If you teach English in Japan, you may notice that an abnormally large number of people say things like:

"I'm sleepy."

The first time I heard one of my students say this, I thought, That sounds a bit off. A bit childish, maybe?

But I couldn't see anything wrong with the sentence, so I didn't mention anything to the student.

Fast forward several months, and I've met hundreds of students who are "sleepy," but none that are "tired."

What gives?

The problem is the difference between these two words in Japanese:

眠い
ねむい
sleepy; drowsy

疲れた
つかれた
worn-out; tired

They kind of match up with the English well.
(That is, assuming that my English is good enough to be talking about this ^_^.)

Typically, English speakers will use the word "sleepy" when yawning, nodding, off, and ready to go to sleep.
Whereas we use the word "tired" any time we lack energy.

This kind of matches up with the Japanese.

1. If you're nodding off, then you're 眠い.
2. If you just finished a long day of work, you're 疲れた.

But what about this...

Say you went out with your coworkers after work last night. Your boss pressured you to stay out drinking and reveling until the first train--like six in the morning! (Welcome to Japan.) The next day, you're at work, looking like a wreck, and you want to express your exhaustion to a coworker.

Which word do you use?
眠い？Or 疲れた？

I think in English, most people would not say "I'm sleepy" in this situation unless they are referring to the fact that they can't keep their eyes open due to lack of sleep (even then I would probably still say "tired," but maybe I'm an anomaly).

But if you're so exhausted that you just want to go home and crawl into bed, then you're tired, right?
Or maybe either one? I don't know.

If you're speaking Japanese, then I think 眠い is more appropriate here.

I've certainly had dozens of hungover students at Saturday morning English lessons telling me they were "sleepy," after all.

Anyways, this is a bigger problem for Japanese people, so I wouldn't worry about it too much.

Rei and I wrote an article about it once on my other website, 英語Boost!

These are the examples we used...

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眠そうだね
ねむそう　だね
You look tired.
Literally: "look sleepy + だね."

You may already know that to say something "looks like" an i-adjective, you just drop the ～い and add ～そう：

熱い（あつい // hot）
忙しい（いそがしい // busy）
高い（たかい // tall; expensive）
悲しい（かなしい // sad）
嬉しい（うれしい // happy）

Remove the final い：

熱-（あつ-）
忙し-（いそがし-）
高-（たか-）
悲し-（かなし-）
嬉し-（うれし-）

Then add ～そう：

熱そう（あつそう // It looks hot.）
忙しそう（いそがしそう // You look busy.）
高そう（たかそう // It looks expensive.）
悲しそう（かなしそう // He looks sad.）
嬉しそう（うれしそう // She looks happy.）

(Note: This is probably obvious, but I'm just guessing the context/subjects for those one-word sentences.)

The same phenomenon above happens with verbs, too.

With verbs, you just take the masu-form, then delete ～ます and add ～そう.

Here is a single, lonely example...

吐く（はく // to spit; to throw up; to vomit）
↓
吐きます（はきます）
↓
吐き‐（はき‐）
↓
吐きそう！（はきそう！ // I think I'm gonna be sick!）

Oh, dear. I got sidetracked again...

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10時間も寝たのにまだ眠い
じゅうじかん　も　ねた　のに　まだ　ねむい
I’m still tired [sleepy] even though I slept ten hours.
Literally: "ten hours + も + slept + although + still + sleepy."

The のに in this sentence is useful for saying "although" or "even though."

You'll hear it a lot after the past tense of a verb.

For example, let's say that you and your friends are planning to go to the beach on Saturday. You're stoked. But you don't have a swimsuit! So you go to the trouble of buying one, and... it rains on Saturday. Bummer!

You could say...

せっかく水着買ったのに…
せっかく　みずぎ　かった　のに…
I even went to the trouble of buying a swimsuit... (This sucks!)
Literally: "with trouble / for the sole purpose of / at great pains + swimsuit + bought + のに…"

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お腹いっぱいになったら眠くなってきた
おなか　いっぱい　に　なったら　ねむくなってきた
Eating made me sleepy [tired].
Literally: "stomach + full + に + if / when became + become sleepy."

I'm guessing that some of you out there will be wondering the difference between:

1) なった（became）
2) なってきた（became [ = te-form of "to become" + past tense of "to come."]）.

Uh...

Mhmm...

I don't know! Stop picking on me. T_T

Well, I kind of know. So here it goes...

Saying なった just means "became," right? So if I said...

寒くなった。
さむく　なった。
It got cold.
Literally: "cold + became."

...then the nuance is that all of this happened in the past. I could even be talking about the weather for an entire month or season last year.

Using ～てきた has a stronger nuance of the process of change.
In other words, the change is still ongoing. So...

寒くなってきた。
さむく　なってきた。
It was getting cold.
Literally: "cold + become + came."
Note: The nuance is that it may continue to get colder from now, as well.

I found a student asking about this on HiNative, and there was on great answer...

彼氏ができてから、人生が楽しくなった。
かれし　が　できて　から、 じんせい　が　たのしく　なった。
Life became more fun after I got a boyfriend.
Literally: "boyfriend + が + was able to make + from, + life + が + fun + なった."

最初のデートは退屈だったけど、だんだんと楽しくなってきた。
さいしょ　の　デート　は　たいくつ　だった　けど、 だんだん　と　たのしく　なってきた。
Our first date was boring, but it's getting more fun now.
Literally: "first + の + date + は + boring / bored + was + but, + gradually + fun + なったきた."

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眠すぎて死にそう
ねむすぎて　しにそう
I’m so tired (I feel like I’m gonna die).
Literally: "too sleepy + looks like I will die."

We just saw how to use this ～そう！

死ぬ（しぬ // to die）
↓
死に‐（しに‐）
↓
死にそう（しにそう // looks like [he; I; it] will die）

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本読んでると眠くなる
ほん　よんでる　と　ねむくなる
Reading makes me sleepy [tired].
Literally: "book + am reading + と + sleepy + become."

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So I may have confused you a lot about "sleepy" and "tired."

Confused myself, at least.

But hopefully you learned some useful Japanese along the way! ^_^