You are all familiar with the feeling of tiredness that makes your body feel like a lead clump, makes your thoughts slower and your speech addle.
You can try driving it away with coffee, or nicotine, or stimulating drugs, but it's still there, only temporarily thrown off by these stimulants. It tears into you again twice stronger the very second the stimulant's effect starts abating.
My idea is inventing a portable stimulating device that would spur on one's vegetative nervous system by influencing it not with artificial chemicals but with mild ultrasounds perceived unconsciously. These ultrasounds would activate it in the same way our favourite music makes us energetic and enthusiastic.