Learn Hardcore Japanese: natu ha situdo ga takai to, eakon wo tukenai to nemuremasen. - In summer, when the humidity is high, I can’t sleep without turning on the air conditioner.

natural results or automatic consequences, e.g.
- 春になると暖かくなります。
  → “When spring comes, it gets warm.”
- ボタンを押すと、ドアが開きます。
  → “If you push the button, the door opens.”

So here, 湿度が高いと sets up the condition that leads to the result in the second part of the sentence.

Does the second と after つけない mean the same thing as the first one?

Yes. The second と (in エアコンをつけないと) is also a conditional と.

エアコンをつけないと眠れません。
Literally: “If (I) don’t turn on the air conditioner, (I) can’t sleep.”

You can often think of ～ないと … as:

“if (I) don’t ~, …”
“unless (I) ~, …”

So the whole structure is:

夏は［湿度が高いと］、［エアコンをつけないと眠れません］。
→ “In summer, when the humidity is high, if I don’t turn on the AC, I can’t sleep.”

Is it okay to have two と conditionals in one sentence like this? Isn’t that confusing?

Grammatically it’s fine and very natural. You just have two nested conditions:

夏は湿度が高いと、…
→ “In summer, when the humidity is high, …”
エアコンをつけないと眠れません。
→ “If I don’t turn on the AC, I can’t sleep.”

Put together:

“In summer, when the humidity is high, if I don’t turn on the AC, I can’t sleep.”

Japanese often stacks conditional clauses like this; context and word order keep it clear.

Could I replace the first と with たら or なら? How would the nuance change?

You can replace it, but the nuance changes slightly.

夏は湿度が高いと、エアコンをつけないと眠れません。
- Neutral statement of a regular / natural condition.
- “In summer, when the humidity is high, I can’t sleep without AC.”
- Feels like a general truth / typical pattern.
夏は湿度が高いとき、エアコンをつけないと眠れません。
- Using とき: “At times when the humidity is high in summer, I can’t sleep without AC.”
- Focuses more on specific situations.
夏は湿度が高いと、 vs 夏は湿度が高いなら、
- なら sounds more like “if it is the case that…”, often in response to something, or setting up a more hypothetical condition.
- It’s not wrong, but と fits better for a regular, automatic relationship like humidity in summer.

So for a general, habitual statement, と is the most natural choice.

What about the second と: can I say エアコンをつけなければ眠れません instead of つけないと眠れません?

Yes, you can:

エアコンをつけないと眠れません。
エアコンをつけなければ眠れません。

Both mean roughly:

“If I don’t turn on the AC, I can’t sleep.” / “I have to turn on the AC or I can’t sleep.”

Nuance:

～ないと
- More casual / conversational.
- Very common in speech.
～なければ
- Slightly more formal / textbook-like.
- Common in writing or more careful speech.

Meaning-wise, in this sentence, they’re effectively the same.

Is つけないと here a shortened form of つけないといけません / つけなくてはいけません (“must turn on”)?

Not exactly in this sentence.

Two common patterns:

A と B (pure conditional)
- A: “if …”
- B: result
  Example: エアコンをつけないと眠れません。
  → “If I don’t turn on the AC, I can’t sleep.”
A ないと (いけない／だめだ) (obligation)
- A: action
- Whole phrase: “must / have to …”
  Example: エアコンをつけないと（いけません）。
  → “I have to turn on the AC.”

In your sentence, because we have つけないと眠れません, the と is clearly connecting to the next clause (眠れません), so it’s the pure conditional pattern (1), not an obligation shortcut.

That said, the practical feeling is similar:
“I need to turn on the AC, otherwise I can’t sleep.”

Why is 夏 marked with は, but 湿度 is marked with が? Could we use が with 夏 instead?

This is a classic は vs が situation: topic vs subject.

夏は → sets the topic: “as for summer / in summer…”
湿度が高い → states what is true of that topic: “the humidity is high.”

So:

夏は（topic）湿度が（subject）高い。
→ “As for summer, the humidity is high.”

Using 夏が here would sound strange, because you’d get two が:

✕ 夏が湿度が高いと…
You’d be saying “summer is the one that has high humidity” in a very awkward structure.

If you really wanted が, you’d normally have just one main subject, like:

夏が来ると、湿度が高くなります。
“When summer comes, the humidity gets high.”

