The ears are active during sleep.

So we sealed their ears (with sleep) in the cave for a number of years.
(Qur'an, 18:11)

The original Arabic of the phrase "We sealed their ears" in the Qur'an is the verb daraba. This bears the metaphorical meaning of "We have made them sleep. Used in reference to the ears, daraba means "preventing the ears from hearing." The fact that only the sense of hearing is mentioned here is very significant.

According to recent scientific discoveries, the ear is the only sensory organ active while a person is sleeping. This is why we need an alarm clock to wake up. The wisdom of the pharse "We sealed their ears" is,in all probability, that Allah closed the hearing of the young people in question, for which reason they remain asleep for many years.