Перед сном я ставлю напоминание в телефоне: «Сделай паузу и дыши глубже».

Сон is the noun “sleep.” After перед you must use the instrumental case, so сон → сном (instrumental singular). Перед сон is ungrammatical.

До сна is possible and correct, but it’s a bit more neutral or general “before sleep / before sleeping.”
Перед сном often feels a bit more like “right before going to bed / as part of the bedtime routine.” In this context, перед сном sounds very natural and idiomatic.

Я ставлю is present tense, imperfective aspect of ставить (to put/place, to set). Imperfective is used for:

• habitual actions (what you usually do)
• ongoing processes

So Перед сном я ставлю напоминание = “Before bed I (usually) set a reminder.”

Я поставлю напоминание (perfective поставить) would mean “I will set a reminder (once, in the future),” focusing on the result of a single action, not on a routine.

With напоминание (“reminder”), the natural verb is ставить / поставить:

• ставить напоминание – to set a reminder (habitually / process)
• поставить напоминание – to set a reminder (once, completed action)

Делать напоминание is not idiomatic for a phone or computer reminder. You might see делать напоминания in a very general sense (“to make reminders for oneself” as a life habit), but for a concrete reminder in a device, ставить/поставить напоминание is the standard collocation.

В телефоне (preposition в + prepositional case) literally means “in the phone,” i.e. inside the phone’s system / app. It’s used for things that are stored or exist inside the device:

• в телефоне много фотографий – there are many photos in the phone
• у меня всё в телефоне – I have everything in my phone

На телефоне is used in a few other meanings, for example:

• говорить по телефону / быть на телефоне – to be on the phone (talking)
• чехол на телефоне – a case on the phone (physically on its surface)

For a software reminder, ставить напоминание в телефоне is the natural choice.

Напоминание is in the accusative case, singular neuter: it’s the direct object of я ставлю.

• nominative (dictionary form): напоминание
• accusative (same form for inanimate neuter): напоминание

So я ставлю (что?) напоминание – “I set (what?) a reminder.”

Пауза is a feminine noun. In the sentence Сделай паузу, паузу is in the accusative singular:

• nominative: пауза
• accusative: паузу

It’s the direct object of the imperative verb сделай:

• сделай (что?) паузу – “take (what?) a pause / break.”

• Сделай паузу is the imperative (command) of the perfective verb сделать. It means “Take a pause (one specific pause, achieve this result).” Perfective imperative = one completed action.

• Делай паузу would be the imperative of the imperfective делать. That would suggest something like “Keep making a pause / Be pausing,” which sounds odd here. Imperfective imperatives are more about ongoing or repeated processes, not a single “Take a break now” type command.

• Сделать паузу is the infinitive (“to take a break”) and is not a command by itself.

So for a reminder that tells you to take a break at a certain moment, Сделай паузу is exactly right.

The infinitive is дышать – “to breathe.”

Дыши is the imperative, 2nd person singular (informal “you”):

• infinitive: дышать
• 2nd person singular present: ты дышишь
• imperative singular: дыши (drop -шь, add -и)

So дыши глубже = “breathe deeper.” It’s talking to one person (the same “you” as in сделай).

Both are grammatically correct, but the meanings differ slightly:

• глубоко – “deeply” (basic adverb)
• глубже – “deeper” (comparative adverb: more deeply)

Дыши глубже means “Breathe more deeply (than you are now / than usual).”
A reminder aimed at relaxation usually wants you to increase the depth of your breathing, so глубже is more natural than just глубоко.

In Russian, when you introduce direct speech with a reporting clause like я говорю, я думаю, я ставлю напоминание, you typically use a colon:

• Я ставлю напоминание: «…»

The part before the colon describes the act (setting a reminder), and the part in quotes is the exact content of the reminder. This is a standard punctuation pattern:

• Он сказал: «Я скоро вернусь».
• Она подумала: «Нужно отдохнуть».

Russian, like many languages, normally drops the subject pronoun with imperatives. The person is understood from the verb ending:

• сделай – imperative, 2nd person singular → “(you) take”
• дыши – imperative, 2nd person singular → “(you) breathe”

Adding ты (ты сделай паузу) is possible, but then it adds emphasis, contrast, or emotion (sometimes irritation). For a neutral written reminder, you just use the bare imperatives without ты.

Yes, Я перед сном ставлю напоминание в телефоне is also correct and natural. Russian word order is relatively flexible.

• Перед сном я ставлю… slightly highlights the time frame перед сном.
• Я перед сном ставлю… slightly highlights the subject я first, then adds перед сном as extra information.

The core meaning doesn’t really change; both sound normal in everyday speech.