How can indefinite article "a" comes with uncounted noun?

[Odessa Dawn]

Yoav, who is also from Eilat, said "I woke up from a deep sleep to the terrible sound of an explosion." Oren said he heard the sound of two rockets falling: "People on the street just froze in their tracks. It was frightening. Maybe it's time that sirens are installed in Eilat as well."

".I woke up from a deep sleep to the terrible sound of an explosion"

Isn't that an error?

[billmcd]

Yoav, who is also from Eilat, said "I woke up from a deep sleep to the terrible sound of an explosion." Oren said he heard the sound of two rockets falling: "People on the street just froze in their tracks. It was frightening. Maybe it's time that sirens are installed in Eilat as well."

".I woke up from a deep sleep to the terrible sound of an explosion"

Isn't that an error?

Some common expressions/idioms that include non-count nouns, especially those preceded by an adjective can/should include the indefinite article, "a/an", e.g. "a deep sleep", "a good education", "a heavy fog", "an extensive vocabulary" etc.

[Tdol]

Countable & uncountable are not fixed absolute rules- when a dictionary labels a word, it doesn't mean that it will be used that way 100% of the time.

[Barb_D]

When you add descriptive information, many uncountable nouns become countable because you've isolated a specific type.

We both like coffee.
I like a mild-flavored coffee called "Breakfast Blend."
She likes dark, bitter coffees that I find unpalatable.

You need more sleep.
He was in a deep sleep when the fire alarm sounded.
I awoke from a restlless sleep.
(Unlike "coffees" you can't really use "sleeps" but you can qualify sleep, and then you use "a/an.")