Утром аппетит вернулся, но бессонница всё ещё мешала мне отдыхать.

Here утром means in the morning.

It is the instrumental form of утро and is very commonly used as an adverb of time:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So in this sentence, Утром аппетит вернулся means In the morning, the appetite returned.

Because the verb here is вернуться, not вернуть.

• вернуть = to return something, to bring something back
• вернуться = to return, to come back

So:

• Я вернул книгу. = I returned the book.
• Он вернулся домой. = He returned home.

In your sentence, аппетит is not returning something else; it itself came back. So Russian uses вернулся.

The -ся is part of the verb вернуться. In many cases, this makes the verb intransitive or gives it the meaning to come back / to return oneself rather than to return something.

Compare:

• вернуть = to return something
• вернуться = to return, to come back

So аппетит вернулся is literally something like the appetite returned / came back.

Because аппетит is a masculine singular noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• masculine: вернулся
• feminine: вернулась
• neuter: вернулось
• plural: вернулись

Since аппетит is masculine, the correct form is вернулся.

Russian often leaves out possessive words like мой, его, её, etc. when the meaning is already obvious from context.

So English might say:

• my appetite returned

But Russian very naturally says just:

• аппетит вернулся

It is understood that this is the speaker’s appetite.

If you said мой аппетит вернулся, it would sound more emphatic or contrastive, not wrong, just less neutral.

Всё ещё means still.

• ещё by itself can mean still, yet, more, or another, depending on context.
• всё ещё is a very common combination that clearly means still in the sense of continuing up to now.

So:

• бессонница всё ещё мешала = insomnia was still interfering

Using both words makes the meaning very natural and clear.

Because Russian often expresses this idea with the thing causing the problem as the subject:

• бессонница мешала мне отдыхать
• literally: insomnia hindered me from resting

This is a very natural Russian structure. English often uses a similar one too:

• insomnia was preventing me from resting

So бессонница is doing the action of мешала in grammatical terms.

The verb мешать normally takes the person affected in the dative case.

Pattern:

• мешать кому? = to bother / hinder / interfere with whom?
• мешать кому делать что-то = to prevent someone from doing something

So:

• мне = to me
• мешала мне = was hindering me / was bothering me

More examples:

• Ты мешаешь мне работать. = You’re preventing me from working.
• Шум мешает детям спать. = The noise prevents the children from sleeping.

Because after мешать кому-то Russian often uses an infinitive to say what action is being prevented.

Pattern:

• мешать кому-то + infinitive

So:

• мешала мне отдыхать = was preventing me from resting

Other examples:

• Он мешал мне читать. = He was preventing me from reading.
• Боль мешала ей ходить. = The pain was preventing her from walking.

Мешала is imperfective, which fits an ongoing or continuing situation.

• мешала = was bothering / was interfering / kept preventing
• помешала = hindered / interfered once, as a completed event

In this sentence, the idea is that insomnia was still interfering with rest over a period of time, so imperfective мешала is the natural choice.

Because отдыхать means to rest, which is broader than спать (to sleep).

Бессонница is strongly connected with sleep, of course, but the sentence is phrased a bit more generally: insomnia was preventing the speaker from getting rest.

So this can suggest:

• not sleeping properly
• not relaxing
• not recovering

If the sentence said мешала мне спать, it would focus more specifically on sleeping.

Because но means but, and it joins two clauses here:

• Утром аппетит вернулся
• но бессонница всё ещё мешала мне отдыхать

In Russian, when но connects two full clauses with their own verbs, a comma is normally required.

Yes. Russian word order is flexible, though different orders can sound more natural in different contexts.

The given sentence is very natural:

• Утром аппетит вернулся, но бессонница всё ещё мешала мне отдыхать.

It starts with утром to set the time right away.

Other possible orders are grammatically possible, for example:

• Аппетит утром вернулся...
• Но бессонница мне всё ещё мешала отдыхать.

These may shift emphasis slightly, but the original version is smooth and neutral.

Historically it is the same word, but in всё ещё it functions as part of a fixed expression meaning still.

So you should understand всё ещё as one unit:

• всё ещё = still

Also, in careful writing it is spelled всё with ё, though many Russian texts write е instead, so you may also see все еще meaning the same thing in practice.

It has two coordinated parts:

1. Утром аппетит вернулся

   • утром = time expression
   • аппетит = subject
   • вернулся = verb

2. но бессонница всё ещё мешала мне отдыхать

   • но = but
   • бессонница = subject
   • всё ещё = still
   • мешала = verb
   • мне = dative object
   • отдыхать = infinitive

So the sentence combines:

• one completed event: the appetite returned
• one ongoing problem: insomnia was still interfering with rest