Why 寝たよう instead of 寝そう in this example


I was going through a JLPT grammar practice book and am confused on a
question:

「一郎、まだ起きているかな。」

「部屋の電気が消えているから、もう＿＿＿＿＿よ。」

I choose 寝そう since ～そう is often used to make conjectures based on visual info
(Ichiro's light is off). But the answer key says 寝たよう is better. Any thoughts
on why?




「寝{ね}そう」 = " _ **It looks like someone is about to go asleep**_ " The person
is still awake, officially.

「寝たよう」 = " _ **It seems one has just gone asleep/gone to bed**_." As far as
you can tell, the person is already asleep.

Thus, the two phrases describe very different situations.

> 「一郎{いちろう}、まだ起{お}きているかな。」 "Would Ichiro be still up?"
>
> 「部屋{へや}の電気{でんき}が消{き}えているから、もう＿＿＿＿＿よ。」 "_______ because the room lights are
> off."

The correct answer should clearly be 「もう寝たようよ」.




「寝{ね}そう」 = " _ **It looks like someone is about to go asleep**_ " The person
is still awake, officially.

「寝たよう」 = " _ **It seems one has just gone asleep/gone to bed**_." As far as
you can tell, the person is already asleep.

Thus, the two phrases describe very different situations.

> 「一郎{いちろう}、まだ起{お}きているかな。」 "Would Ichiro be still up?"
>
> 「部屋{へや}の電気{でんき}が消{き}えているから、もう＿＿＿＿＿よ。」 "_______ because the room lights are
> off."

The correct answer should clearly be 「もう寝たようよ」.

