Learn Hardcore Russian: Плохой сон мешает мне хорошо работать днём. - A bad dream prevents me from working well during the day.

Be careful: мешать also means “to mix / to stir” in other contexts (e.g. мешать суп – to stir soup), but the structure and meaning are different.

Why is сон used for “sleep”? I thought сон also means “dream”.

Сон can mean both:

Sleep as a state (what you need at night)
Dream (what you see while sleeping)

In this sentence плохой сон most naturally means “poor / bad-quality sleep” – not sleeping well, not resting properly.

If the speaker specifically meant a bad dream / a nightmare, they might say:

Плохой сон не даёт мне выспаться. – A bad dream doesn’t let me get enough sleep.
Мне приснился плохой сон. – I had a bad dream.

Context usually makes it clear whether сон is “sleep” or “dream”.

Why is it плохой сон, not something like плохого сна?

Плохой сон is in the nominative case, because it is the subject of the sentence.

Плохой – masculine singular nominative adjective
сон – masculine singular nominative noun

The basic word order is:

Плохой сон (subject)
мешает (verb)
мне (indirect object, dative)
хорошо работать днём (infinitive phrase)

If you used плохого сна, that would be genitive, and you would need a different construction around it (for example, из-за плохого сна… – because of bad sleep).

Why is it работать and not a finite verb like я работаю?

Here работать is in the infinitive, because in Russian after this meaning of мешать you normally use an infinitive to express what is being hindered:

что мешает кому делать что?
- Шум мешает мне спать. – The noise prevents me from sleeping.
- Дождь мешает нам гулять. – The rain stops us from walking.
- Плохой сон мешает мне хорошо работать. – Bad sleep makes it hard for me to work well.

So in Russian you say literally: “Bad sleep hinders me to work well.”, with an infinitive instead of a finite clause like that I work.

Why is работать imperfective? Could I say понработать or something similar?

You use the imperfective работать because you’re talking about an ongoing / habitual ability to work well, not one single completed action.

Imperfective: работать – to work (in general, as a process, regularly)
Perfective: поработать – to work (for a while / get some work done, completed episode)

In this sentence, you want to express a general problem:

Плохой сон мешает мне хорошо работать днём.
= bad sleep is interfering with my general daytime performance.

If you said something like:

Плохой сон помешал мне хорошо поработать днём.

this would be about one specific occasion in the past:
“Bad sleep prevented me from working well during the day (that time).”

What is днём exactly? Is it a separate word, and why is there ё?

Днём is the instrumental singular form of день (day), used adverbially to mean “during the day / in the daytime”.

день – day (nominative)
днём / днем – by day, in the daytime (instrumental, used adverbially)

So you can think of it as similar to “by day / in the day”, but it functions as an adverb: it tells you when the action happens.

About the spelling:

днём – with ё, fully phonetic spelling
днем – often written in print (many publishers write е instead of ё, but it’s pronounced /днём/)

Both spellings refer to the same word; pronunciation stays with ё.

Could I say в днeвное время or днём днём instead of днём?

You can say other expressions, but they sound different:

днём – by day / during the day (most natural, short, neutral)
днём, днём – repeating днём would sound strange here; it’s not natural.
в дневное время – literally “in the daytime hours”; grammatically fine, but more formal / bookish and too heavy for this simple sentence.

The most natural versions for everyday speech would be:

Плохой сон мешает мне хорошо работать днём.
Плохой сон мешает мне хорошо работать в течение дня. (more wordy, “throughout the day”)

Why is the order хорошо работать, not работать хорошо?

Both word orders are grammatically correct, but the nuance differs slightly:

хорошо работать днём – neutral, maybe slight emphasis on хорошо as a general skill: to work well (in general) during the day.
работать днём хорошо – can sound more like you’re emphasizing how you work specifically during the day (as opposed to at night), but in this short sentence it’s just less natural.

Native speakers usually say:

мешает мне хорошо работать днём

That is the most common and natural order here.

Can I change the word order to emphasize something, like Мне плохой сон мешает хорошо работать днём?

Yes, Russian allows flexible word order to shift emphasis. Some possible variations:

Плохой сон мешает мне хорошо работать днём.
– neutral, the most standard version.
Мне плохой сон мешает хорошо работать днём.
– now мне is emphasized (to me personally), a bit more “emotional”, like “It’s bad sleep that’s making it hard for me to work well in the day.”
Плохой сон днём мешает мне хорошо работать.
– sounds like the sleep itself is happening during the day (i.e. napping in the day interferes with work).

Word order changes meaning or focus. Your original version is the clearest and most default.

Is there a difference between Плохой сон мешает мне хорошо работать днём and Плохой сон не даёт мне хорошо работать днём?

They’re very similar in meaning, but with a slight nuance:

мешает мне хорошо работать
– “gets in the way, interferes with my ability to work well”; more about obstruction / hindrance.
не даёт мне хорошо работать
– literally “does not let me work well”; slightly stronger, more like actively preventing / not allowing.

In everyday speech, they’re often interchangeable, but мешает sounds a bit more neutral and is a very typical choice in such sentences.

How do I say “Good sleep helps me work well during the day” using a similar structure?

You can parallel the original sentence exactly:

Хороший сон помогает мне хорошо работать днём.

Structure:

Хороший сон – good sleep (subject, nominative)
помогает – helps
мне – to me (dative)
хорошо работать днём – to work well during the day (infinitive phrase)

So:

Плохой сон мешает мне хорошо работать днём. – Bad sleep hinders me from working well during the day.
Хороший сон помогает мне хорошо работать днём. – Good sleep helps me work well during the day.

How does мешать change with different persons and tenses in this meaning?

For мешать (to hinder / interfere) in the present:

я мешаю – I hinder
ты мешаешь – you hinder (sg., informal)
он / она / оно мешает – he/she/it hinders
мы мешаем – we hinder
вы мешаете – you hinder (pl. or formal)
они мешают – they hinder

In your sentence:

Плохой сон мешает мне… – он мешает

Past tense:

мешал (m), мешала (f), мешало (n), мешали (pl.)

Examples:

Плохой сон мешал мне хорошо работать днём. – Bad sleep (in the past) hindered me from working well during the day.
Шум мешал нам спать. – Noise was disturbing us from sleeping.

Future (imperfective):

будет мешать – will hinder
- Плохой сон будет мешать мне хорошо работать днём. – Bad sleep will keep hindering my daytime work.

For a single completed hindrance, you can use the perfective помешать:

Плохой сон помешал мне хорошо поработать днём. – Bad sleep prevented me from working well that day (on that occasion).