The white "gunk" is just dead skin that is wet. It happens more at night because you drool and don't wipe it off so you wake up in the morning with the line. Try wetting your tongue and keeping it between your lips for just 5 minutes and you'll see that the white line appears since you have dead skin on your lips. once the skin is removed it will not turn white. I don't understand why this is so confusing for everyone.