"Εσείς, αλήθεια, πώς κοιμάστε το βράδυ;" So this sentence without the commas would be wrong?
No, it wouldn’t. As you can see, I just talked about “the correct writing”, that is in a more carefully
written text. Of course, there isn’t any ambiguity, if you omit the commas, as Perseas correctly noted. The commas are just the written marks which indicate a short pause of the voice, happening when speaking or reading aloud the text.
Perhaps, it is worth elaborating on it somehow.
Αλήθεια here is a
sentence adverb (προτασιακό επίρρημα in Greek) and as such it refers to, and defines, the content of the whole sentence in which it occurs, that is the speaker truly wants to find out how they sleep. And the προτασιακό επίρρημα is usually placed at the beginning of the sentence, but often after one (or rarely more) word (s) as well. That is why, when at the beginning, it should be followed by a comma or, else, be placed between two commas, just for its role to be distinct. The same does not apply to other adverbs, e.g. “
Δυστυχώς, ο γιατρός δεν ήρθε
αμέσως”, where Δυστυχώς is a sentence adverb, whereas αμέσως is a non-sentence adverb. See the corresponding English sentence, where the adverbs play the same roles; “
Unfortunately, the doctor didn't come
immediately”
Unfortunately = sentence adverb,
immediately = non-sentence adverb.
What about the following variations?
"Αλήθεια, εσείς, πώς κοιμάστε το βράδυ;"
"Αλήθεια πώς κοιμάστε το βράδυ;"
"Εσείς πώς κοιμάστε το βράδυ;"
"Αλήθεια, εσείς, πώς κοιμάστε το βράδυ;"

"Αλήθεια, εσείς πώς κοιμάστε το βράδυ;"

Here, εσείς should not be followed by a comma, as normally the subject is not separated from its verb and viceversa. "Αλήθεια πώς κοιμάστε το βράδυ;"

"Αλήθεια, πώς κοιμάστε το βράδυ;"

A comma is needed here, because αλήθεια is a sentence-adverb. "Εσείς πώς κοιμάστε το βράδυ;"

There is no sentence-adverb here, no need for a comma. Of course, all the above are just explanatory points regarding punctuation.