itami de nemuremasen.

It is read:

いたみで ねむれません。

• 痛み = いたみ
• で = で
• 眠れません = ねむれません

In normal Japanese writing, it would usually be written without spaces:

痛みで眠れません。

痛み is a noun meaning pain.

• 痛い is an i-adjective meaning painful / hurts
• 痛み is the thing itself: pain

So:

• 痛い = it hurts / painful
• 痛み = pain

In this sentence, the speaker is saying that pain is the reason they cannot sleep, so the noun 痛み fits naturally.

Here, で shows the cause/reason.

So 痛みで means:

• because of pain
• due to pain

This is a very common use of で with nouns, especially when talking about causes such as illness, injury, weather, accidents, and so on.

Examples:

• 風邪で休みました。 = I was absent because of a cold.
• 事故で電車が遅れた。 = The train was delayed because of an accident.

So in 痛みで眠れません, で is not marking a place or a tool; it is marking the reason.

This is one of the most important points in the sentence.

• 寝ません means do not sleep / will not sleep
• 眠れません means cannot sleep

So:

• 寝ません can sound like a choice, habit, or refusal
• 眠れません expresses inability

Because the sentence means that pain is preventing sleep, 眠れません is the natural choice.

Compare:

• 今日は寝ません。 = I’m not going to sleep today.
• 今日は眠れません。 = I can’t sleep today.

眠れません is the polite negative form of 眠れる, which is the potential form of 眠る.

Step by step:

• Dictionary form: 眠る = to sleep
• Potential form: 眠れる = can sleep
• Polite negative: 眠れません = cannot sleep

So the structure is basically:

pain + because of + cannot sleep

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

So even though English usually says I can’t sleep because of the pain, Japanese often just says:

痛みで眠れません。

The subject is understood from the situation. Depending on context, it could mean:

• I can’t sleep because of the pain
• He/She can’t sleep because of the pain
• They can’t sleep because of the pain

But in many real-life situations, it will naturally be understood as I.

Yes, it is natural and complete.

It sounds like something someone might say:

• to a doctor
• to a family member
• when explaining their condition

It is short, direct, and very natural in spoken or written Japanese.

If you wanted to be more specific, you could name the source of the pain:

• 頭痛で眠れません。 = I can’t sleep because of a headache.
• 歯の痛みで眠れません。 = I can’t sleep because of tooth pain.
• 腰の痛みで眠れません。 = I can’t sleep because of lower back pain.

Yes, and it is also very natural.

There is a small difference in structure:

• 痛みで眠れません。 = I can’t sleep because of pain.
• 痛くて眠れません。 = It hurts, and because of that I can’t sleep.

The first uses the noun 痛み.
The second uses the adjective 痛い in its -te form: 痛くて.

Both are natural, but the nuance is slightly different:

• 痛みで眠れません focuses on pain as the cause
• 痛くて眠れません focuses more on the experience of it hurting

Usually, で is more natural here.

• 痛みで眠れません。 sounds concise and natural.
• 痛みだから眠れません。 sounds less natural in most situations.

Why? Because で is commonly used with nouns to mark a direct cause such as illness, pain, injury, weather, or circumstances.

You could use other expressions, but they would be built differently, for example:

• 痛いので眠れません。 = I can’t sleep because it hurts.
• 痛みがあるので眠れません。 = I can’t sleep because I have pain.

So for this exact noun-based pattern, で is the best choice.

眠れません is polite speech.

It is appropriate in many situations, such as:

• speaking to a doctor
• talking to someone you do not know well
• being generally polite

Casual versions would be:

• 痛みで眠れない。 = I can’t sleep because of the pain.
• 痛くて眠れない。 = It hurts and I can’t sleep.

So the sentence you were given is polite but still very everyday and natural.

The spaces are usually there only for learners.

Normal Japanese writing would be:

痛みで眠れません。

Japanese is not normally written with spaces between words. Textbooks and learning materials sometimes add spaces to make the structure easier to see.

So the spaced version is helpful for study, but the standard written form is without spaces.