I do intermittent fasting (12-20 hours w/o food) usually everyday. I find my head feels very clear in the morning before eating.

You may be interested in something called dry fasting, which is explained towards the end of this podcast. I've personally never tried it, but one day I hope to, as well as increase the lengths of my fasts. My problem is that I usually go hypoglycemic around the 13-15 hr mark.

Ian Clark Podcast

I think I remember Mark Sisson saying that it is only "necessary" to do longer fasts at a maximum of once per week.

Thanks for sharing. Keep us updated.