zutuu ga hidoi node, kyou ha hayame ni nemasu.

頭痛がひどい is the natural way to say that the headache is severe.

Here, が marks 頭痛 as the thing being described by ひどい. In other words, the sentence is saying the headache is severe.

If you used 頭痛はひどい, it would sound more contrastive or topic-based, as if you were specifically talking about the headache as a topic, maybe in contrast with something else. For a simple description, 頭痛がひどい is more natural.

In this sentence, ひどい means something like severe, bad, or terrible.

So 頭痛がひどい means the headache is not just present, but quite bad.
This is a very common way to describe symptoms:

• せきがひどい = the cough is bad
• 風邪がひどい = the cold is severe

Both ので and から can mean because, but they feel a little different.

• ので sounds softer, more explanatory, and often a bit more polite
• から can sound more direct and straightforward

In this sentence, 頭痛がひどいので、今日は早めに寝ます sounds natural and polite, like giving a reason for your action.

Compare:

• 頭痛がひどいので、今日は早めに寝ます。
  Softer, explanatory
• 頭痛がひどいから、今日は早めに寝ます。
  More direct, still natural in casual speech

No だ is needed here.

ひどい is an i-adjective, and i-adjectives connect directly to ので:

• 頭痛がひどいので

If it were a na-adjective or a noun, then you would need な before ので:

• 静かなので
• 病気なので

So this sentence is correct as written because ひどい is an i-adjective.

は marks 今日 as the topic, giving the sense of as for today or today, at least.

So 今日は helps frame the sentence like this:

• As for today, I’ll go to bed a little early

Without は, 今日 would just function more like a plain time expression. That can also happen in Japanese, but 今日は sounds very natural here because the speaker is talking about their plan for today specifically.

早めに means a little early or earlier than usual.

It comes from 早め, which means somewhat early or on the early side. The に turns it into an adverb.

So:

• 早く寝ます = I go to bed early
• 早めに寝ます = I go to bed a bit earlier than usual

早めに often suggests adjustment relative to a normal schedule, which fits this sentence well.

Because 早め is functioning like a noun-like expression here, and に is used to make it adverbial.

So 早めに寝ます means go to bed early / earlier than usual.

This pattern is common:

• 多めに入れる = put in a little extra
• 少なめにする = make it a little less
• 早めに行く = go a little early

寝ます means to go to bed or to sleep, and it is the natural choice here.

眠ります focuses more on the act of sleeping itself and is less common in everyday sentences like this one.

So:

• 早めに寝ます = I’ll go to bed early / earlier than usual
• 早めに眠ります is grammatically possible, but less natural in this context

When talking about your bedtime routine, 寝る is usually the better verb.

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

In this sentence, it is obvious that the speaker is talking about their own condition and plan:

• the speaker has a bad headache
• the speaker will go to bed early

So 私は is unnecessary. If you added it, the sentence would still be correct:

• 私は頭痛がひどいので、今日は早めに寝ます。

But in normal conversation, leaving it out sounds more natural.

Yes. The sentence is polite because it ends with 寝ます, the polite ます form, and ので also has a somewhat polite, gentle tone.

So this would sound appropriate in many situations, such as:

• talking to a coworker
• explaining yourself to a teacher
• sending a polite message

A more casual version might be:

• 頭痛がひどいから、今日は早めに寝る。

頭痛 is pronounced ずつう.

This can be surprising because 頭 is often read あたま by itself, but in compounds it can have different readings. Here:

• 頭痛 = ずつう

This is the standard word for headache.

Not exactly.

早めに寝ます does not mainly mean soon. It means earlier than usual or on the early side.

So the emphasis is on being earlier than the normal bedtime, not simply on doing it in the near future.

If you wanted to say I’m going to sleep soon, you would use something more like:

• もうすぐ寝ます
• そろそろ寝ます

Yes, very common.

This pattern is extremely useful:

• reason + ので
• condition + ので
• situation + ので

In this sentence:

• 頭痛がひどいので = because my headache is severe
• 今日は早めに寝ます = I’ll go to bed earlier today

So the whole structure is a standard way to give a reason for what you are about to do.