Hoy estoy lo suficientemente cansado como para acostarme temprano.

Because cansado describes a temporary state, not a permanent characteristic.

In Spanish:

• estar is used for states or conditions
• ser is used for identity, essential traits, and more permanent descriptions

So:

• Estoy cansado = I am tired
• Soy cansado would sound wrong here

Since the speaker is talking about how they feel today, estar is the correct verb.

That is just how Spanish structures this idea.

Spanish uses:

• lo suficientemente + adjective + como para
• so here: lo suficientemente cansado como para

English puts enough after the adjective:

• tired enough to...

So the word order is different:

• Spanish: sufficiently tired
• English: tired enough

This is a very common pattern in Spanish, so it is best learned as a chunk:

• lo suficientemente alto como para...
• lo suficientemente rápido como para...
• lo suficientemente bueno como para...

Here lo is part of a set expression that adds the idea of enough or to a sufficient degree.

So lo suficientemente + adjective/adverb means:

• sufficiently
• enough

Examples:

• lo suficientemente grande = big enough
• lo suficientemente lejos = far enough

You should not try to translate this lo word by word. It is better to treat lo suficientemente as a fixed pattern.

In this sentence, como para helps express the result or consequence:

• lo suficientemente cansado como para acostarme temprano
• tired enough to go to bed early

A useful way to think of it is:

• enough ... to ...

So the full structure is:

• lo suficientemente + adjective + como para + infinitive

Examples:

• Está lo suficientemente oscuro como para ver las estrellas.
• No soy lo suficientemente rico como para comprar eso.

Without como, the sentence would sound incomplete in this structure.

Because adjectives in Spanish must agree with the person they describe.

Here the subject is yo, but Spanish often leaves yo out. The adjective still has to match the speaker:

• a male speaker says cansado
• a female speaker says cansada

So a woman would say:

• Hoy estoy lo suficientemente cansada como para acostarme temprano.

If the subject were plural, the adjective would change too:

• Estamos lo suficientemente cansados...
• Estamos lo suficientemente cansadas...

Because the verb here is acostarse, which is a reflexive verb meaning to go to bed.

Its infinitive is:

• acostarse

When the reflexive pronoun is attached to the infinitive, it becomes:

• acostarme = to put myself to bed / to go to bed
• acostarte
• acostarse
• acostarnos, etc.

So in this sentence, acostarme temprano means to go to bed early.

If you used acostar without the reflexive pronoun, it would usually mean to put someone else to bed.

Because in Spanish, object and reflexive pronouns can attach to an infinitive.

So after para, you get:

• para acostarme temprano

This is normal and natural.

The base verb is acostarse, and the me shows that the subject is also the person going to bed.

With infinitives, Spanish often attaches the pronoun to the end:

• quiero acostarme
• voy a acostarme
• antes de acostarme

Because Spanish often uses para + infinitive when the subject of both actions is the same.

Here, the same person is:

• being tired
• going to bed early

So para acostarme temprano works perfectly.

A useful comparison:

• Estoy listo para salir.
• No tengo tiempo para estudiar.

If the subject changed, Spanish would often use a different structure, such as para que + subjunctive. But in this sentence, the subject stays the same, so the infinitive is the natural choice.

Because temprano is commonly used as an adverb meaning early.

In Spanish, many time-related words can function as adverbs without needing an -mente ending.

So:

• acostarme temprano = go to bed early

This is much more natural than tempranamente, which sounds formal or unusual in this context.

Other similar examples:

• llegar tarde = arrive late
• levantarse pronto = get up soon / early

Yes, Hoy estoy suficientemente cansado para acostarme temprano is understandable and grammatical.

However, lo suficientemente ... como para ... is a very common and very natural pattern, and many speakers prefer it.

So:

• lo suficientemente cansado como para acostarme temprano = very idiomatic
• suficientemente cansado para acostarme temprano = also possible, but slightly less full-sounding to many speakers

For learners, the longer pattern is very useful because it is clear and widely used.

Yes. A very common alternative is:

• Hoy estoy tan cansado que me acostaré temprano.
• Hoy estoy tan cansado que voy a acostarme temprano.

This uses:

• tan + adjective + que

That structure is closer to:

• so tired that...

The difference is mainly structural:

• lo suficientemente cansado como para... = tired enough to...
• tan cansado que... = so tired that...

Both are natural, but they are not exact word-for-word equivalents.

Yes, Spanish word order is flexible.

You can say:

• Hoy estoy lo suficientemente cansado como para acostarme temprano.
• Estoy lo suficientemente cansado hoy como para acostarme temprano.
• Lo suficientemente cansado como para acostarme temprano estoy hoy. — possible, but marked and less natural

The original sentence is the most neutral and natural order.

Putting hoy first is very common because it sets the time frame right away: today.