Learn Hardcore Japanese: natu ha neru toki, senpuuki wo yowaku tukete oku to suimin ga kimotiyoku narimasu. - In summer, if I leave the fan on low while sleeping, my sleep feels better.

夏は = As for summer / In summer
It sets up the general situation or time frame you’re going to talk about.
The subject of the main clause is actually 睡眠 (sleep), marked by が:
睡眠が気持ちよくなります。 = Sleep becomes pleasant.

So the overall structure is:

夏は – as for summer / in summer (topic)
寝るとき – when (you) sleep
扇風機を弱くつけておくと – if you keep the fan on low
睡眠が気持ちよくなります – your sleep becomes more pleasant.

In English we often say “In summer, when you sleep…”, and Japanese uses 夏は to play that role.

Could you say 夏に instead of 夏は? What’s the difference?

You can say 夏に, but the nuance shifts slightly.

夏は
- Treats summer as the topic, a broad situation you’re talking about.
- Feels like: “In summer (as a general situation)…”
- Very natural when making a general statement or tip.
夏に
- Uses に as a time particle, like “at/in (time).”
- Feels closer to: “In the summer (at that time)” as a more neutral time expression.
- Slightly more concrete, less “topic-like”.

For a general rule or piece of advice (like this sentence), 夏は is more natural and idiomatic.

What exactly does 寝るとき mean? Is it “when you sleep” or “when you are going to sleep”?

寝るとき literally is “when (you) sleep”, but in everyday use it often means “when you go to bed / at bedtime.”

寝る – to sleep / to go to bed
～とき – “when ~ / at the time (that) ~”

In context:

夏は寝るとき、扇風機を…
→ “In summer, when you go to bed, if you keep the fan on low…”

So it’s not about the entire period while you are already asleep; it’s more like at the time you’re going to bed (when you set things up before sleeping).

Why is it 寝るとき and not 寝たとき? What’s the difference between ～るとき and ～たとき?

The form before とき changes the time relationship.

寝るとき (dictionary form + とき)
- Means “when (you are about to) sleep / when you go to bed.”
- The action in the main clause (turning on the fan) happens around or just before the sleeping.
寝たとき (past form + とき)
- Means “when (you had) slept / after you had gone to sleep.”
- Suggests the sleeping already started, then something else happened.

In this sentence, you turn on the fan as part of getting ready to sleep, so 寝るとき is correct and natural.
If you said 寝たとき、扇風機をつけておく, it would sound like you somehow turned the fan on after you already fell asleep, which doesn’t really make sense.

Why is 扇風機 marked with を and not で? I thought で can mean “with” or “by means of.”

Here, を is used because 扇風機 is the direct object of the verb つける.

扇風機をつける = “to turn on the fan”
- 扇風機 is the thing you are directly acting on.

で is used when something is an instrument, means, or place:

ナイフで切る – cut with a knife
バスで行く – go by bus
部屋で寝る – sleep in the room

But turning on the fan is not “doing something by means of the fan”; it’s a direct action on the fan. So it must be 扇風機をつける, not 扇風機でつける.

Why is it 弱く and not 弱い before つけておく?

弱い is the adjective form (“weak”), and 弱く is the adverb form (“weakly / in a weak way”).

In Japanese:

Adjective (い-form) modifies nouns:
- 弱い風 – weak wind
Adverb (～く form) modifies verbs:
- 弱くする – make (something) weak
- 弱くつける – turn it on weakly / at a low setting

So:

扇風機を弱いつけておく → ungrammatical
扇風機を弱くつけておく → correct: “keep the fan on low.”

弱く here means “on a weak setting / gently / not strong.”

What does つけておく mean exactly? How is it different from just つける or つけている?

つけておく is て-form of つける + おく, and ～ておく has a special nuance.

Basic meanings:

つける – to turn on (a light, fan, etc.)
つけている – to have it on / it is on (state)
つけておく – to turn it on and leave it that way / keep it on in advance

～ておく often means:

Do something in advance / as preparation, or
Do something and leave it in that state.

