Hairline crack in Ipod screen

I woke up this morning with a hairline crack in my iPod screen. It's just a single line through the middle of the screen- no spider webbed cracks or anything like that. My iPod is still under a one year warranty. I had read somewhere that Apple doesn't charge for hairline crack repairs because it's considered a defect in the glass. Is that true? Because the crack in my screen appeared overnight, it was fine when I went to bed, and then not fine when I woke up! Thank you!