tā shuō zìjǐ shuìjiào de shíhou xǐhuan kāizhe kōngtiáo, bú shì yīnwèi wàimiàn hěn rè, érshì yīnwèi zhèyàng bǐjiào ānxīn.

自己 means oneself / self and often refers back to the subject of the sentence.

So in 她说自己睡觉的时候..., 自己 refers to 她. It means:

• She says that when she sleeps...
• more literally: She says that she herself, when sleeping...

Using 自己 helps make it clear that the person who likes sleeping with the AC on is the same person as 她.

If you used 她 instead, it could still mean her, but 自己 is very natural when the subject refers back to itself in reported speech.

睡觉的时候 means when sleeping or at the time of sleeping.

Breakdown:

• 睡觉 = to sleep
• 的时候 = when, at the time that

So:

• 自己睡觉的时候喜欢开着空调 = she likes to have the air conditioner on when she sleeps

This is a very common pattern:

• 吃饭的时候 = when eating
• 上班的时候 = when going to work / while at work
• 小时候 = when one was little

In Verb + 的时候, 的 links the action to 时候, which means time.

So:

• 睡觉的时候 = the time when one sleeps / when sleeping

This is a standard structure in Mandarin:

• 我来的时候 = when I came
• 下雨的时候 = when it rains
• 看书的时候 = while reading

In many common time expressions, 的时候 functions almost like a fixed phrase.

This is an important point.

• 开空调 usually means turn on the air conditioner
• 开着空调 means have the air conditioner on / leave it running

The 着 shows a continuing state.

So in this sentence, 喜欢开着空调 does not mainly mean she likes the action of switching it on. It means she likes the AC to be on while she sleeps.

Compare:

• 请开空调。 = Please turn on the AC.
• 空调开着。 = The AC is on.
• 我睡觉的时候喜欢开着空调。 = I like sleeping with the AC on.

着 often indicates a continuing state or ongoing situation.

Here, 开着 means in the state of being on.

So 开着空调 means:

• the AC is kept on
• the AC remains on

This is different from 在, which usually marks an ongoing action, and different from 了, which often marks completion or change.

Examples:

• 门开着。 = The door is open.
• 灯亮着。 = The light is on.
• 她穿着红衣服。 = She is wearing red clothes.

This pattern means:

• not because..., but because...

In the sentence:

• 不是因为外面很热，而是因为这样比较安心。

it means:

• It is not because it is hot outside, but because this feels more reassuring / more secure.

This is a very common contrast structure in Mandarin.

Examples:

• 他不是不想去，而是太忙了。 = It is not that he does not want to go, but rather that he is too busy.

• 我学中文不是为了考试，而是为了工作。 = I study Chinese not for exams, but for work.

Because each side of the contrast has its own reason:

• 不是因为外面很热
• 而是因为这样比较安心

Repeating 因为 makes the structure balanced and clear.

In English we also often do this:

• not because..., but because...

Could you sometimes omit the second 因为 in casual speech? Sometimes people do shorten things in conversation, but in standard, clear Mandarin, repeating it here is very natural.

这样 means like this, this way, or doing it this way.

Here it refers to the action just mentioned: sleeping with the AC on.

So:

• 因为这样比较安心 = because doing this feels more reassuring = because this way she feels more at ease

It points back to the previous idea rather than introducing a brand-new one.

比较 often means relatively, rather, or quite.

So 比较安心 means:

• more at ease
• relatively reassured
• feel safer / more settled

In everyday speech, 比较 is often used in a softer, less absolute way than simply saying something strongly.

Compare:

• 安心 = at ease, reassured
• 比较安心 = more at ease / fairly reassured

It can sometimes imply a comparison, even if the other side is not stated directly.

安心 means to feel at ease, to feel reassured, or to feel secure in one’s mind.

Here it does not mean physical safety only. It is more about mental comfort.

So the idea is:

• having the AC on helps her feel calmer or more settled while sleeping

Depending on context, 安心 can be translated as:

• at ease
• reassured
• secure
• able to relax without worry

Yes, 外面 here means outside or outdoors.

In this sentence:

• 外面很热 = it is very hot outside

It refers to the environment outside the room or building.

Common uses:

• 外面下雨了。 = It is raining outside.
• 外面有人。 = There is someone outside.
• 外面很冷。 = It is cold outside.

This is a tone change rule.

不 is normally fourth tone: bù.
But before another fourth-tone syllable, it changes to second tone: bú.

Since 是 is fourth tone (shì), we get:

• 不是 → bú shì

More examples:

• 不要 → bú yào
• 不能 → bú néng? Actually 能 is second tone, so this stays bù néng
• 不对 → bú duì

So the tone changes only before another fourth tone.

Here it introduces reported speech or reported content.

Mandarin often does not need a word like that after 说.

So:

• 她说自己睡觉的时候喜欢开着空调...

means:

• She said that she likes to sleep with the AC on...

This is very common in Chinese:

• 他说他不去。 = He said that he is not going.

• 我妈说今天会下雨。 = My mom said it will rain today.

Mandarin often just places the spoken content right after 说.

Chinese uses commas very often to separate clauses and make the sentence easier to read.

Here the commas divide the sentence into clear parts:

• 她说自己睡觉的时候喜欢开着空调，
• 不是因为外面很热，
• 而是因为这样比较安心。

This helps show the structure:

1. main statement
2. rejected reason
3. real reason

So the punctuation supports the grammar and rhythm of the sentence.