Learn Hardcore German: Zu wenig Schlaf kann fast jede Krankheit schlimmer machen. - Too little sleep can make almost any illness worse.

zu wenig Schlaf = too little sleep (there is less than what is healthy / needed). It implies a clear deficit.
nicht genug Schlaf = not enough sleep (you don’t reach the “enough” threshold). Slightly weaker, more neutral.

In everyday speech, zu wenig Schlaf is very common when talking about health consequences, because it emphasizes that the amount is too low and is causing problems.

You could say:

Nicht genug Schlaf kann fast jede Krankheit schlimmer machen. – grammatically fine, but the original sounds a bit more pointed and natural.

What case is Schlaf in, and why is there no article?

Schlaf is in the nominative case, because zu wenig Schlaf is the subject of the sentence:

Wer oder was kann Krankheiten schlimmer machen? → zu wenig Schlaf.

There is no article because German often omits articles with uncountable, abstract nouns when talking in general:

Schlaf (sleep)
Wasser (water)
Geduld (patience)

Compare:

Zu viel Zucker ist ungesund. – Too much sugar is unhealthy.
Zu wenig Schlaf kann fast jede Krankheit schlimmer machen.

You could add an article, e.g. der Schlaf, but that usually refers to a more specific, identifiable sleep (for example, someone’s sleep on a particular night). Here we mean sleep in general.

What exactly does zu mean in zu wenig Schlaf?

Here zu means “too” in the sense of excess / deficiency beyond a healthy or normal range.

zu wenig = too little
zu viel = too much
zu spät = too late

So zu wenig Schlaf is not just “a small amount of sleep”; it is an amount that is insufficient and problematic.

Why is Schlaf capitalized? Isn’t it related to the verb schlafen?

Yes, Schlaf is related to the verb schlafen (to sleep), but in this sentence it’s used as a noun, and all nouns in German are capitalized.

This kind of noun is a “substantivized” form, originally derived from the verb:

schlafen → der Schlaf (sleep)
essen → das Essen (food, the act of eating)
trinken → das Trinken (drinking)

Because Schlaf is a noun here (subject of the sentence), it must be capitalized.

Is Schlaf countable in German? Could you say zwei Schläfe like “two sleeps”?

Normally, Schlaf is treated as an uncountable mass noun in German, just like sleep in English:

Ich brauche mehr Schlaf. – I need more sleep.

The plural Schläfe exists, but it does not mean sleeps; die Schläfe (plural die Schläfen) actually means temple (the sides of your head).

To express countable units of sleep, German uses other words:

zwei Nächte Schlaf – two nights of sleep
ein Mittagsschlaf – a nap
drei Stunden Schlaf – three hours of sleep

So zu wenig Schlaf is the normal way to say too little sleep.

Why is the verb split as kann … machen? Why is machen at the end?

Kann is a modal verb (like can, must, should). In German main clauses:

The conjugated verb (here kann) goes in second position.
The main verb infinitive (here machen) goes at the end of the clause.

Pattern:

Subjekt – konjugiertes Modalverb – (Mittelfeld) – Vollverb (Infinitiv)
Zu wenig Schlaf – kann – fast jede Krankheit – schlimmer machen.

So:

Zu wenig Schlaf kann fast jede Krankheit schlimmer machen.
– literally: Too little sleep can almost every illness worse make.

This is standard word order with modal verbs in a main clause.

Why is it schlimmer machen and not just verschlimmern?

You could say:

Zu wenig Schlaf kann fast jede Krankheit verschlimmern.

That is grammatically correct and means almost the same. The difference:

schlimmer machen = literally make (something) worse
- more colloquial, very common
verschlimmern = to worsen, to aggravate
- more formal or concise

schlimmer machen uses the comparative adjective schlimmer plus machen, which is very transparent and common in spoken German.

What does schlimmer mean exactly, and why not mehr schlimm?

Schlimmer is the comparative form of the adjective schlimm (bad, serious).