In this sentence:

扇風機を弱くつけておく
→ “turn the fan on low and leave it on (while you sleep).”

So the nuance is closer to:

“If you keep the fan on low…”
rather than just “If you turn the fan on low (once).”

What is the と after つけておく doing? Is this the same と as in quotations?

Here, と is not a quotation marker. It’s a conditional particle meaning “if / when.”

扇風機を弱くつけておくと、睡眠が～
→ “If you keep the fan on low, (then) your sleep becomes more pleasant.”

This と has the sense:

“When X happens, Y (naturally) happens.”
It often implies a natural or automatic result.

Compare with other conditionals:

～たら – if/when (more general, often used in conversation)
～ば – if (somewhat more formal/abstract)
～なら – if (given that / in that case / as for)

Here, つけておくと nicely expresses “when you do this, this good effect happens.”

Why is 睡眠 marked with が? Could it be 睡眠は instead?

睡眠が marks 睡眠 (sleep) as the subject of the verb なる (to become).

睡眠が気持ちよくなります。
→ “Sleep becomes pleasant.”

You could use 睡眠は in some contexts, but it would change the nuance:

睡眠は気持ちよくなります。
- Treats 睡眠 as the topic (“as for sleep…”).
- Could sound like you’re contrasting sleep with something else, or talking about sleep in general in a broader discussion.

Using が:

Clearly says “It is the sleep that becomes pleasant” as the result of using the fan.
Feels more like a straightforward description of cause and effect, which fits this sentence.

So 睡眠が気持ちよくなります is the most natural here.

Why is it 気持ちよくなります and not 気持ちがよくなります or just 気持ちいいです?

All three exist, but they behave differently.

気持ちいいです
- Adjective phrase: “It feels good.”
- Very common in speech.
- Directly describes your feeling:
  - 扇風機をつけると気持ちいいです。 – It feels good when you turn on the fan.
気持ちがいい
- More “textbook-style” form of 気持ちいい (with the が explicitly).
- 気持ちがよくなる = “(my) feeling becomes good / I start to feel good.”
気持ちよくなる
- Here, 気持ちよく is the adverb form of 気持ちいい.
- Literally: “to become in a pleasant way.”
- In practice, 気持ちよくなる and 気持ちがよくなる are both used to mean:
  - “to feel better / to feel more pleasant.”

Why 気持ちよくなります here?

The subject is 睡眠 (“sleep”).
So: 睡眠が気持ちよくなります =
“Your sleep becomes (more) pleasant / more comfortable.”

If you said:

睡眠は気持ちいいです。
→ “Sleep feels good.” (a general statement about sleep itself)
気持ちいいです。
→ “It feels good.” (but subject is less clear)

Using 気持ちよくなります with が ties it nicely to the change in your sleep caused by using the fan: it becomes more pleasant.

Can I say 寝る instead of 眠る, or vice versa? What’s the difference?

In this sentence, 寝る is used, and it’s natural. The difference:

寝る
- Everyday, common verb.
- Means “to go to bed / lie down to sleep / sleep.”
- Used for both going to bed and sleeping as an activity.
- Very frequent in daily speech.
眠る
- Slightly more literary/neutral; focuses more on the state of being asleep.
- Often used in written language, set phrases, or when you emphasize the “sleep state” itself.

In 夏は寝るとき、…, the nuance is:

“When you go to bed (in summer)…”

You could say 夏は眠るとき and it would still be understood, but it sounds more formal or a bit stiff in this everyday context. 寝るとき is the most natural choice here.

What is the overall politeness level of this sentence? Is it okay in conversation?

The sentence ends with ～なります (polite ます form), so it is polite but not overly formal.

気持ちよくなります – polite, suitable for:
- Talking to someone you don’t know well
- Giving advice in general
- Writing explanatory text, learning materials, etc.

A more casual version (among friends) might be:

夏は寝るとき、扇風機を弱くつけておくと、睡眠が気持ちよくなるよ。

And a slightly more “advice-style” version:

夏は寝るとき、扇風機を弱くつけておくと、よく眠れますよ。

But your original sentence is perfectly natural, polite Japanese.