I overslept this morning.

I overslept this morning because I helped with my friend's artworks again last night.

Another friend of mine and I were helping him until 23:30 pm

Therefore I got back home at 1:30 am.

He's always awake till midnight or stays up all night.

Several days ago, I asked him that how was his work and whether it would be successful.

I overslept this morning because I helped with my friend's artworks again.

My friend and I were helping him until 11:30 pm,

You could also say until 2330, or until 2330 hours. But most people in America would say 11:30 (assuming pm), or 11:30pm.

He is always awake till midnight or he stays up all night.

Several days ago, I asked him that how was his work was going and whether will be successful or not.

He seemed tired.

After that, I heard that he had been awake since 5 days ago.

My friend and I were helping We helped him until (23:30 11:30pm).

AM/PM separates morning and evening and is unnecessary with 24-hour time format. Suggest either using 24-hour time (23:30) or 12-hour time (11:30pm). English speakers in America almost always use 12-hour time except for soldiers and police.

Therefore I got back home at 1:30 am.

Could also say, "I didn't get home until 1:30am."

He always awake till 'til midnight or stay up all night.

'til is a contraction of "until." Always use an apostrophe with contractions. Best way is to just memorize, they usually only "kind of" make sense.

e.g.,
can not == can't
do not == don't
will not == won't
until == 'til

Several days ago, I asked him that how was his work was going and whether he will succeed.

He seems apparently tired.

Fun words for tired: drained, exhausted, sleepy, weary, worn out.

After that, I heard that he hadn't slept for 5 days.

Agree with the previous correction, but this is another way.

心配ー・・・

I know what you mean by 心配 but the average English speaker might not. Might say, "I'm so worried" or something along those lines. :)

I overslept this morning because I helped my friend with his artwork again last night.

Assuming that it was the cause of you sleeping late, and hence sleeping in.

My other pal and I helped him until 23:30 pm

pal - friend (More casual expression)

Therefore I got back home at 1:30 am.

woah, you must live really far from him!! How lucky is he to have a friend like you?^^

He is always awake till midnight or pulls all night-ers.

"pulls all night-ers" is colloquial language and means that he doesn't sleep at all. (all night)

Example Sentence: I pulled an all night-er last week to complete my physics assignment due the day after.

Several days ago, I asked him about how was his work was and whether he succeeded or not.

He seems tired.

or: Apparently, he is tired.

After that, I heard that he hadn't slept for five days.

Wow, that could be really damaging on his health!

心配ー・・・

If you wanted that:

"I'm so worried..."