Re: How can indefinite article "a" comes with uncounted noun?
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.")
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.