I could not sleep at night/in the night.

[thehammer]

Hello, which one should I use?

- Yesterday I had a deadly headache. I took painkillers but it didn't work. I could not sleep at night/in the night.

 

[Barque]

Neither, in this case.

I couldn't sleep the whole night.
I didn't get much sleep last night.
I slept just a couple of hours last night.
I couldn't/didn't sleep well.

Yesterday I had a deadly headache.

You wouldn't be here to tell us about it if you did. "Deadly" means something that kills or can kill.

I had a very bad headache.
I had a splitting headache.

I took painkillers but it didn't work.

I took painkillers but they didn't work.
I took a painkiller but it didn't work.

 

[thehammer]

Here is another example taken from the internet.

- Someone stole my car in the night/at night.

 

[jutfrank]

If you want to learn how to use the phrases at night and in the night accurately, it will help to find examples written by teachers, or examples from dictionaries, rather than making them up yourself.

Here is another example taken from the internet.

- Someone stole my car in the night/at night.

You found something on the internet that said exactly that? Where?

 

[thehammer]

If you want to learn how to use the phrases at night and in the night accurately, it will help to find examples written by teachers, or examples from dictionaries, rather than making them up yourself.



You found something on the internet that said exactly that? Where?

Difference between AT NIGHT and IN THE NIGHT - MyEnglishTeacher.eu Blog

When something happens between the evening and the morning, you can use both ‘at night’ and ‘in the night’. However, ‘in the night’ is much less common and is used to mean 'during the night'. AT NIGHT

www.myenglishteacher.eu

I think both are correct in this regard.

 

[Barque]

Someone stole my car in the night. Possible, if you're saying this the next morning or afternoon.
Someone stole my car at night. Grammatical, but unlikely.

Here is another example taken from the internet.

- Someone stole my car in the night/at night.

That webpage only refers to "in the night" for this particular example.

 

[Tarheel]

Try:

Somebody stole my car last night.

Please notice how much information that sentence conveys. (Loads.)