No sabes lo bueno que es dormir bien cuando estás cansado.

In Spanish, No sabes... is often used idiomatically to mean something like:

• You have no idea...
• You don’t know how...
• You can’t imagine...

So No sabes lo bueno que es... is a very natural way to say You have no idea how good it is... rather than a cold, literal statement about knowledge.

Lo bueno is a very common Spanish structure: lo + adjective.

It turns an adjective into an abstract idea, so:

• bueno = good
• lo bueno = the good thing, what’s good, or here more naturally how good

In this sentence, lo bueno que es dormir bien means something like:

• how good sleeping well is
• the good thing about sleeping well

This lo is a neuter article, so it does not refer to a masculine noun. It refers to an idea or quality.

Other examples:

• No sabes lo difícil que fue. = You don’t know how difficult it was.
• Mira lo bonito que es. = Look how beautiful it is.

Because this is not a direct exclamation like How good it is!

Spanish often uses the pattern:

• lo + adjective + que + ser

to mean how + adjective + something is

So:

• No sabes lo bueno que es dormir bien = You have no idea how good sleeping well is

If you said qué bueno es, that would sound more like a direct exclamation or question, not the structure used after No sabes.

Compare:

• No sabes lo bueno que es descansar. = You have no idea how good resting is.
• ¡Qué bueno es descansar! = How good it is to rest!

Both are possible in Spanish, but they are different structures.

Because in Spanish, the infinitive can act like a noun.

So dormir bien means:

• to sleep well
• or more naturally in English, sleeping well

In this sentence, dormir bien is the thing being described as good:

• Dormir bien es bueno. = Sleeping well is good.

This is very normal in Spanish. English can also do this with sleeping well.

Other examples:

• Fumar es malo. = Smoking is bad.
• Viajar me gusta. = I like traveling.

Because the sentence needs the action as a general idea, not an action in progress.

• dormir bien = sleeping well in general
• durmiendo bien = sleeping well right now / while sleeping

Here, Spanish uses the infinitive because it is talking about the general act of sleeping well.

So:

• No sabes lo bueno que es dormir bien... = You have no idea how good it is to sleep well...

Using durmiendo here would not fit the structure.

Because estar is used for states or conditions, while ser is used for identity or more permanent characteristics.

Being tired is a temporary condition, so Spanish uses estar:

• estás cansado = you are tired

Not:

• eres cansado

In fact, ser cansado would mean something more like to be tiring / annoying / a bore, depending on context, not to feel tired.

Examples:

• Estoy cansado. = I’m tired.
• Es una persona cansada. can describe someone as weary-looking, but that is different from saying someone feels tired right now.

Spanish often uses the tú form to talk about people in general, just like English can use you in a general sense.

So here:

• cuando estás cansado = when you’re tired

This does not have to mean one specific person. It can mean when a person is tired or when people are tired.

English does the same:

• You sleep better when you’re tired.

That does not necessarily refer to one particular person.

A more impersonal version in Spanish would be:

• cuando uno está cansado
• cuando se está cansado

But cuando estás cansado sounds natural and conversational.

Because the sentence is using the default masculine singular form.

When Spanish addresses someone with tú, adjectives often change depending on who is being spoken to:

• to a man: estás cansado
• to a woman: estás cansada

If you are speaking to a mixed group in a general statement, Spanish often uses the masculine as the default. But if the speaker is clearly talking to a woman, cansada would be expected.

So both are possible depending on who is being addressed:

• No sabes lo bueno que es dormir bien cuando estás cansado.
• No sabes lo bueno que es dormir bien cuando estás cansada.

Because cuando takes the indicative when referring to something habitual, general, or known.

Here the meaning is general:

• when you are tired

This is a normal, recurring situation, so Spanish uses estás.

Use the subjunctive after cuando when referring to a future or not-yet-real event:

• Cuando estés cansado, descansa. = When you get tired / when you are tired, rest.

Compare:

• cuando estás cansado = when you are tired, in general
• cuando estés cansado = when you are tired, at some future time

In this structure, que links the quality to the thing being described.

So:

• lo bueno que es dormir bien

literally works like:

• the good-ness that sleeping well is

But that sounds unnatural in English. A better English rendering is:

• how good sleeping well is

This pattern is very common:

• lo importante que es = how important it is
• lo difícil que fue = how difficult it was
• lo útil que resulta = how useful it turns out to be

So que is part of a set expression, not a separate word you should translate literally every time.

Normally, no. In this kind of sentence, standard Spanish strongly prefers:

• No sabes lo bueno que es...

not:

• No sabes qué bueno es...

The lo + adjective + que + ser pattern is the normal one after expressions like no sabes, mira, imagina, etc.

So learners should treat lo bueno que es as the standard structure to use here.

Yes, it sounds natural and idiomatic in Spain.

It has a conversational tone and means something like:

• You have no idea how good it is to sleep well when you’re tired.

A Spaniard might also say similar things like:

• No sabes lo bien que sienta dormir bien cuando estás cansado.
• No sabes lo bueno que es descansar cuando estás agotado.

But your original sentence is perfectly natural.

Not normally. Cuando means when, not because.

So the sentence means:

• when you’re tired

not:

• because you’re tired

If you wanted because, you would use something like:

• porque estás cansado

So:

• duermes bien cuando estás cansado = you sleep well when you’re tired
• duermes bien porque estás cansado = you sleep well because you’re tired

The meaning may be similar in some situations, but the grammar is different.

Yes. A very natural English translation would often be:

• You have no idea how good it is to sleep well when you’re tired.

That captures the idiomatic force of No sabes...

A more literal version would be:

• You don’t know how good it is...

Both are possible, but You have no idea... often sounds more natural in English.

Yes. The sentence is built around the fixed pattern:

• No sabes + lo + adjective + que + es + infinitive phrase + cuando...

So it breaks down like this:

• No sabes = you have no idea
• lo bueno que es = how good it is
• dormir bien = to sleep well / sleeping well
• cuando estás cansado = when you’re tired

Spanish word order here is very typical and sounds natural as written. English usually keeps a similar order:

• You have no idea how good it is to sleep well when you’re tired.

So this is a useful sentence pattern to remember as a whole.