sorosoro neru zikan dakedo, nikki dake ha kaite kara netai.

そろそろ means something like soon, before long, or it’s about time.

In this sentence, そろそろ寝る時間 means:

• it’s about time to sleep
• it’s getting to be bedtime

So そろそろ adds the feeling that the time has more or less arrived.

寝る時間 means the time to sleep or bedtime.

Here, 寝る is in its plain dictionary form because it is modifying the noun 時間. In Japanese, verbs in plain form can come before a noun and describe it.

So:

• 寝る時間 = the time when one sleeps / time to sleep
• 食べる時間 = time to eat
• 勉強する時間 = time to study

Why not 寝た時間?

• 寝た時間 would mean the time when someone slept or the time at which someone went to sleep, so it refers to a completed action in the past.

Why not 寝たい時間?

• 寝たい時間 would mean the time when I want to sleep, which is grammatically possible in some contexts, but that is not the normal way to say bedtime here.

So 寝る時間 is the natural expression.

だけど means but or though.

It connects the first idea and the second idea:

• そろそろ寝る時間だけど = It’s about time to sleep, but...
• 日記だけは書いてから寝たい = I want to write at least my diary before sleeping.

So the whole sentence has a contrast:

• It’s already bedtime
• but I still want to do one thing first

だけど is a casual spoken connector. More formal versions would be:

• ですが
• でも in some contexts

This is a very common thing learners ask about.

• だけ = only
• は = topic marker, but here it often adds contrast or emphasis

So 日記だけは does not just mean only the diary in a plain factual way. It has a nuance like:

• at least the diary
• the diary, at least
• the diary is the one thing I want to make sure I write

It suggests contrast with other possible things the speaker might do before bed.

For example, the feeling is something like:

• I may not do other things,
• but I do want to write my diary.

So だけは often carries an emphasized or contrastive at least this one thing nuance.

書いてから means after writing.

The pattern is:

• Vてから = after doing V

So:

• 書いてから寝たい = I want to sleep after writing
• more naturally in English: I want to write first, and then go to sleep

If you said only 書いて寝たい, it would sound less complete and less natural for expressing a clear sequence. Vてから specifically tells us the order:

1. write the diary
2. sleep

That ordering is important here.

Both can mean I want to sleep after writing, but the nuance is a little different.

• 書いてから寝たい is very common in everyday speech and strongly emphasizes sequence: write first, then sleep
• 書いたあとで寝たい also means after writing, I want to sleep, but it can sound a little more explicit or slightly more formal/descriptive depending on context

In this sentence, 書いてから寝たい is very natural because the speaker is talking about the next step in a simple sequence.

寝たい is the -tai form of 寝る, and it means want to sleep.

So the second part of the sentence is not just stating what will happen. It expresses the speaker’s desire:

• 書いてから寝たい = I want to go to sleep after writing

This is important because the speaker is talking about intention or preference, not just a schedule.

Compare:

• 寝る = sleep / will sleep
• 寝たい = want to sleep

Yes, the subject is omitted, which is very normal in Japanese.

From context, the subject is understood to be the speaker:

• (私は) そろそろ寝る時間だけど、日記だけは書いてから寝たい。
• (I) know it’s about time to sleep, but I want to write at least my diary before going to bed.

Japanese often leaves out subjects when they are obvious.

Also, because the sentence uses ～たい, it naturally sounds like the speaker talking about their own desire. Using ～たい for another person usually requires special context or a different phrasing.

The overall nuance is:

• It’s already getting late / it’s bedtime
• but there is one thing I still want to do before sleeping
• that one thing is writing my diary

Because of 日記だけは, it feels a little like:

• I may skip other things, but I at least want to write my diary
• Even if I’m tired, I want to make sure I write my diary first

So the tone is natural and personal, and it suggests a small sense of priority or determination.

This sentence is basically casual to neutral conversational Japanese.

Signs of that include:

• plain form 寝る
• だけど instead of more formal ですが
• plain-style ending 寝たい rather than 寝たいです

A more polite version could be:

• そろそろ寝る時間ですが、日記だけは書いてから寝たいです。

That version is more suitable for polite conversation, while the original sounds natural in everyday speech or inner thoughts.

A very natural translation would be:

• It’s about time for bed, but I want to at least write my diary before going to sleep.

Other natural possibilities are:

• It’s getting to be bedtime, but I want to write at least my diary before I go to sleep.
• It’s almost time to sleep, but I want to make sure I write my diary first.

The exact English depends on how strongly you want to bring out the nuance of だけは:

• only the diary
• at least my diary
• my diary, at least

In this sentence, at least my diary is usually the best fit.