It is an old saying that “Khali dimag shaitan ka ghar”.
Empty mind means when you are doing nothing and Shaitan means negative thoughts. In terms of Vedic Sciences, negative thoughts mean absence of positive thoughts and they are often equated to darkness which is absence of light.
Positive thoughts always need efforts and exertions while negative thoughts are spontaneous and without exertion.
It is recommended that one should think differently and positive otherwise there will be spontaneous appearance of negative thoughts.
Darkness is spontaneous and naturally present and to bring light one has to make efforts by switching on the light or the nature has to ask the Sun to come and give the light.