yoru ni moufu o kakete mo samui toki wa, futon no naka de shinkokyuu o suru to sukoshi ochitsuku.

That’s a very common question.

In Japanese, some time expressions often appear without に, especially very common ones like:

• 今日
• 明日
• 昨日
• 毎日

But other time expressions often do take に, especially when they point to a more specific time frame. In this sentence, 夜に means at night / in the night.

So:

• 夜に = at night

You may also sometimes hear 夜 without に in other contexts, but 夜に is completely natural here.

かける has many meanings, and one of them is to put something over someone/something, like a blanket.

So:

• 毛布 = blanket
• 毛布をかける = to put a blanket on oneself/someone

Here, 毛布をかけて is the て-form, connecting it to what comes next:

• 夜に毛布をかけても寒い
  = Even if you put on a blanket at night, it’s still cold

The particle を marks 毛布 as the thing being put over the body.

～ても means even if or even though.

It is made from the て-form of a verb plus も:

• かけて
  • も = かけても

So:

• 毛布をかけても寒い
  = Even if you put on a blanket, it’s cold or = Even though you have a blanket on, it’s cold

This pattern is very common:

• 食べても = even if you eat
• 行っても = even if you go
• 読んでも = even if you read

Both は and に can appear after とき, but they do different jobs.

Here, 寒いときは means something like:

• when it’s cold, ...
• in cold times, ...
• as for when it’s cold, ...

The は sets up the situation as the topic or condition.

So the sentence structure is basically:

• As for times when it’s cold, ...

If you used ときに, it would sound more like at the time when..., focusing more directly on timing.
But ときは is very natural when introducing a general situation and what to do in it.

Before nouns like とき, Japanese uses the plain form.

Since とき is a noun meaning time, the clause before it modifies it:

• 寒いとき = the time when it is cold / when it is cold

This is just like other noun-modifying clauses:

• 食べる人 = the person who eats
• 行った場所 = the place I went to
• 面白い本 = an interesting book

So 寒いとき is normal because 寒い directly modifies とき.

They are different bedding items.

• 毛布 = blanket
• 布団 = futon, bedding, quilt-like bedcover, or the bed setup itself depending on context

In this sentence:

• 毛布をかけて = putting on a blanket
• 布団の中で = inside the futon / under the bedding

So the sentence suggests:

1. Even with a blanket, you still feel cold.
2. Then, while inside the futon, taking deep breaths helps you calm down a little.

A learner may notice that English futon often means a fold-out sofa-bed, but Japanese 布団 usually refers to traditional bedding.

で marks the place where an action happens.

So:

• 布団の中 = inside the futon / inside the bedding
• 布団の中で = in the futon / under the covers

Since 深呼吸をする is an action, で is used for the location of that action.

Compare:

• 部屋で勉強する = study in the room
• ベッドで寝る = sleep in bed
• 布団の中で深呼吸をする = take deep breaths under the covers

Japanese often uses noun + する to make verbs.

• 深呼吸 = deep breathing / deep breath
• 深呼吸をする = to take a deep breath / to breathe deeply

This is a very common pattern:

• 勉強する = to study
• 運動する = to exercise
• 電話する = to call

So 深呼吸をする is completely natural Japanese.

Here, と does not mean simple and.

In this sentence, Vる と ... expresses a result that happens when you do something.
So:

• 深呼吸をすると少し落ち着く
  = If you take deep breaths, you calm down a little = When you take deep breaths, you feel a little calmer

This と often means:

• if
• when
• once

It can sound like a natural or expected result.

Examples:

• 春になると暖かくなる = When spring comes, it gets warm.
• このボタンを押すとドアが開く = If you press this button, the door opens.

So here it suggests that deep breathing brings about the result of calming down.

Sometimes yes, but the nuance changes.

すると

This sounds like a general or natural result:

• If/when you do deep breathing, you calm down a little.

したら

This would sound more like:

• If you do it, then...
• a more situational or conditional feel

するなら

This means more like:

• if you are going to do it
• if that’s the case

In this sentence, すると works well because it presents the result as something that generally happens: deep breathing tends to help.

少し means a little or a bit.

Here it modifies 落ち着く:

• 少し落ち着く = calm down a little / feel a bit more settled

So it does not mean the deep breathing is small. It means the result is limited or gentle:

• you feel a little better
• you calm down somewhat

落ち着く often means:

• to calm down
• to settle down
• to become composed
• to feel more settled

In this sentence, it can include both mental and physical relief. Since the context is being cold at night, 少し落ち着く can mean something like:

• you feel a little calmer
• the discomfort becomes a bit easier to handle
• you settle down a little

It does not necessarily mean the cold disappears. It means your state improves somewhat.

Inside a sentence like this, Japanese often uses plain forms in subordinate clauses and general statements.

For example:

• 深呼吸をすると
• 寒いときは

These are not sentence-ending polite forms. They are part of the sentence structure.

Also, the whole sentence sounds like a general observation or piece of advice, and plain style is natural for that.

If you wanted to make the entire sentence polite, you could say something like:

• 夜に毛布をかけても寒いときは、布団の中で深呼吸をすると少し落ち着きます。

Only the final verb usually needs to become polite in that version.

Yes, the subject is omitted, which is very common in Japanese.

The sentence does not explicitly say you, I, or one. Japanese often leaves that unstated when it is clear from context.

So depending on context, this could mean:

• When you’re still cold even with a blanket at night, taking deep breaths under the futon helps a little.
• When I’m still cold even with a blanket at night, taking deep breaths under the futon helps me calm down a little.
• If one is still cold at night even under a blanket, deep breathing under the futon helps a little.

In English, we usually have to choose a subject, but Japanese often does not.

A helpful way to break it down is:

1. 夜に
   = at night

2. 毛布をかけても寒いときは
   = when it’s cold even if you put on a blanket

3. 布団の中で
   = in the futon / under the covers

4. 深呼吸をすると
   = if/when you take deep breaths

5. 少し落ち着く
   = you calm down a little

So the overall structure is:

• When it’s cold even with a blanket at night, if you take deep breaths under the covers, you calm down a little.

Japanese often stacks conditions and setting information before the main result, so the full meaning becomes clearer as you reach the end.