Yes it's hard to say. Sometimes it's when I feel like it's a natural time for a break, but also maybe whatever it is (that the audio tickles in my brain) becomes fatigued or used up.

Either way the silence gives a great feeling of relief. I sleep with a noise generator in my room and can't really do without it (tiny noises fill me with adrenaline at night) but when I wake up and turn it off my body loves me for returning the silence.