Saya sahaja yang belum tidur.

Sahaja here means only or just.

So Saya sahaja means only me / I alone.

In everyday Malaysian Malay, sahaja is very often shortened to saja:

• Saya sahaja yang belum tidur.
• Saya saja yang belum tidur.

Both mean the same thing, but sahaja looks more formal or standard in writing.

Yang links saya sahaja to the description belum tidur.

A very natural way to understand it is:

• Saya sahaja = only me
• yang belum tidur = who has not slept yet

So the whole sentence works like:

I am the only one who hasn’t slept yet.

This is a very common Malay structure:

• Ali yang datang = Ali is the one who came
• Dia yang menang = He/She is the one who won

In your sentence, yang helps turn the second part into something like a relative clause: the one who...

Because belum means not yet, while tidak means a more general not.

• belum tidur = have not slept yet
• tidak tidur = do not sleep / did not sleep / are not sleeping, depending on context

Since the idea is that sleeping is still expected to happen later, belum is the natural choice.

So:

• Saya sahaja yang belum tidur = I’m the only one who hasn’t slept yet

If you used tidak, it would sound different in meaning.

Tidur can cover several related ideas in Malay, depending on context:

• to sleep
• to go to sleep
• to be asleep

In belum tidur, the most natural English translation is haven’t slept yet or aren’t asleep yet, depending on the situation.

For example:

• If everyone is supposed to be in bed, it may mean I’m the only one who isn’t asleep yet.
• If people are talking about whether they have gone to sleep at all, it may mean I’m the only one who hasn’t slept yet.

Malay often leaves this kind of distinction to context.

Both are possible.

• Saya sahaja yang belum tidur
• Hanya saya yang belum tidur

Both mean Only I am the one who hasn’t slept yet / I’m the only one who hasn’t slept yet.

Differences:

• sahaja / saja is extremely common in everyday speech
• hanya can sound a little more formal or emphatic

You may also hear:

• Cuma saya yang belum tidur

That also means roughly the same thing.

Because sahaja is modifying saya.

• Saya sahaja = only me

If you moved sahaja somewhere else, the meaning or naturalness would change.

For example:

• Saya sahaja yang belum tidur = Only I haven’t slept yet
• Saya yang belum tidur sahaja is not the natural way to express this meaning

So the placement matters: sahaja usually goes right after the word or phrase it limits.

Yes, you can think of an implied idea like orang or seorang.

The sentence can be understood as something like:

• Saya sahaja (orang) yang belum tidur
• Saya seorang sahaja yang belum tidur

That is why the English translation often becomes: I’m the only one who hasn’t slept yet.

Malay often leaves out nouns like this when they are obvious from context.

They are different in focus.

• Saya belum tidur lagi = I haven’t slept yet
• Saya sahaja yang belum tidur = I’m the only one who hasn’t slept yet

The first sentence only talks about you. The second sentence compares you with others and says you are the only one in that situation.

So sahaja and yang together add the idea of exclusivity.

Yes, you may hear:

• Saya sahaja yang belum tidur lagi

This is possible, but often belum already carries the idea of not yet, so lagi can feel optional or slightly emphatic.

Compare:

• belum tidur = not asleep / haven’t slept yet
• belum tidur lagi = still haven’t slept yet

In everyday speech, both can occur, depending on emphasis and regional habits.

Yes, in most everyday contexts they mean the same thing.

• sahaja = standard/full form
• saja = shortened, very common spoken and informal written form

So these are equivalent:

• Saya sahaja yang belum tidur
• Saya saja yang belum tidur

A learner should recognize both.

Yes. Both are good translations, and the best one depends on context.

Possible natural English translations include:

• Only I haven’t slept yet.
• I’m the only one who hasn’t slept yet.
• I alone haven’t slept yet.

The Malay sentence strongly focuses on saya as the only person in that situation. In most contexts, I’m the only one who hasn’t slept yet is the clearest English version.

Yes, it is natural and understandable.

That said, depending on style and region, speakers might also say:

• Saya saja yang belum tidur.
• Hanya saya yang belum tidur.
• Saya seorang saja yang belum tidur.

All of these are natural. The original sentence is perfectly fine, especially in standard written Malay.