Раз ты устал, тебе лучше лечь спать пораньше.

Here раз does not mean one time or times.

In this sentence, раз is a conjunction meaning something like:

• since
• if
• seeing that
• given that

So Раз ты устал... means Since/if you’re tired...

This use of раз is common in conversational Russian. It often introduces a reason that is already accepted as true.

Compare:

• Раз ты устал, тебе лучше лечь спать пораньше.
  Since you’re tired, you’d better go to bed early.

It is a bit more colloquial than поскольку or так как.

Устал is the past-tense masculine singular form of the verb устать (to get tired / to become tired), but in Russian it is very often used in a present-result sense:

• Я устал = I am tired
• Ты устал = You are tired (said to a man or boy)

This is because the idea is you have become tired, and the result is that you are now tired.

Russian often expresses states this way, especially with verbs of change:

• Я замёрз = I’m cold / I got cold
• Я проголодался = I’m hungry / I got hungry

If speaking to a woman, it would be:

• Раз ты устала, тебе лучше лечь спать пораньше.

The structure кому-то лучше + infinitive is a very common Russian way to say:

• someone had better do something
• it would be better for someone to do something

So:

• тебе лучше лечь спать = you’d better go to bed

Here тебе is in the dative case because the expression is built that way:

• мне лучше... = I’d better...
• тебе лучше... = you’d better...
• ему лучше... = he’d better...

If you said ты лучше, it would usually mean you are better or would need a different structure entirely.

It usually sounds like advice or a recommendation, though depending on tone it can be fairly strong.

• тебе лучше... = you’d better...

This can range from gentle concern to firm warning.

In your sentence, it most naturally sounds caring and practical:

• Since you’re tired, you should go to bed earlier.

If someone wanted to sound softer, they might say:

• тебе стоит лечь спать пораньше = you should go to bed a bit earlier
• тебе нужно лечь спать пораньше = you need to go to bed earlier

This is about verbal aspect.

• лечь = perfective
• ложиться = imperfective

After лучше, Russian often uses the infinitive to describe the recommended action. Here лечь is used because it refers to a single, complete action:

• лечь спать = to go to bed / to lie down to sleep

The speaker means: it would be better for you to go to bed now or tonight, as one concrete action.

If you used ложиться, it would sound more like a habitual or repeated action in the right context:

• тебе лучше ложиться спать пораньше = you’d better go to bed earlier (as a habit)

So:

• лечь спать пораньше = go to bed early this time / on this occasion
• ложиться спать пораньше = make a habit of going to bed earlier

This is a very common Russian pattern.

• лечь literally means to lie down
• спать means to sleep

Together, лечь спать means to go to bed to sleep or simply to go to bed.

Russian often uses combinations like this, where one verb shows the movement/start of an action and the second verb names the action itself.

Common examples:

• сесть отдохнуть = sit down to rest
• пойти гулять = go for a walk
• лечь спать = go to bed

So while English uses one expression, Russian uses a verb of position/change plus the main activity.

Пораньше means a bit earlier or earlier than usual / earlier than now expected.

Compare:

• рано = early
• пораньше = earlier, a little earlier

So:

• лечь спать рано = go to bed early
• лечь спать пораньше = go to bed earlier than usual / a bit earlier

The prefix по- in words like this often softens the meaning and makes it sound more natural in advice.

Other similar words:

• побольше = a bit more / more
• поменьше = a bit less
• пораньше = a bit earlier
• позже = later
• попозже = a bit later

In this sentence, пораньше sounds very natural because the speaker is recommending an earlier bedtime relative to the usual one.

It can overlap with both, but here it is usually closer to since you’re tired.

The speaker is treating you’re tired as an established fact, and then drawing a conclusion from it.

So the logic is:

• Given that you’re tired, it would be better for you to go to bed earlier.

If the speaker were unsure whether the person was tired, если would be more natural:

• Если ты устал, тебе лучше лечь спать пораньше.
  If you’re tired, you’d better go to bed earlier.

So:

• раз = the condition/reason is basically accepted
• если = a more neutral or uncertain if

The given word order is completely natural:

• Раз ты устал, тебе лучше лечь спать пораньше.

Russian word order is flexible, but changes can affect emphasis.

For example:

• Тебе лучше лечь спать пораньше, раз ты устал.
  This is also natural. It puts the advice first and the reason second.

• Раз устал, тебе лучше лечь спать пораньше.
  Also possible in разговорная речь (colloquial speech). The pronoun ты can be omitted if it is obvious.

In your sentence, the original order is straightforward and clear: first the reason, then the recommendation.

Yes, but the meaning and tone would change.

• тебе лучше лечь спать пораньше = you’d better go to bed earlier
  Advice; sounds reasonable and caring.

• тебе надо / нужно лечь спать пораньше = you need to go to bed earlier
  Stronger, more direct, more necessity-focused.

• тебе стоит лечь спать пораньше = you should go to bed earlier
  Mild, polite advice.

So лучше is a good choice when the speaker is making a recommendation based on the situation.

Устал agrees with the subject in gender and number.

Forms:

• ты устал = you are tired (to a man)
• ты устала = you are tired (to a woman)
• вы устали = you are tired (to several people, or formal singular)

So the whole sentence would become:

• Раз ты устала, тебе лучше лечь спать пораньше.
• Раз вы устали, вам лучше лечь спать пораньше.

Notice that with вы, the dative pronoun also changes:

• тебе лучше
• вам лучше