J’ai sommeil, donc je vais dormir.

In French, sommeil is used with avoir, not être.

French often says to have for physical states where English uses to be. So:

• j’ai faim = I am hungry
• j’ai soif = I am thirsty
• j’ai sommeil = I am sleepy

So je suis sommeil is not correct French.

This is because of elision.

When je comes before a word starting with a vowel sound, the e drops and is replaced by an apostrophe:

• je ai → j’ai
• je aime → j’aime
• je habite → j’habite

This makes pronunciation smoother.

Here, sommeil refers to sleepiness or the need for sleep, not sleep as an action.

So j’ai sommeil means you feel sleepy.

It is different from:

• dormir = to sleep
• le sommeil = sleep, the state of sleeping, or sleepiness depending on context

In this sentence, avoir sommeil is a fixed expression meaning to be sleepy.

They are similar, but not identical.

• j’ai sommeil = I feel sleepy / I want to sleep
• je suis fatigué = I am tired

You can be fatigué without being ready to fall asleep, and you can have sommeil specifically because you need sleep.

So j’ai sommeil is more precise here.

Donc means so, therefore, or thus.

It connects the two ideas:

• J’ai sommeil = first idea
• je vais dormir = result or consequence

So donc shows logic: because I’m sleepy, I’m going to sleep.

It is a very common connector in French.

Because donc joins two clauses, and each clause needs its own subject.

So French says:

• J’ai sommeil, donc je vais dormir.

Not:

• J’ai sommeil, donc vais dormir.

In standard French, the subject pronoun je must be stated again.

Je vais dormir is the near future: aller + infinitive.

It is very common in everyday spoken French and often feels natural for an immediate plan or intention.

• je vais dormir = I’m going to sleep
• je dormirai = I will sleep

Both are grammatical, but je vais dormir sounds especially natural when someone is about to go to bed soon.

Because French uses aller + infinitive directly.

So:

• je vais dormir
• tu vas manger
• nous allons partir

There is no extra word equivalent to English to between vais and dormir in this structure.

A simple pronunciation guide is:

zhay so-may, donk zhuh vay dor-meer

A few details:

• j’ai sounds like zhay
• sommeil sounds roughly like so-may
• donc is usually pronounced with a nasal vowel; in careful speech the final c may be heard, but often it is not strongly pronounced depending on accent and context
• je is a soft zhuh
• vais sounds like vay
• dormir sounds like dor-meer

The final r in dormir is pronounced, unlike many final consonants in French.

Yes, and it is slightly different.

• je vais dormir = I’m going to sleep
• je vais me coucher = I’m going to go to bed

These are close, but not exactly the same:

• me coucher focuses on going to bed
• dormir focuses on sleeping

In real life, both can work depending on what you want to emphasize.

It is normal and recommended here.

The comma helps separate the cause from the result:

• J’ai sommeil, donc je vais dormir.

In informal writing, people may sometimes leave punctuation out, but in standard writing the comma is a good choice.

Yes. It is completely natural French.

A native speaker might say it in everyday conversation when they feel sleepy. Other common possibilities are:

• Je suis fatigué = I’m tired
• J’ai envie de dormir = I feel like sleeping
• Je tombe de sommeil = I’m बेहद sleepy / I’m falling asleep

But j’ai sommeil is standard, common, and very useful to know.