Saya mandi sebelum tidur supaya saya merasa lebih tenang.

Mandi is a general word for to bathe / wash oneself, and in everyday Indonesian it often covers what English speakers would call take a shower.

So Saya mandi sebelum tidur can naturally be understood as I shower before sleeping / before bed.

A few useful related words:

• mandi = bathe, wash oneself
• mandi air hangat = take a warm bath / wash with warm water
• mandi pancuran = shower, though this is less common in casual speech than just mandi
• berendam = soak in water

In normal conversation, Indonesians very often just say mandi, even when a shower is meant.

In Indonesian, verbs do not need a marker like English to in expressions like before sleeping.

So:

• sebelum tidur = before sleeping / before bed
• literally: before sleep

This is very normal in Indonesian. After time words or conjunctions like sebelum and sesudah, a bare verb is often enough.

Examples:

• sebelum makan = before eating
• sesudah pulang = after going home / after returning
• tanpa bicara = without speaking

So sebelum tidur is completely natural.

Yes, the second saya can often be omitted if the subject is clearly the same.

Your sentence is:

• Saya mandi sebelum tidur supaya saya merasa lebih tenang.

This is correct, but many speakers would also say:

• Saya mandi sebelum tidur supaya merasa lebih tenang.

Both are natural.

Why repeat saya at all? Because Indonesian often allows the subject to be stated again for clarity, especially when a new clause begins. It is not wrong or unnatural.

A rough comparison:

• supaya saya merasa lebih tenang = so that I feel calmer
• supaya merasa lebih tenang = so as to feel calmer

The repeated saya sounds a bit more explicit.

Here supaya introduces a purpose:

• supaya saya merasa lebih tenang = so that I feel calmer

So the structure is:

• action + supaya
  • intended result

Examples:

• Saya belajar supaya lulus. = I study so that I pass.
• Dia datang lebih awal supaya tidak terlambat. = He came earlier so that he would not be late.

As for supaya vs agar:

• supaya = so that, in order that
• agar = so that, in order that

They are very similar. In many contexts they are interchangeable.

• supaya often sounds a bit more conversational
• agar can sound a bit more formal or written

So this sentence could also be:

• Saya mandi sebelum tidur agar saya merasa lebih tenang.

That is also correct.

Because the sentence is about an internal feeling, not a change of identity or state in the stronger sense of become.

• merasa = to feel
• menjadi = to become

So:

• saya merasa lebih tenang = I feel calmer
• saya menjadi lebih tenang = I become calmer

Both can be grammatical, but merasa is more natural when talking about an emotional or mental sensation.

Compare:

• Saya merasa lelah. = I feel tired.
• Saya merasa senang. = I feel happy.
• Saya merasa lebih tenang. = I feel calmer.

Using menjadi would shift the idea slightly toward a change of state rather than a personal feeling.

Lebih means more, and tenang means calm, peaceful, or relaxed.

So:

• lebih tenang = more calm = calmer

This is the normal Indonesian way to make a comparative adjective:

• lebih besar = bigger
• lebih kecil = smaller
• lebih cepat = faster
• lebih tenang = calmer

Indonesian does not usually add a special ending like English -er. Instead, it uses lebih.

Yes. Tenang can describe both:

1. a person's emotional state

   • Saya merasa tenang. = I feel calm.

2. a situation or place

   • Tempat ini tenang. = This place is quiet / peaceful.

In your sentence, because it follows merasa, it clearly refers to an emotional state:

• merasa lebih tenang = feel calmer / more at ease

So here tenang is about how the speaker feels internally.

Yes, you can absolutely say:

• Sebelum tidur, saya mandi supaya saya merasa lebih tenang.

This is very natural.

Both versions work:

• Saya mandi sebelum tidur supaya saya merasa lebih tenang.
• Sebelum tidur, saya mandi supaya saya merasa lebih tenang.

The difference is mainly one of focus:

• starting with Saya mandi emphasizes the action
• starting with Sebelum tidur emphasizes the time

Indonesian is fairly flexible with this kind of time expression.

Both are possible.

Full version:

• Saya mandi sebelum tidur.

More casual, if the subject is already understood:

• Mandi sebelum tidur.

Indonesian often drops subjects when they are obvious from context, especially in casual speech. But in a standalone example sentence for learners, keeping saya is very normal and helpful.

So:

• Saya mandi sebelum tidur = complete and clear
• Mandi sebelum tidur = natural in conversation, but less explicit on its own

Yes. In many contexts, sebelum tidur can mean either:

• before actually going to sleep
• before bed / as part of a bedtime routine

Just like in English, the exact timing depends on context.

So this sentence can suggest:

• I shower just before sleeping, or
• I shower at night before going to bed

That slight flexibility is normal.

Yes. Here are a few natural alternatives, with slightly different nuance:

• Saya mandi sebelum tidur supaya lebih tenang.
  Very natural; the second saya is omitted.

• Saya mandi sebelum tidur agar merasa lebih tenang.
  Same meaning, with agar instead of supaya.

• Sebelum tidur, saya mandi supaya merasa lebih tenang.
  Same meaning, but the time phrase comes first.

• Saya mandi malam supaya merasa lebih tenang sebelum tidur.
  This emphasizes that the shower happens at night.

Your original sentence is already correct and natural. These are just variations a learner may encounter.