Regarding your Question For people who listen, it means and is generally referring to those people who give a thought to things, why they happen, when they happen, how they happen and such questions. If you go through these questions and reach their bottom you'll find that it is a perfect mechanism which is being sustained and is not sustaining itself, and the sustainer of all things is Allah.
Signs in Sleep, well there is a hadith which mentions that Sleep is similar to death and going to sleep is similar to dying and and waking up from sleep is similar to ressurection. But waking from sleep has other meaning as well, i.e, Allah azawajjal made you to come back from a death like situation(It is quite relative to those people who don't wake from sleep because of some physical problem like some person in coma(hospital one!), they may wake now, the next moment or never even if their heart is beating.). It follows from this verse(that you mentioned) that we should be thankful to Allah that he gives us sleep and then wakes us up too.
"It is God that takes the souls (of men) at death; and those that die not (He takes) during their sleep: those on whom He has passed the decree of death, He keeps back (from returning to life), but the rest He sends (to their bodies) for a term appointed, verily in this are Signs for those who reflect." (Quran 39:42)
There is a record of few people mentioned in Quran as "People of the Cave", who retreated to a cave to seek protection from being persecuted for their belief in Allah
Or have you thought that the companions of the cave and the
inscription were, among Our signs, a wonder?
[Mention] when the youths retreated to the cave and said, "Our Lord,
grant us from Yourself mercy and prepare for us from our affair right
So We cast [a cover of sleep] over their ears within the cave for a
number of years.
Then We awakened them that We might show which of the two factions was
most precise in calculating what [extent] they had remained in time.
It is We who relate to you, [O Muhammad], their story in truth.
Indeed, they were youths who believed in their Lord, and We increased
them in guidance.
And We made firm their hearts when they stood up and said, "Our Lord
is the Lord of the heavens and the earth. Never will we invoke besides
Him any deity. We would have certainly spoken, then, an excessive
These, our people, have taken besides Him deities. Why do they not
bring for [worship of] them a clear authority? And who is more unjust
than one who invents about Allah a lie?"
[The youths said to one another], "And when you have withdrawn from
them and that which they worship other than Allah , retreat to the
cave. Your Lord will spread out for you of His mercy and will prepare
for you from your affair facility."
And [had you been present], you would see the sun when it rose,
inclining away from their cave on the right, and when it set, passing
away from them on the left, while they were [laying] within an open
space thereof. That was from the signs of Allah . He whom Allah guides
is the [rightly] guided, but he whom He leaves astray - never will you
find for him a protecting guide.
And you would think them awake, while they were asleep. And We turned
them to the right and to the left, while their dog stretched his
forelegs at the entrance. If you had looked at them, you would have
turned from them in flight and been filled by them with terror.
And similarly, We awakened them that they might question one another.
Said a speaker from among them, "How long have you remained [here]?"
They said, "We have remained a day or part of a day." They said, "Your
Lord is most knowing of how long you remained. So send one of you with
this silver coin of yours to the city and let him look to which is the
best of food and bring you provision from it and let him be cautious.
And let no one be aware of you.
Indeed, if they come to know of you, they will stone you or return you
to their religion. And never would you succeed, then - ever."
And similarly, We caused them to be found that they [who found them]
would know that the promise of Allah is truth and that of the Hour
there is no doubt. [That was] when they disputed among themselves
about their affair and [then] said, "Construct over them a structure.
Their Lord is most knowing about them." Said those who prevailed in
the matter, "We will surely take [for ourselves] over them a masjid."
They will say there were three, the fourth of them being their dog;
and they will say there were five, the sixth of them being their dog -
guessing at the unseen; and they will say there were seven, and the
eighth of them was their dog. Say, [O Muhammad], "My Lord is most
knowing of their number. None knows them except a few. So do not argue
about them except with an obvious argument and do not inquire about
them among [the speculators] from anyone."
And never say of anything, "Indeed, I will do that tomorrow,"
Except [when adding], "If Allah wills." And remember your Lord when
you forget [it] and say, "Perhaps my Lord will guide me to what is
nearer than this to right conduct."
And they remained in their cave for three hundred years and exceeded
Say, " Allah is most knowing of how long they remained. He has
[knowledge of] the unseen [aspects] of the heavens and the earth. How
Seeing is He and how Hearing! They have not besides Him any protector,
and He shares not His legislation with anyone."
Sura Kahf - 18, Verses (9 to 26)
This shows us that sleep is actually quiet similar to death the only difference being your internal body functions are working while asleep and they too stop when one is dead.
These things require you to research and think about these verses and search your knowledge base, and ask help of Allah in understanding things, this is why Quran generally refers signs to people who have knowledge(wisdom in particular and knowledge in general sense), similarly Quran says about changing of the shadow length throughout the day, it requires you to understand how and why shadows are formed and when you reach to the depth of it you'll see none but Allah.