When I am having trouble falling asleep. I use the sound of my breath and the mantra So Hum to make a better connection with my mind and body and to more easily fall asleep.
It helps me to keep my mind occupied with the sound of Sa for each inhale and the sound hum for each exhale. Then I will not get as easily distracted by my thoughts, as I try to rest and go to sleep.
It filles me up with prana (energy) so my mind can rest, and my body and mind can become integrated.
A mantra is a sound/words/vibration that are having a special power - when said or whispered in the mind repeatedly. It can give you special vibrations that can give you certain effects.
It is a natural mantra because the vibrations of the mantra are already a part of your nature. It is the sound of your breath. So is the sound of your inhalation and hum is the sound of your of your exhalation.
The Sa Hum mantra means I am that I am.
So it is also a mantra with a deeper meaning for accepting yourself for what you are.
As you inhale you can whisper in your mind Sa and as you exhale whisper hum.
After sometime you may no longer hear your breath and instead only hear and feel your breath with your mind and body.
The mantra can be used for resting your mind doing the day as a meditation or when you have to sleep.