If you enter a Zen master's hut, he may suddenly give a scream. You will become startled, but if you know why he is doing this you know he is doing it just to make you awake. Any sudden thing makes you awake. Any sudden sound can make you awake.
Suddenness breaks your sleep, and ordinarily we are asleep. Unless something goes wrong, we are not out of sleep, we go on doing things sleepily. That is why we never feel the sleep. You go to your office, you drive the car, you come back to your home, you love your children, you talk to your wife, so you think you are not sleepy at all.
In a deeper sense, we are all sleepwalkers. This sleepiness can be broken.
