nemuke ga tuyoi toki ha, tumetai mizu wo nomu to sukosi me ga samemasu.

眠気 is a noun meaning sleepiness or drowsiness.

• 眠い = sleepy (an i-adjective)
• 眠気 = sleepiness / drowsiness (a noun)

So:

• 眠いとき = when you are sleepy
• 眠気が強いとき = when the feeling of sleepiness is strong

The sentence uses 眠気が強い because it is describing the degree of drowsiness in a slightly more objective way.

Because 強い describes 眠気 as the thing that is strong.

In Japanese, with many adjectives, the thing being described often takes が.

So:

• 眠気が強い = sleepiness is strong

This is very natural Japanese. Similar patterns are:

• 雨が強い = the rain is heavy
• 風が強い = the wind is strong
• 興味が強い = interest is strong

Here, が marks what has the quality of being strong.

Yes, literally it means strong, but in this sentence it is being used naturally to mean:

• severe
• intense
• strong

So 眠気が強い means something like:

• the sleepiness is strong
• you feel very sleepy
• your drowsiness is intense

Japanese often uses strong/weak with feelings, conditions, and sensations in ways that English does not always copy directly.

とき means time or when.

So:

• 眠気が強いとき = when you are very sleepy / at times when your drowsiness is strong

Then は marks that whole phrase as the topic:

• 眠気が強いときは、... = as for when you feel very sleepy, ...

It sets up the situation or context for the rest of the sentence.

Because the sentence is talking about a general situation, not one specific past time.

• 強いとき = when it is strong / when you are very sleepy
• 強かったとき = when it was strong / when you were very sleepy

Here the speaker is giving general advice or describing a regular result, so the non-past form 強い is the natural choice.

The は highlights the condition or situation as the topic.

Compare:

• 眠気が強いとき、冷たい水を飲むと…
  = When you are very sleepy, if you drink cold water...
• 眠気が強いときは、冷たい水を飲むと…
  = As for when you are very sleepy, drinking cold water...

The version with は feels a little more like:

• in that case
• when that happens
• as for those times

It adds a contrast/topic nuance, even if the contrast is only implicit.

冷たい is the normal adjective for something that feels cold to the touch or when consumed.

So:

• 冷たい水 = cold water

Japanese often distinguishes:

• 冷たい = cold things, cold drinks, cold surfaces
• 寒い = cold weather / cold air / feeling cold

So you say:

• 冷たい水 = cold water
• 寒い日 = a cold day

Yes, と can work like if/when, but it has a special nuance.

In this sentence:

• 冷たい水を飲むと、少し目が覚めます
• = if/when you drink cold water, you wake up a little / become a little more alert

This と is often used for:

• natural results
• habitual results
• things that generally happen

So it sounds like:

• When you drink cold water, the result is that you feel a bit more awake.

It is less about a deliberate decision and more about a typical outcome.

Because と fits well when the speaker is describing a regular or expected result.

と

Used for:

• general truths
• habitual results
• automatic consequences

Example:

• 春になると暖かくなる。
• When spring comes, it gets warm.

たら

Used for:

• if/when something happens
• more flexible conditions
• one-time situations too

Example:

• 眠かったら、冷たい水を飲んでみて。
• If you're sleepy, try drinking cold water.

なら

Used for:

• if it is the case that...
• given that...
• based on some assumption/topic

Example:

• 眠いなら、少し休んだほうがいい。
• If you're sleepy, you'd better rest a bit.

So 飲むと here sounds like: drinking cold water tends to make you feel a little more awake.

This is a very common question.

There are two related expressions:

目が覚める

An intransitive expression:

• to wake up
• to become awake
• to become alert

目を覚ます

A transitive expression:

• to wake someone up
• to awaken
• sometimes: to make oneself fully wake up

In this sentence, the idea is that your eyes/awareness wake up as a result, so Japanese naturally uses:

• 目が覚めます

It can mean not only literally wake up from sleep, but also become more alert.

Literally, yes, the structure looks like that:

• 目 = eye(s)
• が覚める = wake up / become awake

But as an expression, 目が覚める means:

• to wake up
• to become alert
• to feel more awake

It is an idiomatic phrase, so you should learn it as a set expression rather than translating each word too literally.

In this sentence, it means something like:

• you feel a little more awake
• you perk up a bit

少し means a little or slightly.

Here it modifies the whole result:

• 少し目が覚めます
• = you become a little more awake
• = it wakes you up a little

So it does not mean:

• drink a little water

That would be different, such as:

• 冷たい水を少し飲む = drink a little cold water

Word order helps show what 少し is modifying.

Because the whole sentence is written in polite style.

• 飲む
• 覚めます

This kind of mixing is normal because verbs before とき and と usually stay in their plain form, even in polite sentences.

So:

• 眠気が強いときは → plain form before とき
• 冷たい水を飲むと → plain form before と
• 少し目が覚めます → polite main sentence ending

This is standard Japanese grammar.

Because verbs and adjectives that come inside a clause usually appear in their plain form, even in polite speech.

Examples from this sentence:

• 眠気が強いとき
  not 強いですとき
• 冷たい水を飲むと
  not 飲みますと

The main sentence ending is where politeness usually shows up:

• 目が覚めます

So the rule is basically:

• inside modifying/conditional clauses → plain form
• final sentence ending → polite or plain depending on style

It can be understood as either, depending on context, but by itself it sounds most like a general statement about what tends to happen.

It could mean:

• When you're very sleepy, drinking cold water helps a little.
• If you drink cold water, you become a little more alert.

Without extra context, it sounds less like a command and more like an observation or a piece of practical advice.

If the speaker wanted to give more direct advice, they might say something like:

• 眠気が強いときは、冷たい水を飲むといいです。
• When you're very sleepy, it's good to drink cold water.

Yes. A helpful breakdown is:

1. 眠気が強い
   = the sleepiness is strong / you are very sleepy

2. 眠気が強いときは
   = when you are very sleepy / as for times when you are very sleepy

3. 冷たい水を飲む
   = drink cold water

4. 冷たい水を飲むと
   = if/when you drink cold water

5. 少し目が覚めます
   = you become a little more awake

So the full structure is:

• [When you are very sleepy], [if you drink cold water], [you become a little more awake].

That is why the sentence may feel like it has both when and if/when in it: one part sets the situation, and the next part describes the result.