I built a circuit that flashes a 6 by 6 array of LED lights at
40 Hz (40 flashes per second). This 40 Hz frequency
stimulates the brain to strengthen the level gamma waves
that it normally produces at that frequency. This is said to
have a calming effect on your mood and to generate a sense
of well-being. And is said to be an aid to meditation. Also,
a 40 Hz flashing LED array, might alleviate the symptoms of
Alzheimer's disease, a small study has suggested.
Here is a link to the circuit:
The LED array I use in the circuit is this product on Amazon:
You could power it with a power bank, external battery for
charging smartphones. There are many of these on Amazon.
Also you could power it with a 5 volt voltage-regulated wall wart.