בקיץ יש לפעמים לחות גבוהה גם בלילה, ולכן קשה לי לישון.

In Hebrew, several very common prepositions are written as prefixes, attached directly to the next word. One of them is ב־, which usually means in, at, or during.

So בקיץ is ב־ + קיץ. In context, it means in summer or in the summer.

A useful extra point: with words that are definite, Hebrew often absorbs the ה־ article after ב־, ל־, and כ־. So ב + הקיץ becomes בקיץ.

Yes, the same prefix rule is happening here. בלילה is ב־ + לילה, and in context it means at night.

Grammatically, it is often understood as ב + הלילה, with the article absorbed. Hebrew often uses this kind of singular time expression in a general way, so בלילה can mean at night in general, not just on one specific night.

יש is the standard Hebrew word for there is / there are. It is used to express existence.

So יש לפעמים לחות גבוהה literally means something like there is sometimes high humidity.

Unlike English, יש does not change for singular vs. plural. The same word works for both.

לפעמים means sometimes, and here it modifies the whole idea of existence: there is sometimes high humidity.

That placement is very natural in Hebrew. It comes right after יש to show that the existence of this condition is occasional.

Hebrew adverb placement can be somewhat flexible, but יש לפעמים... is a common and smooth order.

Because Hebrew adjectives must agree with the nouns they describe in gender and number.

לחות is a feminine singular noun, so the adjective must also be feminine singular: גבוהה.

A detail that can confuse learners: even though לחות ends in ־ות, it is not plural here. It is a singular abstract noun meaning humidity.

In Hebrew, adjectives normally come after the noun they describe.

So:

• לחות גבוהה = high humidity
• literally: humidity high

This is the normal Hebrew word order for noun + adjective combinations.

גם usually means also, too, or sometimes even, depending on context.

In גם בלילה, it means that the statement is true at night as well. In natural English, this is often best understood as even at night or also at night.

Its position before בלילה shows that nighttime is the part being added or emphasized.

ולכן means and therefore, so, or that’s why. It introduces a result.

So the sentence structure is:

• situation: there is sometimes high humidity, even at night
• result: ולכן קשה לי לישון = therefore it’s hard for me to sleep

This is different from כי, which usually introduces a cause and means because. So כי points backward to the reason, while ולכן points forward to the result.

קשה לי לישון is a very common Hebrew pattern:
adjective + ל־ person + infinitive

Literally, it means hard for me to sleep. That is how Hebrew often expresses ideas like I have trouble sleeping.

You can say אני מתקשה לישון, and that also means I have difficulty sleeping, but קשה לי לישון is extremely natural and everyday.

לי means to me or for me. In this kind of structure, it marks the person who experiences the difficulty.

So:

• קשה לי = it is hard for me
• קשה לו = it is hard for him
• קשה להם = it is hard for them

English usually makes that person the subject: I have a hard time... Hebrew often does not.

Because לישון is the infinitive form of the verb, meaning to sleep.

In Hebrew, infinitives usually begin with ל־, so this prefix often corresponds to English to:

• לישון = to sleep
• לאכול = to eat
• ללכת = to go / to walk

After expressions like קשה לי, Hebrew normally uses an infinitive: קשה לי לישון.

You can think of it that way in English, but Hebrew does not need to say it explicitly here.

קשה לי לישון already works as a complete clause meaning it is hard for me to sleep. Hebrew often leaves out the equivalent of English it in this kind of impersonal sentence.

So there is no missing word that must be supplied; the sentence is complete as written.