Page 130 - Baby Bargains 7th Edition
"We know that some parents figure it's easier to change crib sheets at 2 am if they simply pile on several layers of sheets. While we admire the creative thinking, we suggest NOT doing this. One sheet over a waterproof liner is enough. Or use the Ultimate Crib Sheet over your sheet - you won't need an additional liner since the Ultimate Crib Sheet is waterproof. The take-home message: any more than TWO layers on top of a mattress is dangerous."