Learn Hardcore Greek: Πρόσφατα έχω λιγότερο άγχος, γιατί κοιμάμαι καλύτερα. - Recently I have less stress because I sleep better.

Έχω λιγότερο άγχος = I have less anxiety / I experience less anxiety.
You are talking about the amount of anxiety as a kind of “thing” you have.
Είμαι λιγότερο αγχωμένος = I am less anxious.
You are describing your state / quality as a person.

In everyday Greek, έχω άγχος (“I have anxiety / I’m anxious”) is extremely common and often preferred over είμαι αγχωμένος. So έχω λιγότερο άγχος sounds very natural and idiomatic.

Why is there no article before άγχος? Why not το άγχος?

Άγχος is an abstract, mass-like noun (anxiety, stress). In Greek, abstract or mass nouns often appear without the article when you talk about them in a general or measurable way:

Έχω άγχος. = I have anxiety / I’m anxious.
Έχω λιγότερο άγχος. = I have less anxiety.

You would use το άγχος when you refer to a specific, already-known kind or episode of anxiety:

Το άγχος της δουλειάς είναι μεγάλο.
The stress of work is big.

In this sentence, we mean anxiety in general as a quantity, so the article is not used: λιγότερο άγχος.

What exactly is λιγότερο here? Is it an adjective or an adverb, and why doesn’t it change form?

Λιγότερο is the comparative form of λίγο (little, few). It can act as:

An adjective: agreeing in gender/number/case
An adverb: invariable

In λιγότερο άγχος, it functions like an adjective meaning less, but:

Άγχος is neuter singular
The form λιγότερο already matches neuter singular

So you see λιγότερο άγχος and not λιγότερη άγχος etc. The form λιγότερο is also the one you use adverbially (e.g. δουλεύω λιγότερο = I work less), which is why learners often feel it looks “invariable”, even though it can technically agree with neuter nouns here.

What kind of noun is άγχος? How is it used in Greek?

Άγχος:

Gender: neuter
Basic form: το άγχος (singular)
Meaning: anxiety, stress

Typical patterns:

Έχω άγχος. = I’m anxious / I have anxiety.
Έχω πολύ άγχος. = I have a lot of anxiety.
Έχω λιγότερο άγχος. = I have less anxiety.

It is usually treated like a mass/uncountable noun (you don’t normally say “one anxiety, two anxieties” in Greek). That’s why you’ll see it mostly with quantifiers like πολύ, λίγο, καθόλου, λιγότερο, περισσότερο rather than with numbers.

Why is there a comma before γιατί? Is it necessary?

The comma before γιατί separates the main clause from the reason clause:

Πρόσφατα έχω λιγότερο άγχος, γιατί κοιμάμαι καλύτερα.
Lately I have less anxiety, because I sleep better.

In written Greek, it is standard and recommended to use this comma, just like in English before “because” when it introduces a full clause. In short sentences, especially informally, you might occasionally see it omitted, but correct punctuation includes it here.

Does γιατί mean both “why” and “because”? How do I know which one it is here?

Yes, γιατί can mean:

why (in questions)
because (in answers or in the middle of a sentence giving a reason)

You can tell by position and intonation (or, in writing, punctuation):

Question:
Γιατί έχεις λιγότερο άγχος;
Why do you have less anxiety?
Answer / reason:
Έχω λιγότερο άγχος, γιατί κοιμάμαι καλύτερα.
I have less anxiety, because I sleep better.

When γιατί introduces a subordinate clause explaining the reason, it means because. If it appears at the start of a direct question, it means why.

Also, επειδή always means because, never why, so you could also say:

... επειδή κοιμάμαι καλύτερα. = because I sleep better.

What tense is έχω here, and why is the simple present used with Πρόσφατα (“recently”)?

Έχω is in the present tense (simple present):

έχω = I have

In Greek, the present tense often covers what English would say with “have been … lately / recently”, especially with adverbs like πρόσφατα, τελευταία, τον τελευταίο καιρό:

Πρόσφατα έχω λιγότερο άγχος.
= Lately I have less anxiety / I’ve been having less anxiety.

The idea is that this is a current state extending into the recent past, and Greek uses the present naturally for that, without needing a separate present perfect form like English does.

What kind of verb is κοιμάμαι? Why does it end in -μαι?

Κοιμάμαι is a middle/passive form that is used with active meaning. In modern Greek, many common verbs exist only (or mainly) in this -μαι form, even though they are active in meaning. These are often called deponent verbs by learners.

κοιμάμαι = I sleep
There is no commonly used κοιμάω in everyday modern Greek.

Other verbs of this type:

φοβάμαι = I’m afraid
θυμάμαι = I remember
κάθομαι = I sit / I am sitting

So you just learn κοιμάμαι as the normal present form meaning I sleep.

What is καλύτερα here? Why not καλύτερος?

Καλύτερα is the comparative adverb of καλά (well):

καλά = well
καλύτερα = better (as an adverb)

You use καλύτερα with verbs to say do something better:

κοιμάμαι καλύτερα = I sleep better
τρέχω καλύτερα = I run better

Καλύτερος, -η, -ο is the comparative adjective of καλός (good):

καλύτερος ύπνος = better sleep
είμαι καλύτερος = I am better (as an adjective describing “I”)

So:

κοιμάμαι καλύτερα = I sleep better (adverb, modifies the verb)
έχω καλύτερο ύπνο = I have better sleep (adjective, modifies the noun ύπνο)

Can I change the word order, for example say Έχω λιγότερο άγχος πρόσφατα, γιατί κοιμάμαι καλύτερα?

Yes, Greek word order is relatively flexible, especially for adverbs like πρόσφατα. All of these are acceptable:

Πρόσφατα έχω λιγότερο άγχος, γιατί κοιμάμαι καλύτερα.
Έχω λιγότερο άγχος πρόσφατα, γιατί κοιμάμαι καλύτερα.
Έχω πρόσφατα λιγότερο άγχος, γιατί κοιμάμαι καλύτερα.

The difference is mainly in rhythm and emphasis. Starting with Πρόσφατα slightly emphasizes the time frame (“Recently…”), which matches the English style of the sentence.