sakuya ha gussuri reta node, kesa ha kibun ga yokatta.

ぐっすり means soundly, deeply, or a good, solid amount when talking about sleep.

It is an adverb, so it describes how the sleeping happened.

• ぐっすり眠る = to sleep soundly
• ぐっすり眠れた = was able to sleep soundly / slept well

It is a very common word with sleep, and it gives the sentence a natural, everyday feeling.

眠れた is the potential past form of 眠る, so literally it means was able to sleep.

• 眠った = slept
• 眠れた = could sleep / was able to sleep

In English, both may be translated simply as slept, but in Japanese the choice matters.

Here, 眠れた suggests:

• sleep happened successfully
• the speaker had no trouble sleeping
• this successful sleep is relevant to why they felt good in the morning

So ぐっすり眠れた is very natural as I slept soundly or more literally I was able to sleep soundly.

It comes from the verb 眠る.

The steps are:

• dictionary form: 眠る
• potential form: 眠れる
• past tense: 眠れた

So:

• 眠る = to sleep
• 眠れる = can sleep
• 眠れた = could sleep / was able to sleep

This pattern is common for many -u verbs.

The は marks 昨夜 as the topic of the first part of the sentence.

• 昨夜はぐっすり眠れた = As for last night, I slept soundly

This does not mean last night is the subject in the English sense. It is just setting the scene or frame.

It also helps create a contrast with the next phrase:

• 昨夜は ...
• 今朝は ...

So the sentence has a nice structure:

• As for last night → slept well
• As for this morning → felt good

Again, は marks 今朝 as the topic of that clause.

• 今朝は気分がよかった = As for this morning, my mood/feeling was good

Using は with both 昨夜 and 今朝 creates a natural comparison between two time frames:

• last night
• this morning

This is very common in Japanese when talking about different times or situations.

In this sentence, 今朝は is already the topic, so 気分 is treated as the thing being described within that topic.

• 今朝は = as for this morning
• 気分がよかった = my feeling/mood was good

The が marks 気分 as the subject of よかった.

So the structure is roughly:

• As for this morning, my mood felt good

If you said 気分はよかった, that would shift the focus and create more contrast, like:

• As for my mood, it was good
  possibly implying something else was not good.

So 気分がよかった is the most neutral and natural choice here.

ので means because or since.

It connects the reason and the result:

• 昨夜はぐっすり眠れたので = because I slept soundly last night
• 今朝は気分がよかった = I felt good this morning

So the whole sentence means:

• Because I slept soundly last night, I felt good this morning.

Compared with から, ので often sounds:

• a bit softer
• a bit more explanatory
• slightly more formal or gentle

Yes, you could say:

• 昨夜はぐっすり眠れたから、今朝は気分がよかった。

This is also natural.

The difference is mostly nuance:

• ので = softer, more explanatory, a little gentler
• から = more direct, more straightforward

In everyday speech, both are common. In this sentence, ので sounds very natural and smooth.

Yes, literally it is something like my mood/feeling was good.

But in natural English, depending on context, it can mean:

• I felt good
• I felt refreshed
• I was in a good mood
• I felt well

Here, because the cause is sleeping well, I felt good this morning or I felt refreshed this morning is probably the most natural interpretation.

So 気分 is broader than just mood. It can refer to:

• emotional state
• physical feeling
• general sense of well-being

Because the sentence is talking about this morning, which is a completed time in the past.

• よい / いい = good
• よかった = was good

So:

• 今朝は気分がよかった = My feeling was good this morning / I felt good this morning

Even though this morning may feel recent in English, Japanese still normally uses the past tense because that time period has already happened.

This is because いい has an irregular pattern.

Although the dictionary form is usually いい, its inflected forms come from よい:

• いい / よい = good
• よかった = was good
• よくない = not good
• よくなかった = was not good

So 気分がいい becomes 気分がよかった in the past.

Yes. Japanese often omits subjects when they are obvious from context.

A natural understood subject here is probably I:

• 昨夜はぐっすり眠れたので、今朝は気分がよかった。
• Because I slept soundly last night, I felt good this morning.

But Japanese does not need to say 私は if the context already makes it clear.

This is very normal. In fact, adding 私は here could sound unnecessary unless you specifically want contrast, such as:

• I felt good, unlike someone else

Yes. All of these can refer to last night, but the nuance is a little different.

• 昨夜 = last night, slightly written or neutral
• 昨晩 = last night, very common and natural in speech
• 昨日の夜 = yesterday night / last night, very clear and straightforward

So you might also hear:

• 昨晩はぐっすり眠れたので、今朝は気分がよかった。

That is perfectly natural too.

The sentence has two main parts:

1. 昨夜はぐっすり眠れたので

   • 昨夜は = as for last night
   • ぐっすり = soundly
   • 眠れた = was able to sleep / slept
   • ので = because

2. 今朝は気分がよかった

   • 今朝は = as for this morning
   • 気分が = my feeling/mood
   • よかった = was good

So the pattern is:

• [reason] ので、[result]

This is a very common and useful structure in Japanese.

No, it is not strictly necessary, but it is very natural.

The comma helps separate:

• the reason part
• the result part

So this is easy to read:

• 昨夜はぐっすり眠れたので、今朝は気分がよかった。

Without the comma, it is still understandable:

• 昨夜はぐっすり眠れたので今朝は気分がよかった。

Japanese punctuation is often more flexible than English punctuation.

Sometimes, but the nuance changes.

• 気分がよかった = I felt good / refreshed / in a good mood
• 元気だった = I was energetic / well / lively

After a good night’s sleep, 気分がよかった focuses more on how the speaker felt internally.
元気だった would emphasize energy or physical liveliness more.

So both can make sense, but 気分がよかった fits especially well if the speaker means they felt pleasant or refreshed.

It is basically neutral plain style.

• 眠れた
• よかった

These are plain forms, not polite -ます forms. That makes the sentence suitable for:

• casual conversation
• diary writing
• narration
• informal written Japanese

A polite version would be:

• 昨夜はぐっすり眠れたので、今朝は気分がよかったです。

Or even more consistently polite:

• 昨夜はぐっすり眠れましたので、今朝は気分がよかったです。

Some natural options are:

• I slept soundly last night, so I felt good this morning.
• Because I slept really well last night, I felt good this morning.
• I got a good night’s sleep, so I felt great this morning.
• Since I slept well last night, I felt refreshed this morning.

The best choice depends on tone, but all of these capture the Japanese naturally.