German usually makes comparatives by adding -er:

kalt → kälter (cold → colder)
schön → schöner (beautiful → more beautiful)
schlimm → schlimmer (bad/serious → worse)

Using mehr schlimm would be ungrammatical or at least sound very wrong. Just as in English you say worse rather than more bad, German uses schlimmer, not mehr schlimm.

What case is jede Krankheit in, and why is it jede and not jeder?

Jede Krankheit is in the accusative case, because it is the direct object of schlimmer machen:

Was (what) kann zu wenig Schlaf schlimmer machen? → fast jede Krankheit.

Krankheit is a feminine noun (die Krankheit). In the singular accusative, the article-like word jede takes the ending -e:

Nominative/Accusative feminine singular: jede Krankheit

Jeder would be masculine (for example: jeder Hund), so jeder Krankheit would be wrong here.

What exactly does fast mean in fast jede Krankheit, and what does it modify?

Fast means almost / nearly.

In fast jede Krankheit, fast modifies jede, so the phrase means:

fast jede Krankheit = almost every illness.

It does not mean that the verb kann is “almost” happening; it’s about the quantity of illnesses:

Zu wenig Schlaf kann fast jede Krankheit schlimmer machen.
→ Too little sleep can make almost every illness worse.

If you moved fast elsewhere, the meaning would shift. For example (though unnatural):

Zu wenig Schlaf kann jede Krankheit fast schlimmer machen.
would suggest “can almost make every illness worse”, which is odd and unclear.

Could I say alle Krankheiten instead of jede Krankheit?

You can, but the meaning changes slightly:

fast jede Krankheit – almost every illness (emphasizes each individual illness; very broad, but spoken of individually)
fast alle Krankheiten – almost all illnesses (more about the set as a whole)

Both are grammatically fine:

Zu wenig Schlaf kann fast jede Krankheit schlimmer machen.
Zu wenig Schlaf kann fast alle Krankheiten schlimmer machen.

The original with jede Krankheit sounds a bit more universal and emphatic, because it stresses that any given illness can be made worse by lack of sleep.

Why is a singular used: jede Krankheit instead of Krankheiten?

This is the generic singular in German: a singular noun used to talk about a whole category in general.

Jede Krankheit = any illness, every illness (as a type).
Krankheiten is the normal plural (illnesses).

Both forms are possible:

Zu wenig Schlaf kann fast jede Krankheit schlimmer machen.
– focuses on each illness individually.
Zu wenig Schlaf kann fast alle Krankheiten schlimmer machen.
– focuses on the group of illnesses.

German often uses the singular for general statements:

Der Mensch braucht Schlaf. – Humans need sleep.
Jede Krankheit hat Ursachen. – Every illness has causes.

Can I change the word order to Fast jede Krankheit kann zu wenig Schlaf schlimmer machen?

Yes, that sentence is grammatically correct, but the emphasis changes.

Original:

Zu wenig Schlaf kann fast jede Krankheit schlimmer machen.
– Topic/emphasis: too little sleep and what it does.

Variant:

Fast jede Krankheit kann zu wenig Schlaf schlimmer machen.
– Topic/emphasis: almost every illness and what it does to lack of sleep (makes it worse).

The second one suggests that the illness itself can worsen the problem of too little sleep, which is almost the opposite meaning. So changing word order here actually changes who affects whom, not just emphasis.

Why is it Krankheit schlimmer machen and not something like schlimmer werden?

This is a difference between causing a change and undergoing a change:

schlimmer machen – to make (something) worse → causative
schlimmer werden – to become worse → the illness changes on its own

In the sentence:

Zu wenig Schlaf kann fast jede Krankheit schlimmer machen.
→ Lack of sleep actively worsens the illness.

If you said:

Mit der Zeit kann fast jede Krankheit schlimmer werden.
→ Over time, almost every illness can become worse (no explicit external cause given).

So machen is used because zu wenig Schlaf is presented as the cause that makes the illness worse.