But for “in summer, the humidity is high,” 夏は湿度が高い is the natural pattern: topic は + subject が.

Is 夏は湿度が高いと a complete sentence by itself?

On its own, 夏は湿度が高いと is incomplete; you’re left waiting for the result:

夏は湿度が高いと…（どうなるの？）
“In summer, when the humidity is high, … (what happens?)”

You need something after that と to complete the meaning, like:

夏は湿度が高いと、体がだるくなります。
“In summer, when the humidity is high, I feel sluggish.”

In casual spoken Japanese, people sometimes trail off with a と-clause, but grammatically, it wants a following result clause.

Why isn’t there a subject like 私 or 僕? How do we know who can’t sleep?

Japanese very often omits the subject if it’s obvious from context.

眠れません。
Just means “(someone) can’t sleep,” and context tells you it’s “I” in this case.

If you explicitly add it:

私は夏は湿度が高いと、エアコンをつけないと眠れません。
→ “As for me, when the humidity is high in summer, I can’t sleep if I don’t turn on the AC.”

That’s grammatically fine, but feels heavier and more formal. Normally, Japanese speakers leave out 私 when talking about themselves, unless they need to contrast it (e.g. “I can’t, but my brother can”) or avoid confusion.

Why is it 眠れません and not 眠りません? What does the れ mean?

眠れません is the potential form of 眠る (“to sleep / to fall asleep”).

眠る → “to sleep / to fall asleep”
眠れる → “to be able to sleep / to be able to fall asleep” (potential)
眠れません → polite negative potential
→ “cannot sleep / can’t fall asleep”

If you said 眠りません, it would mean:

“(I) don’t sleep / won’t sleep” (simple negative), not “can’t.”

So:

眠れません = cannot sleep (inability)
眠りません = do not / will not sleep (refusal or habit)

In your sentence, can’t is the intended meaning, so 眠れません is correct.

What’s the difference between 眠れない and 寝られない? Could I use 寝られません here instead?

Both are common and understandable, but there is a nuance:

眠る (ねむる)
- Focuses on being asleep / falling asleep.
- 眠れない → “(I) can’t fall asleep / can’t sleep (even though I want to).”
寝る (ねる)
- More about going to bed / lying down / sleeping in general.
- Potential: 寝られない
  → “(I) can’t sleep / can’t go to bed / can’t lie down (for some reason).”

In practice, both 眠れない and 寝られない often get translated as “I can’t sleep,” and:

エアコンをつけないと寝られません。
is natural and means basically the same as
エアコンをつけないと眠れません。

If you want to emphasize difficulty actually falling asleep, 眠れない / 眠れません fits very well.

Why is it エアコンをつける? Doesn’t つける mean “to attach”? How does it mean “to turn on”?

つける has several meanings, including:

付ける – to attach / put on / add
点ける – to turn on (a light, TV, AC, etc.)

In everyday writing, people usually write it in hiragana (つける) and let context decide the meaning.

With electrical appliances, つける almost always means “to turn on / switch on”:

電気をつける → turn on the light
テレビをつける → turn on the TV
エアコンをつける → turn on the air conditioner

You might also hear エアコンをかける, which is another natural verb for “run / use the AC,” but エアコンをつける is very common and correct.

Is the comma after the first と necessary? How does punctuation work here?

The comma （、） after 高いと is optional; it’s there mainly for readability and to show a pause:

夏は湿度が高いと、エアコンをつけないと眠れません。
夏は湿度が高いとエアコンをつけないと眠れません。

Both are grammatically fine. The comma just makes it easier to see:

Clause 1: 夏は湿度が高いと (“when humidity is high in summer”)
Clause 2: エアコンをつけないと眠れません (“if I don’t turn on the AC, I can’t sleep”)

Japanese commas don’t usually change the grammar; they help with rhythm and clarity.

Is it okay to mix つけない (plain) with 眠れません (polite)? Shouldn’t both be polite?

In this pattern, it’s actually very normal.

エアコンをつけないと眠れません。

Here’s why it’s acceptable:

The ない in ～ないと tends to stay in plain form even in overall polite sentences; it’s part of a fixed conditional pattern.
Trying to “politeness-conjugate” it like つけませんと is unnatural in modern Japanese.

Other examples:

時間がないと、間に合いません。
宿題をしないと、先生に怒られます。

Both are common even in polite speech. So plain ない before と, followed by a polite verb, is standard and natural.