Увлажнитель работает почти всю ночь, и утром мне легче дышать.

Because увлажнитель is the subject of the sentence — it is the thing that works / runs.

• увлажнитель = humidifier
• работает = works / is running

So увлажнитель работает means the humidifier is running.

Yes, работать can mean to work for a person, but it is also very commonly used for machines, devices, systems, and appliances.

So Russian speakers naturally say things like:

• телефон не работает = the phone isn’t working
• лифт работает = the elevator is working
• увлажнитель работает = the humidifier is running / working

For appliances, работает often sounds more natural than a literal English-style verb like functions.

Because всю ночь expresses duration of time: for the whole night / all night.
In Russian, duration without a preposition is often shown with the accusative case.

• вся ночь = the whole night (nominative)
• всю ночь = all night / for the whole night (accusative)

So:

• Он спал всю ночь. = He slept all night.
• Увлажнитель работает почти всю ночь. = The humidifier runs almost all night.

The word почти (almost) modifies the whole phrase всю ночь.

Because ночь is a feminine noun whose accusative singular looks the same as the nominative singular:

• nominative: ночь
• accusative: ночь

But the adjective весь does change:

• feminine nominative: вся
• feminine accusative: всю

So the phrase becomes:

• вся ночь = the whole night
• всю ночь = all night / for the whole night

This is very normal in Russian: sometimes the adjective shows the case change more clearly than the noun.

Утром means in the morning.

It is the instrumental case of утро, but in this kind of expression it functions like an adverb of time:

• утром = in the morning
• вечером = in the evening
• днём = during the day
• ночью = at night

So утром is a fixed, very common way to say in the morning.
You generally would not say в утро here.

This is a very important Russian pattern.

Russian often expresses feelings and states with:

• dative pronoun/person
• plus a state word such as легко, трудно, холодно, жарко, лучше, легче

So:

• мне легко = it is easy for me
• мне трудно = it is difficult for me
• мне легче = it is easier for me / I feel better

Then Russian adds the infinitive:

• мне легче дышать = it’s easier for me to breathe

This is more natural in Russian than я легче дышу, which would sound more like I breathe more lightly rather than it is easier for me to breathe.

Легче is the comparative form of легко / лёгкий, and here it means more easily / easier.

In this sentence:

• мне легче дышать = it is easier for me to breathe

The comparative often implies comparison with an earlier state, even if that comparison is not stated directly. Here the idea is:

• during the night or before using the humidifier, breathing is harder
• in the morning, breathing is easier

So легче works very naturally even without saying exactly than what.

Because it depends on the expression мне легче.

Russian often uses this structure:

• кому?
  • state/comparative
    • infinitive

Examples:

• Мне трудно понять. = It’s hard for me to understand.
• Ему легко говорить. = It’s easy for him to speak.
• Нам лучше уйти. = It’s better for us to leave.
• Мне легче дышать. = It’s easier for me to breathe.

So дышать stays in the infinitive because it names the action that is easy or easier.

Both are possible, but the word order changes the emphasis.

• Увлажнитель работает почти всю ночь...
  Neutral: the sentence starts with the subject.

• Почти всю ночь увлажнитель работает...
  This emphasizes the time span almost all night.

Russian word order is more flexible than English word order, but the default neutral order often puts the subject and verb early unless something else is being highlighted.

Because и here connects two full clauses:

1. Увлажнитель работает почти всю ночь
2. утром мне легче дышать

Each part has its own grammatical center:

• увлажнитель работает
• мне легче дышать (an impersonal predicate construction)

In Russian, when и joins two independent clauses, a comma is usually required.

Yes, very often:

• всю ночь
• целую ночь

Both can mean all night.

A small nuance:

• всю ночь is very common and neutral
• целую ночь can sometimes feel a bit more emphatic, like the entire night

In this sentence, почти всю ночь sounds especially natural.

Russian can say that, but the chosen version is more natural for expressing a state rather than a simple action.

Compare:

• утром мне легче дышать = in the morning, it is easier for me to breathe
• утром я дышу легче = in the morning, I breathe more easily

Both are possible, but the first version focuses on your physical condition or sensation. It sounds especially natural when talking about comfort, health, or how your body feels.

So мне легче дышать is a very idiomatic choice here.