Learn Hardcore German: Die Müdigkeit macht mich heute langsam.

In German, all nouns are capitalized, regardless of their position in the sentence.
Müdigkeit is a noun meaning tiredness / fatigue, so it must be written with a capital M.

Every German noun has a grammatical gender.
Müdigkeit happens to be feminine, so in the nominative singular it takes the article die:

die Müdigkeit – the tiredness/fatigue (subject form)

So Die Müdigkeit is simply “tiredness/fatigue” as the subject of the sentence.

What role does Die Müdigkeit play in the sentence?

Die Müdigkeit is the subject of the sentence – the thing that is doing the action of the verb.

Sentence structure:

Die Müdigkeit → subject (nominative)
macht → verb
mich → direct object (accusative)
heute → adverb of time
langsam → predicative adjective/adverb (“slow” / “slowly”)

So the literal structure is:
Tiredness / The fatigue makes me slow today.

Why is it macht mich langsam and not something like macht mich langsam sein?

German often uses machen + object + adjective to express a causative meaning:
X macht Y [adjective] = “X makes Y [adjective].”

Examples:

Das macht mich traurig. – That makes me sad.
Die Hitze macht mich müde. – The heat makes me tired.
Die Müdigkeit macht mich langsam. – The tiredness makes me slow.

You do not add sein here.
The adjective (langsam) directly describes the object (mich) as a result of the action of machen. So macht mich langsam is the correct and natural structure.

Why is it mich and not mir?

Mich is the accusative (direct object) form of ich.
Mir is the dative (indirect object) form.

The verb machen in this causative structure takes a direct object:

jemanden (Akk.) [etwas] machen – to make someone [something]

So:

Die Müdigkeit macht mich langsam.
→ mich is the direct object, so accusative is required.

Using mir (dative) here would be ungrammatical:
✗ Die Müdigkeit macht mir langsam. – wrong

Could you also say Die Müdigkeit macht mich heute müde instead of langsam?

Yes, grammatically you can say:

Die Müdigkeit macht mich heute müde. – The tiredness makes me tired today.

However, this sounds a bit tautological (tiredness makes me tired).
The original sentence with langsam focuses on the effect of tiredness:

Not just that you feel tired, but that you are moving/working/thinking more slowly.

So:

müde → focuses on your state/feeling (tired).
langsam → focuses on your performance/speed (slow).

Is langsam here an adjective or an adverb?

In German, the form is the same for adjectives and adverbs; langsam can be both.
In this sentence, it’s used predicatively to describe me (mich):

Die Müdigkeit macht mich langsam.
→ Tiredness makes me (a person) slow.

So syntactically, you can think of it as an adjective used predicatively about the object (mich), even though it could also be seen as “slowly” in English. German doesn’t distinguish forms like slow vs. slowly here; langsam covers both.

Why doesn’t langsam have an ending, like langsame?

Adjectives in German only take endings when they are used attributively, that is, directly before a noun:

ein langsamer Zug – a slow train
die langsame Musik – the slow music

In your sentence, langsam is not placed before a noun. It’s used predicatively after a verb:

Die Müdigkeit macht mich langsam.

Predicative adjectives (after sein, werden, bleiben, or in this causative pattern with machen) do not take endings:

Ich bin müde.
Er wird nervös.
Sie bleibt ruhig.
Das macht mich glücklich.
Die Müdigkeit macht mich langsam.

Can I change the word order and say Heute macht mich die Müdigkeit langsam?

Yes, that is perfectly correct:

Die Müdigkeit macht mich heute langsam.
Heute macht mich die Müdigkeit langsam.

Both are grammatical. The difference is mainly in emphasis:

Die Müdigkeit macht mich heute langsam.
→ neutral; slight emphasis on Müdigkeit as the subject.
Heute macht mich die Müdigkeit langsam.
→ slightly emphasizes heute (“Today, tiredness is making me slow.”), maybe contrasting with other days.

German allows relatively flexible word order in the middle field, as long as you respect the rule that in a main clause the finite verb (here, macht) must be in second position (V2 rule). Both versions keep macht as the second element.

Could I also say Die Müdigkeit macht mich langsam heute?

This is grammatically possible but sounds less natural and a bit clumsy in standard German. Time adverbs like heute generally prefer to be:

near the beginning: Heute macht mich die Müdigkeit langsam.
or before other adverbials/predicative elements: Die Müdigkeit macht mich heute langsam.

Putting heute at the very end is not wrong, but it’s stylistically weaker and less idiomatic than the common patterns.

Why is the verb macht and not machen or something else?

The base verb is machen (“to make, to do”).
It is conjugated according to the subject:

ich mache
du machst
er/sie/es macht
wir machen
ihr macht
sie/Sie machen

The subject in the sentence is die Müdigkeit (3rd person singular, like sie = she/it), so you need:

sie macht → die Müdigkeit macht

Therefore, Die Müdigkeit macht mich heute langsam. is the correct present tense form.

Could I drop the article and say just Müdigkeit macht mich heute langsam?

You can say Müdigkeit macht mich heute langsam, and it would be understandable, but:

With the article Die Müdigkeit, it sounds more like a concrete, current state of tiredness that you’re experiencing today.
Without the article, Müdigkeit sounds more general/abstract, like “Fatigue (as a concept) makes me slow today.”

In everyday speech, Die Müdigkeit macht mich heute langsam is more idiomatic.
The version without die is more stylistic or poetic, or something you might see in headlines.

Why is the subject expressed as Die Müdigkeit instead of just saying Ich bin heute langsam, weil ich müde bin?

These sentences focus on slightly different things:

Ich bin heute langsam, weil ich müde bin.
→ “I am slow today because I am tired.”
Focus: you and your state, with an explicit reason.
Die Müdigkeit macht mich heute langsam.
→ “The tiredness is making me slow today.”
Focus: Müdigkeit as an active cause that affects you.

The second version personifies or highlights tiredness as the agent that changes your condition. It’s a common and natural way in German to express such cause–effect relationships.

Does Die Müdigkeit macht mich heute langsam sound natural in German, or is there a more common way to say this?

The sentence is natural and correct. It would commonly be understood as:

“Fatigue is slowing me down today.”
“My tiredness is making me slow today.”

You could also hear alternatives like:

Ich bin heute wegen der Müdigkeit ganz schön langsam. – I’m pretty slow today because of the tiredness.
Die Müdigkeit bremst mich heute richtig aus. – The tiredness is really slowing me down today. (more informal/colloquial)

But your original sentence is perfectly idiomatic and clear.