Приступ начался ночью.

Приступ can mean an attack, fit, seizure, or sudden episode, depending on context.

Very common uses include:

• a medical attack: приступ астмы = an asthma attack
• a heart attack-like episode or pain attack
• a panic attack
• a seizure/fit

So in this sentence, the exact English translation depends on the larger context. On its own, приступ is a fairly broad word for a sudden attack or episode.

Начался is the past tense, masculine singular form of начаться (to begin, to start).

It agrees with приступ, which is:

• singular
• masculine

So:

• приступ начался = the attack began

If the noun were feminine, you would get a different form:

• боль началась = the pain began

If it were plural:

• приступы начались = the attacks began

The infinitive is начаться.

This verb is:

• perfective
• reflexive (because of -ся)

So:

• начинаться = to be beginning / to begin in an ongoing or repeated sense
• начаться = to begin / to start as a completed event

In this sentence, начался is used because the speaker is referring to the event of the attack starting at a particular time.

Because Russian often uses the instrumental case to express time in phrases like at night, in the morning, during the day.

So:

• ночь = night (dictionary form, nominative)
• ночью = at night / during the night

This is a very common pattern:

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

So начался ночью literally means began at night.

It can mean either, depending on context.

In English, possible translations include:

• at night
• during the night
• in the night (less common in ordinary speech)

With this sentence, during the night often sounds the most natural if you are talking about when an attack or episode started:

• The attack started during the night.

But at night is also perfectly possible.

Russian has no articles like a or the.

So приступ can mean:

• an attack
• the attack

Which one sounds best depends on context.

In a full conversation, the listener usually understands whether the noun is definite or indefinite from the situation, previous sentences, or tone.

Both are correct.

• began is a little more neutral or formal
• started is often more natural in everyday English

So:

• Приступ начался ночью. = The attack began at night.
• Приступ начался ночью. = The attack started at night.

Both work well.

This is a normal, neutral word order:

• subject: приступ
• verb: начался
• time expression: ночью

Russian word order is flexible, so other versions are possible:

• Ночью приступ начался.
• Ночью начался приступ.

These all mean roughly the same thing, but the emphasis changes.

For example:

• Приступ начался ночью. = neutral statement
• Ночью начался приступ. = more emphasis on ночью or on the fact that the event happened then

Not always, but it very often is.

Приступ is commonly used for:

• an illness episode
• a seizure
• a coughing fit
• a panic attack
• a pain attack/spasm

It can also be used more broadly for a sudden burst of something:

• приступ смеха = a fit of laughter
• приступ гнева = a fit/burst of anger

So the word has a wider range than just medical language.

The stress is:

При́ступ начался́ но́чью.

Approximate pronunciation:

• ПРИ-stoop
• nuh-chal-SYA
• NO-chyu

A few notes:

• приступ has stress on the first syllable: При́ступ
• начался has stress on the last syllable: начался́
• ночью has stress on the first syllable: но́чью

The -ся marks the verb as reflexive in form.

In many Russian verbs, that does not mean a literal reflexive action like himself/herself. Often it is just part of the verb’s meaning.

Compare:

• начать = to start something
• начаться = to start / to begin (for something itself)

So:

• Врач начал лечение. = The doctor started the treatment.
• Лечение началось. = The treatment started.

In your sentence, the attack itself began, so Russian uses начаться:

• Приступ начался.

Yes. Depending on context, natural English versions could include:

• The attack started at night.
• The attack began during the night.
• The seizure started at night.
• The episode began during the night.
• The fit came on during the night.

The best choice depends on what kind of приступ is meant.