Binaural Beats can cure Mental Fatigue, as well as many other ailments. Mental Fatigue is caused by unbalanced levels of Sodium and potassium in the brain. Sodium and Potassium are used in osmosis, which helps transport chemicals into and out of the brain.
This fatigue is usually caused by an extended period in the Beta state (normal waking state). The quickest and easiest way to balance up the ratio between the Sodium and Potassium levels is to meditate in the Theta State for between 5 – 15 minutes.
If you are experienced in mediation and can reach this state easily then there is no problem. But most of us cannot enter Theta at will. This is another example of when Binaural Beats can be used. There are many downloadable Theta State Binaural Beats available (pay per download), or of course you could create your own!