אתמול נרדמתי מאוחר, ולכן היה לי קשה להירדם שוב אחרי הסיוט.

נרדמתי means I fell asleep. It refers to the moment or process of entering sleep.

That is different from:

• ישנתי = I slept / I was sleeping
• הלכתי לישון = I went to sleep / I went to bed to sleep

So in this sentence, אתמול נרדמתי מאוחר means that the speaker did not fall asleep until late, not just that they slept late.

They are two forms of the same verb: להירדם = to fall asleep.

• נרדמתי = I fell asleep
• להירדם = to fall asleep (infinitive)

So the sentence first says:

• אתמול נרדמתי מאוחר = Yesterday I fell asleep late

and then:

• היה לי קשה להירדם שוב = it was hard for me to fall asleep again

English does this too:
I fell asleep late, so it was hard to fall asleep again.

The root is ר-ד-ם, which is related to sleep/falling asleep.

This verb is in the נפעל pattern:

• past: נרדמתי
• infinitive: להירדם

For many learners, the important thing is to recognize it as a standard dictionary form:

• להירדם = to fall asleep

You do not need to fully master binyanim right away to understand the sentence, but it helps explain why the forms look the way they do.

In Hebrew, מאוחר can work very naturally as an adverb meaning late.

So:

• נרדמתי מאוחר = I fell asleep late

This is normal and idiomatic.

You could add more detail if you wanted:

• נרדמתי מאוחר בלילה = I fell asleep late at night

But מאוחר by itself is perfectly fine.

ולכן means and therefore, and so, or which is why.

It connects the first clause to the result:

• אתמול נרדמתי מאוחר, ולכן היה לי קשה...
• Yesterday I fell asleep late, and therefore / so it was hard for me...

It is very common and completely natural. It can sound a little more structured than just אז in some contexts, but it is not overly formal.

Compare:

• ולכן = therefore / so
• אז = so / then

Both can appear in everyday Hebrew, but ולכן often sounds a bit more explicit and logical.

Literally, היה לי קשה is something like:

• was to-me difficult

In natural English, that becomes:

• it was hard for me

This is a very common Hebrew structure:

• היה לי קשה להבין = it was hard for me to understand
• היה לי קל לזכור = it was easy for me to remember

So in your sentence:

• היה לי קשה להירדם שוב
• It was hard for me to fall asleep again

Hebrew often uses this was to me hard structure where English uses it was hard for me.

Because קשה here does not describe the speaker as a person. It describes the situation.

• הייתי קשה would mean something like I was difficult / harsh / hard, describing me
• היה לי קשה means it was hard for me, describing my experience

So the Hebrew is not saying I was difficult. It is saying that the act of falling asleep again was difficult for the speaker.

Yes. Both are possible:

• היה לי קשה להירדם שוב
• היה קשה לי להירדם שוב

Both mean the same thing: it was hard for me to fall asleep again.

The version in your sentence, היה לי קשה, is very common and neutral.
The alternative, היה קשה לי, may put slightly more emphasis on hard or on for me, depending on context and intonation.

שוב means again.

So:

• להירדם שוב = to fall asleep again

This placement is natural in Hebrew. In many cases, adverbs like שוב can move around a bit, but this order is very common and clear.

Compare:

• קשה להירדם שוב = hard to fall asleep again
• שוב היה לי קשה להירדם = again, it was hard for me to fall asleep

Those do not mean exactly the same thing in emphasis, so the word order matters somewhat. In your sentence, שוב specifically goes with להירדם.

הסיוט means the nightmare.

Hebrew often uses the definite article when referring to a specific, known event. Here, the speaker is talking about the nightmare they just had, so the nightmare is natural.

• אחרי הסיוט = after the nightmare

If you said אחרי סיוט, that would sound more general, more like after a nightmare.

So the definite form fits the situation better.

Because אחרי is a preposition meaning after. The definite article ה־ is attached to the noun, not the preposition.

So:

• אחרי = after
• הסיוט = the nightmare

Together:

• אחרי הסיוט = after the nightmare

This is normal Hebrew structure.

Yes. Hebrew often places time expressions near the beginning of the sentence.

• אתמול נרדמתי מאוחר = Yesterday I fell asleep late

This is very natural word order. Hebrew is somewhat flexible, but starting with a time word like אתמול, היום, מחר, בלילה, etc. is extremely common.

Yes, in a sense. English requires it:

• It was hard for me to fall asleep again

Hebrew usually does not need an explicit dummy subject like English it in this kind of sentence. So:

• היה לי קשה להירדם שוב

works without any extra word corresponding to English it.

This is a common difference between the two languages.

Yes, very natural.

It sounds like something a native speaker might say when describing poor sleep:

• אתמול נרדמתי מאוחר, ולכן היה לי קשה להירדם שוב אחרי הסיוט.

A speaker might also choose slightly different wording in casual conversation, but this version is fully normal, clear, and idiomatic.