Перед сном я делаю лёгкую растяжку, чтобы расслабить спину.

Because перед requires the instrumental case.

• сон = sleep
• сном = instrumental singular of сон

So:

• перед сном = before sleep / before bed

This is a fixed and very common pattern in Russian:

• перед работой = before work
• перед ужином = before dinner
• перед сном = before sleep

It means before going to sleep or before bed.

It does not usually mean in front of sleep in any literal sense. It is an idiomatic time expression.

In natural English, you would often translate it as:

• before bed
• before going to sleep
• at bedtime (depending on context)

So Перед сном я делаю... is a very natural way to say Before bed, I do...

Russian often uses the expression делать растяжку for to do stretching / to do a stretch routine.

• делать = to do
• растяжка = stretching, stretch routine, stretching exercise

So я делаю растяжку means something like:

• I do some stretching
• I do a stretching routine

You could also sometimes hear verbs related to stretching in other contexts, but делать растяжку is a very common and natural way to express this idea.

In this sentence, растяжка means stretching or a stretching exercise/routine.

A good way to understand it is:

• лёгкая растяжка = light stretching / a gentle stretch routine

Depending on context, растяжка can have other meanings too, but here it clearly refers to physical stretching for the body.

Because растяжку is the direct object of делаю, so it is in the accusative case.

Base form:

• лёгкая растяжка = light stretching

In the sentence:

• я делаю лёгкую растяжку
• I do light stretching

Since растяжка is a feminine singular noun, its accusative form is:

• растяжка → растяжку

And the adjective must agree with it:

• лёгкая → лёгкую

So:

• nominative: лёгкая растяжка
• accusative: лёгкую растяжку

Not exactly. Here лёгкую means more like:

• light
• gentle
• mild

So лёгкая растяжка is gentle stretching, not necessarily easy stretching in the sense of low difficulty.

Russian лёгкий can mean both light and easy, and the exact meaning depends on context. In this sentence, gentle/light is the most natural interpretation.

Because this clause expresses purpose: in order to relax the back.

• чтобы = so that / in order to
• расслабить = to relax something
• расслабиться = to relax oneself / to become relaxed

Here the speaker wants to relax the back, which is a direct object:

• расслабить спину = to relax the back

So чтобы расслабить спину means:

• to relax the back
• in order to relax the back

If you said чтобы расслабиться, that would mean to relax oneself / to unwind, which is a bit different.

After чтобы, Russian very often uses the infinitive when the meaning is purpose.

So:

• чтобы расслабить спину = to relax the back / in order to relax the back

This is a very common structure:

• Я читаю, чтобы учить новые слова.

• I read to learn new words.

• Я делаю упражнения, чтобы укрепить спину.
• I do exercises to strengthen my back.

So the infinitive after чтобы is completely normal here.

Because спина is the direct object of расслабить, so it takes the accusative case.

• nominative: спина = back
• accusative: спину

So:

• расслабить спину = to relax the back

This is the same pattern as:

• поднять руку = raise the hand/arm
• повернуть голову = turn the head
• мыть лицо = wash the face

Russian often omits possessive words like мой / моя / мою when it is obvious whose body part is meant.

So instead of saying:

• расслабить мою спину

Russian naturally says:

• расслабить спину

Because from context, it is already clear that the speaker means their own back.

This is very common with body parts in Russian.

For example:

• Я мыл руки. = I washed my hands.
• Он закрыл глаза. = He closed his eyes.
• Она повернула голову. = She turned her head.

Adding мою is possible, but it often sounds unnecessary unless you want emphasis.

Yes, the word order can be changed, because Russian word order is fairly flexible.

The given sentence:

• Перед сном я делаю лёгкую растяжку, чтобы расслабить спину.

is very natural and puts the time expression first: Before bed...

You could also say:

• Я перед сном делаю лёгкую растяжку, чтобы расслабить спину.

This is also natural and means the same thing.

The original version sounds smooth because it starts with the time frame and then gives the action.

The correct spelling is лёгкую, with ё, from лёгкий.

In many Russian texts, especially informal ones, ё is often written as е, so you may also see:

• легкую

But the actual word is still pronounced and understood as лёгкую.

For learners, it is helpful to remember the dictionary form:

• лёгкий

and the sentence form:

• лёгкую растяжку

Yes, расслабить is a perfective verb.

In this sentence, the idea is to achieve a result:

• do some stretching
• so that the back becomes relaxed

That is why the perfective infinitive расслабить fits well.

A rough contrast:

• расслаблять = to relax something, in an ongoing/repeated process
• расслабить = to relax something successfully, to bring it into a relaxed state

After чтобы, Russian often uses the perfective infinitive when the goal is a completed result.

Yes, it sounds natural.

• Перед сном = a very common time expression
• я делаю лёгкую растяжку = a natural way to say I do some light stretching
• чтобы расслабить спину = a clear and natural purpose phrase

So the whole sentence is idiomatic and would sound normal in everyday Russian.