Have you experienced additional mental acuity/focus first hand?

Fasting for 24-36 hours for me isn't bad. But days 2 to 3 when I do a 3 day fast I find it extremely difficult to concentrate/focus on anything difficult. By day 3 my energy is usually shot as well and I just don't want to do anything but watch tv.

If anyone has advice beyond sugar I'd love to hear it.