nebukuro de neru.

Here, で marks the place or means in which the action happens.

So 寝袋で寝る means something like sleep in a sleeping bag.

In Japanese, で is often used for:

• the place where an action happens
  
  • 公園で遊ぶ = play in the park
• the means/tool used to do something
  
  • バスで行く = go by bus

In this sentence, 寝袋 is treated as the place/context in which you sleep, so で is natural.

That is a very common question, because English in can correspond to either で or に depending on the situation.

• で focuses on where the action happens
• に often marks destination, existence, or a point of result

So:

• 寝袋で寝る = sleep in a sleeping bag
  This is the standard simple sentence.
• 寝袋に入る = get into a sleeping bag
  Here に marks where you enter/go into.

If you want to express both ideas, you could say:

• 寝袋に入って寝る = get into a sleeping bag and sleep

So に is not impossible in related sentences, but in 寝る, the natural particle for the place of the action is usually で.

The sentence breaks down like this:

• 寝袋 = sleeping bag
• で = in / at / by means of; here it marks the place/context of the action
• 寝る = to sleep

So grammatically, it is:

• noun + で + verb

This is a very common pattern in Japanese.

It is not strange at all.

• 寝袋 literally contains the kanji 寝, which relates to sleeping
• 寝る is the verb to sleep

So yes, the sentence repeats the idea of sleep:

• 寝袋 = sleeping bag
• 寝る = sleep

Japanese does this kind of thing naturally, and it does not sound weird just because the same kanji appears twice. English does the same sometimes too, like sleep in a sleeping bag.

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

So 寝袋で寝る can mean:

• I sleep in a sleeping bag
• We sleep in sleeping bags
• They sleep in a sleeping bag
• or just sleep in a sleeping bag

The subject does not need to be stated unless it matters.

If you want to include one, you can:

• 私は寝袋で寝る。 = I sleep in a sleeping bag.

Yes. 寝る is the plain/dictionary form.

A polite version is:

• 寝袋で寝ます。

Very roughly:

• 寝袋で寝る。 = plain / casual
• 寝袋で寝ます。 = polite

Both are correct; the choice depends on who you are speaking to and the situation.

No. In normal Japanese writing, this would usually be written without spaces:

• 寝袋で寝る。

Spaces are often added in beginner materials to make the structure easier to see. So the version with spaces is helpful for learning, but not standard everyday writing.

Yes, it is natural and simple.

However, depending on what nuance you want, Japanese also has other natural expressions:

• 寝袋で寝る = sleep in a sleeping bag
• 寝袋に入って寝る = get into a sleeping bag and sleep
• 寝袋の中で寝る = sleep inside a sleeping bag

The shortest and most basic version is 寝袋で寝る.

That is a good instinct, because で can mark both place and means.

In this sentence, the most natural interpretation is place/context:

• sleep in a sleeping bag

But Japanese sometimes allows a little overlap in how で feels. A sleeping bag is not exactly a location like a park, but it is also not just a tool like a pen. So で works because it marks the setting or medium of the action.

In other words, you do not need to force a strict English category here. Just learn Xで寝る as a natural way to say sleep in X for things like:

• ベッドで寝る = sleep in a bed
• 車で寝る = sleep in a car
• テントで寝る = sleep in a tent
• 寝袋で寝る = sleep in a sleeping bag

Normally, no. The natural reading is sleep in a sleeping bag.

If you wanted to emphasize with in the sense of having one with you, Japanese would usually say it differently. For example, the sentence 寝袋で寝る strongly suggests that the sleeping bag is what you are sleeping in / using as your sleeping environment.