I don't mean to be rude, but you actually won't sleep. Your mind will still be awake and you will enter something known as Sleep Paralysis. Many people get lucid dreams from Sleep Paralysis, but you will not actually be sleeping.

k

btw it takes around 15-30 minutes, it doesn't always happen in 15 minutes.. also it's extremely hard to do so since you can't move anything... I've tried.