asita ha sihatu ni noru node, konya ha hayaku nenai wake ni ha ikimasen.

始発 means the first departure of the day, and in this sentence it most naturally means the first train.

So 始発に乗る means to take / catch the first train.

A helpful nuance:

• 始発 can refer to the first bus, train, etc.
• In everyday Japanese, when no vehicle is stated, people often understand it from context.

The verb 乗る（のる） uses に for the thing you board or ride.

So:

• 電車に乗る = ride a train
• バスに乗る = ride a bus
• 始発に乗る = catch the first train

This is just the normal pattern for 乗る.

The は marks 明日 as the topic: as for tomorrow or tomorrow in the context of what follows.

So:

• 明日は始発に乗るので… = Since tomorrow I’m taking the first train…

It is not necessarily adding strong contrast by itself, though topic は can sometimes feel contrastive depending on context.

This second は marks 今夜 as the topic: as for tonight.

The sentence is structured like this:

• As for tomorrow, I’m taking the first train
• so as for tonight, I can’t not go to bed early

Using は twice helps set up a contrast between tomorrow and tonight:

• tomorrow = the reason
• tonight = the action that follows from that reason

ので means because / since, and it gives the reason for the second part of the sentence.

So:

• 始発に乗るので = because I’m taking the first train

Compared with から:

• ので often sounds a bit softer, more explanatory, and more objective
• から can sound more direct or assertive

In this sentence, ので fits well because the speaker is calmly explaining the reason for needing to sleep early.

寝ないわけにはいきません means I can’t very well not go to bed early or more naturally I have to go to bed early.

This pattern is used when the speaker feels there is some circumstance, logic, duty, or social pressure that makes not doing something impossible.

So the nuance is stronger than just:

• 早く寝ます = I’ll go to bed early

It suggests:

• Given the situation, I really have to
• I don’t have much choice

That matches the first clause well:

• because I’m taking the first train tomorrow, I have to sleep early tonight

This is a set grammar pattern:

dictionary form of verb → verbないわけにはいかない

Meaning:

• cannot avoid doing
• have no choice but to do
• must do

In this sentence:

• base verb: 寝る
• negative form: 寝ない
• pattern added: 寝ないわけにはいきません

So literally:

• it won’t do to not sleep early
• I can’t go without sleeping early
• natural English: I have to go to bed early

It is usually written as:

• Vないわけにはいかない
• polite: Vないわけにはいきません

Because the structure is built around the idea cannot not do something.

So:

• 寝ない = not sleep
• 寝ないわけにはいかない = it’s not acceptable / not possible to not sleep

That double-negative logic produces a positive obligation:

• I have to sleep

This is very common in Japanese grammar. The surface form is negative, but the overall meaning is must / have to.

In this pattern, わけ does not mean a simple concrete reason in the way beginners often learn first.

Here, わけにはいかない is best learned as a fixed expression. The whole pattern means:

• cannot do
• cannot very well do
• it wouldn’t be appropriate/possible to do

So while わけ can relate to ideas like reason, circumstance, or sense, in this sentence it is better not to translate it word by word. Treat ないわけにはいかない as one grammar unit.

In this grammar pattern, いく does not carry its usual literal meaning to go.

わけにはいかない is a fixed expression, so いかない should not be translated literally here.

So:

• 寝ないわけにはいきません does not mean anything like it does not go to not sleep
• it simply means I have to sleep / I can’t avoid sleeping

The いきません form is just the polite present-negative style of いかない.

Here it means early.

早い can relate to time, so:

• 早く寝る = go to bed early

In other contexts, 早く can sometimes feel like quickly, but with 寝る the natural meaning is clearly early, not quickly go to sleep.

It expresses obligation based on circumstances, not just a casual personal choice.

The nuance is:

• Because I’m taking the first train tomorrow, I really need to go to bed early tonight.

So it sounds more like:

• I have to
• I don’t really have a choice than simply:
• I want to
• I decided to

That nuance comes mainly from 寝ないわけにはいきません.

Yes, that is a very natural translation.

Possible translations include:

• Since I’m taking the first train tomorrow, I have to go to bed early tonight.
• Because I’m catching the first train tomorrow, I need to go to bed early tonight.
• Since I’m taking the first train tomorrow, I can’t stay up tonight.

The exact English wording can vary, but the core idea is obligation caused by tomorrow’s early travel.

They are similar, because both can mean must / have to, but the nuance is a little different.

• 寝なければなりません = a straightforward obligation
• 寝ないわけにはいきません = I can’t avoid it; given the situation, I have no choice

So in this sentence:

• 寝なければなりません would sound like a plain statement of necessity
• 寝ないわけにはいきません sounds a bit more like well, under the circumstances, I really have to

That makes the original sentence feel slightly more natural and expressive.