数秒で寝れる、は以下のように英訳できます。
１）I can fall asleep in a second.
＝一秒以内に寝れる＝数秒で寝れる
Fall asleep = 眠りにつく
In a second＝すぐに
２）I can sleep really fast.
＝早く寝れる
形容詞のFastは、何かのスピードの速さに使います。Quick も同様です。
対象に、Early（形容詞・早い）やLate（形容詞・遅い）は、時間帯の早い・遅いに使われます。
例文：
I can sleep fast =早く寝れる（眠りに落ちるスピードが速い）
I can sleep early = 早く寝れる（床に入る時間帯が早い）
I hope it helps :)
mmtsさん、こんばんは。ご質問ありがとうございます。
一例をご紹介しますね。
《英訳例》
fall asleep as soon as one's head hits the pillow
→横になるとすぐに眠りに落ちる
fall asleep
→眠りに落ちる
one's head hits the pillow
→頭が枕に触れる
as soon as one's head hits the pillow
→頭が枕に触れるとすぐに
《語句の確認》
〈英辞郎〉
fall asleep as soon as one's head hits the pillow
とても寝付きが良い、バタンキューで眠る
fall asleep
眠りに落ちる、眠り込む、寝入る、寝込む、眠りにつく
as soon as
～するとすぐに、～するや否や
pillow
【名】枕
☆ 解説
「頭が枕に触れたらすぐに眠りに落ちる」→「横になったらすぐに眠りに落ちる」。
直訳ではありませんね。よく使われる「決まり文句」ということで、ご紹介しました。
as soon as ... は「…するとすぐ(に), …したとたん(に)」(スーパー・アンカー英和辞典)という意味です。
《例》
I fell asleep as soon as my head hit the pillow.
→私は横になったらすぐに眠りに落ちた。
(出典：Cambridge Dictionary)
He fell asleep as soon as his head hit the pillow.
→彼は床につくとすぐに眠りに落ちた。
(出典：Bangor Daily News-Feb 10, 2012)
Do you fall asleep as soon as your head hits the pillow?
→(あなたは)横になったらすぐに眠れますか。
(出典：The Natural Makeover Diet)
If you fall asleep as soon as your head hits the pillow, it may be a sign that you aren't getting enough sleep and that you're sleep deprived.
→もしあなたが、横になったらすぐに眠れるとしたら、それは睡眠不足のサインかもしれません。
(出典：Huffington Post-Mar 18, 2016)
お役に立てば幸いです。
ありがとうございました。
A good slang expression to describe feeling really tired is:
'I'm shattered'
It has the same meaning as:
'I'm exhausted'
These expressions imply that you are really tired and will fall asleep as soon as your head touches the pillow.
When I am really tired, I fall asleep very quickly.
It sometimes happens that you can come back home from work feeling extremely tired or exhausted. This feeling will make you feel very relaxed the moment you sit down on the couch. The more you feel relaxed, the more you doze off. Dozing off is the tendency to fall asleep at short intervals. But, because you are very tired and sitting on a comfortable couch, you can quickly fall asleep for a much longer period of time.
So, you may say to a friend:
When I am really tired, I doze off very easily.
or
When I am really tired, I fall asleep very quickly.
仕事からとても疲れて家に帰ってきたときにこうなることがあると思います。こういうときにソファに座ったらとても気持ちよく感じ、リラックスするでしょう。リラックスするほど眠りに落ちてしまいます。Doze off(うたた寝する)とは短い時間眠りに落ちてしまうことです。けれどもとても疲れているときに快適なソファに座っているとすぐ眠りに落ち、長い時間眠ることができます。
友達に以下のように言えます。
When I am really tired, I doze off very easily.
とても疲れているときはすぐに寝ることができます。
When I am really tired, I fall asleep very quickly.
とても疲れているときはとても早く寝てしまいます。
When I am super sleepy, it only takes me a couple seconds to fall asleep.
It only takes a couple seconds for me to fall asleep when i am super tired.
If you would like to express to someone that you can fall asleep within seconds when tired, you can say something like "I fall asleep within seconds when I am super tired.". or "It only takes me a couple seconds to knock out after a long and tiring day.".
I'm (contraction) - I am
'I' and 'am' are combined to form 'I'm'
_______________________________________________________________
'Exhausted' and 'worn out'
These terms are used to describe extreme tiredness.
It is not necessary to let people know that you will fall asleep in a few seconds if you use these terms.
People will understand that you are extremely tired.
___________________________________________________________________________
Examples
A: You don't look to great.
B: I'm exhausted.
A: You need some rest.
A; How was the marathon?
B: It was fun, but I'm worn out.
I'm (短縮形) - I am
'I' と 'am' は'I'm'に組み合わされます
_______________________________________________________________
'Exhausted' と 'worn out'
これらの単語はとても疲れていることを表すのに使われます。これらの単語を使ったら、数秒後に寝てしまうことを知らせる必要はありません、人々はあなたがとても疲れていると理解します。
___________________________________________________________________________
Examples
A: You don't look to great.体調よくなさそうだね
B: I'm exhausted.疲れきってる
A: You need some rest.少し休んだほうがいいよ
A; How was the marathon?マラソンはどうだった
B: It was fun, but I'm worn out.楽しかったけど疲れ果てた
A) I'm worn out i can fall asleep any second now.
*Worn out -extremely tired; exhausted.
Example -"you look worn out"
B) I am bone-tired !
*Bone-tired -extremely tired.
Example -I was really Bone-tired last night
I hope this helps :-)
A) I'm worn out i can fall asleep any second now.
*Worn out -とっても疲れている、疲れ果てる
Example -"you look worn out"疲れ果てているように見えるよ！
B) I am bone-tired !
*Bone-tired -とっても疲れている
Example -I was really Bone-tired last night
昨日の夜はとっても疲れた。
役に立ちますように！＾＾
To nod off - fall asleep, especially briefly or unintentionally, fall asleep, go to sleep, get to sleep, doze off, drop off; informalgo off, drift off, crash out, flake out, go out like a light, conk out; informalsack out, zone out
"He nodded off during the sermon"
"The audience began to nod off as the comedian's jokes were quite boring."
At the drop of a hat - When you use the expression 'At the Drop of a Hat' you mean that something is. happening instantly, without any delay. Example of use:
“We're all packed and ready to go; we can leave at the drop of a hat.”
＊to nod off= ふいに眠りに落ちる
他にも
doze off, drop off; go off, drift off, crash out, flake out, go out like a light, conk out, sack out, zone out
等が使えます。
例文
"The audience began to nod off as the comedian's jokes were quite boring."
（コメディアンのジョークが面白くなかったので、観客は居眠りを始めた）
＊At the drop of a hat =何かが瞬時に起きる様子
例文
“We're all packed and ready to go; we can leave at the drop of a hat.”
（荷造りと準備は整ったので、すぐに出発できる）
日本 
イギリス 
南アフリカ 
アメリカ合衆国 
