asita ha siken ga aru node, konya ha hayame ni nerukoto ni simasu.

は marks the topic or sets the scene.

In this sentence:

• 明日は = as for tomorrow / tomorrow
• 今夜は = as for tonight / tonight

Using は here helps contrast the two time expressions:

• Tomorrow there is an exam,
• so tonight I’ll go to bed early.

This kind of contrast is very natural in Japanese. English often does this just through word order and context, but Japanese often uses は to frame each part clearly.

試験がある means there is an exam or we have an exam.

Japanese often uses ある for events, appointments, classes, meetings, tests, and so on:

• 会議がある = there is a meeting
• 授業がある = there is class
• 試験がある = there is an exam

If you said 試験です, that would mean something more like it is an exam or it’s the exam, which is a different kind of statement and does not naturally express having an exam scheduled.

So 試験がある is the normal way to say there’s an exam tomorrow.

In 試験がある, が marks the thing that exists.

The pattern is:

• Noun + が + ある = there is/are noun

So:

• 試験がある = an exam exists / there is an exam

This is just how ある usually works. The exam is the thing being presented as existing, so が is natural here.

ので means because / since, and it gives a reason.

So:

• 試験があるので = because there is an exam

Compared with から, ので usually sounds a bit softer, more explanatory, and a little more formal or polite in tone.

Compare:

• 試験があるので、今夜は早めに寝ることにします。
• 試験があるから、今夜は早めに寝ることにします。

Both are correct.
But ので feels slightly calmer and more matter-of-fact, while から can sound a bit more direct.

Before ので, Japanese often uses the plain form:

• あるので
• 行くので
• 忙しいので

Then the whole sentence can still end politely:

• 寝ることにします

This mix is completely normal:

• plain form in the reason clause
• polite form at the end

You can also say ありますので, and that sounds more formal. But あるので ... します is very common in everyday polite speech.

早め means relatively early, a bit early, or earlier than usual.

It is different from just 早く:

• 早く寝る = go to bed early
• 早めに寝る = go to bed a little earlier than usual / on the early side

So 早めに often suggests adjustment relative to a normal routine.

In this sentence, it sounds like:

• I’ll go to bed somewhat earlier than I normally do, because of the exam tomorrow.

早め behaves like a noun-like expression here, and に turns it into an adverbial phrase.

So:

• 早めに寝る = to sleep early / a bit earlier than usual

This is similar to other expressions like:

• 静かに話す = speak quietly
• きれいに書く = write neatly

You do not usually say 早め寝る. The に is needed.

寝ます simply means I go to bed / I will go to bed.

But 寝ることにします means I decide to go to bed or I’ll make it a point to go to bed.

The pattern is:

• dictionary form + ことにする
• dictionary form + ことにします (polite)

It expresses a decision, choice, or resolution.

So the sentence is not just stating the action. It emphasizes the speaker’s decision:

• Because there’s an exam tomorrow, I’ve decided to go to bed early tonight.

That decision nuance is important.

ことにする means to decide to do something.

Examples:

• 毎日運動することにする。 = I’ll decide to exercise every day.
• 今日は電車で行くことにする。 = I’ll decide to go by train today.

In your sentence:

• 寝ることにします = I decide to go to bed

It can sound like:

• a decision made right now,
• or a deliberate choice based on the situation.

It is often used when the speaker is choosing among options or setting a plan.

Because ことにする attaches to the plain dictionary form of the verb.

Pattern:

• 行くことにする
• 食べることにする
• 寝ることにする

You do not say:

• 寝ますことにします ✘

The polite part comes at the end with します.

So the structure is:

• 寝る = plain verb
• ことにします = polite ending of the grammar pattern

They are similar, but not exactly the same.

• 寝ることにします = I decide to go to bed
• 寝ようと思います = I think I’ll go to bed / I’m thinking of going to bed

Nuance:

• ことにします sounds more like a definite decision
• ようと思います sounds more like an intention or thought

In this sentence, ことにします works well because the speaker is making a practical decision based on tomorrow’s exam.

Japanese often leaves out the subject when it is clear from context.

In this sentence, the subject is understood as I:

• (I) have an exam tomorrow, so (I) will decide to go to bed early tonight.

Because 寝ることにします is the speaker’s own decision, it naturally points to I unless context suggests otherwise.

Omitting obvious subjects is extremely common in Japanese.

Japanese non-past forms often cover both present and future.

So します can mean:

• do
• will do

Here it refers to a future action/decision for tonight.

This is very normal in Japanese:

• 明日行きます = I will go tomorrow
• 今夜早めに寝ます = I will go to bed early tonight

So 寝ることにします naturally means I’ll decide to go to bed early tonight or more naturally in English, I’ve decided to go to bed early tonight.

Yes, you can.

• 早く寝ることにします = I’ll decide to go to bed early
• 早めに寝ることにします = I’ll decide to go to bed a little earlier than usual

The difference is nuance:

• 早く is a general early
• 早めに suggests earlier than normal or on the early side

Both are natural, but 早めに sounds especially good when the speaker is adjusting their normal schedule because of the exam.

The comma after ので is not grammatically required, but it is helpful.

It shows the break between:

• the reason: 明日は試験があるので
• the result/decision: 今夜は早めに寝ることにします

Japanese punctuation is somewhat flexible, especially in casual writing, but this comma makes the sentence easier to read.

Time words often do not need に, especially with common expressions like:

• 今日
• 明日
• 昨日
• 今夜
• 毎日

So:

• 明日は試験がある
• 今夜は早めに寝る

are completely natural.

Also, in this sentence, は is being used to mark those time expressions as topics/contrast points, so に is not needed.

It is polite everyday Japanese.

Why:

• あるので uses the plain form inside the reason clause
• します gives a polite sentence ending

This is a very common style in spoken and written Japanese: not stiffly formal, but still polite.

A casual version might be:

• 明日は試験があるので、今夜は早めに寝ることにする。

A more casual, conversational version could also be:

• 明日試験があるから、今夜は早めに寝る。

So your original sentence sounds natural and polite.