – starts the first comparative clause (Što manje spavam)
to – starts the second comparative clause (to sam nervozniji)

If you took this sentence out of the pattern, to could mean that/it, but here you should mainly see it as a grammatical marker of the second half of the comparison.

Why is the word order to sam nervozniji and not sam to nervozniji?

Because sam is a clitic (an unstressed short word) and Croatian has strict rules about clitic placement.

Key points:

sam is the 1st person singular of biti (to be) and behaves as a clitic.
Clitics normally go in second position in their clause, after the first stressed word or phrase.

In to sam nervozniji:

to is the first stressed element of the clause.
sam comes right after it, in second position.
Then comes the rest: nervozniji.

So:

to sam nervozniji – correct and natural
sam to nervozniji – wrong word order, because the clitic sam is not in second position.

Why is there no ja in the sentence? How do we know it means I?

Croatian is a pro-drop language: subject pronouns (like ja, ti, on) are normally omitted because the verb ending already shows the person and number.

In spavam, sam, koncentriram:

-m endings show 1st person singular → I sleep, I am, I concentrate.

You can add ja for emphasis or contrast:

Što manje ja spavam, to sam nervozniji… – I in particular, when I sleep less…

But in neutral sentences, leaving ja out is more natural:

Što manje spavam, to sam nervozniji…

What exactly is nervozniji, and how is it formed?

Nervozniji is the comparative form of the adjective nervozan (nervous).

Pattern:

nervozan – nervous
nervozniji – more nervous

In general, Croatian forms comparatives of many adjectives with -iji / -ji:

brz → brži – fast → faster
pametan → pametniji – smart → smarter
skup → skuplji – expensive → more expensive

So to sam nervozniji literally means I am more nervous (than before / when I sleep more).

What does teže mean here?

Here teže is the comparative adverb of teško (with difficulty, hard).

teško se koncentriram – I concentrate with difficulty / I have a hard time concentrating
teže se koncentriram – I concentrate with more difficulty / it is harder for me to concentrate

Be aware that težak (heavy, difficult) also has forms like:

teži (heavier / more difficult – adjective)
teže (comparative adverb: more difficultly / harder, or neuter plural adjective form in other contexts)

In this sentence it is clearly the adverb: I concentrate harder / with more difficulty, not heavier in the physical sense.

Why is it teže se koncentriram and not se teže koncentriram?

Because se is also a clitic, and like sam, it prefers the second position in the clause.

In teže se koncentriram:

teže is the first stressed word.
se goes right after it (second position).
Then comes koncentriram.

So:

teže se koncentriram – neutral and preferred.
se teže koncentriram – sounds wrong in standard Croatian; se shouldn’t stand first.
koncentriram se teže – grammatically possible, but changes the rhythm and slightly emphasizes teže at the end, and it’s not what you normally say in this pattern.

For the neutral sentence corresponding to the English here, teže se koncentriram is the natural choice.

Why is koncentriram in the present tense if we’re talking about a general tendency?

Croatian uses the present tense for:

Actions happening now, and
General truths / habits / tendencies, just like English does.

So:

Što manje spavam, to sam nervozniji i teže se koncentriram.
→ expresses a general rule about you, not just at this moment.

Other examples:

Kad sam umoran, sporije razmišljam. – When I am tired, I think more slowly.
Ako puno pijem kavu, lošije spavam. – If I drink a lot of coffee, I sleep worse.

So the present tense here naturally covers the idea of whenever this happens, this is what tends to follow.

Can I drop to and say Što manje spavam, nervozniji sam i teže se koncentriram?

Yes, you can say:

Što manje spavam, nervozniji sam i teže se koncentriram.

It is grammatically correct and understandable. However:

With to (Što manje spavam, to sam nervozniji…) the sentence uses the standard correlative pattern, which sounds very natural and slightly more formal/emphatic.
Without to, it becomes more like “The less I sleep, I am more nervous…”, which is still fine, but stylistically a bit plainer.

In everyday speech you will hear both, but the Što X, to Y form is a very common and idiomatic way to express this kind of proportional relationship.