I once wondered if one could lose weight by thinking harder, but it seems like the energy consumption in the brain is not a simple matter of greater mental effort sapping more of the body’s available energy.
Even though a more difficult mental task requires more energy because of more neural activity, you won’t see a large increase of glucose consumption. Claude Messier of the University of Ottawa explains that the reason for this is that the brain at base level already is consuming quite a lot of energy, even in slow-wave sleep with very little activity there is a high baseline consumption of glucose. Most organs do not require so much energy for basic housekeeping, while the brain must keep the all of the cells in its billions of neurons charged even when those cells are not firing. Because of this continuous maintenance, the brain usually has all the energy it needs, and you won’t lose weight by thinking harder.
Read more here: http://www.scientificamerican.com/article/thinking-hard-calories/
diplopi 
That answers a long-standing question of mine that I never got around to researching – many thanks Jarle; and Happy New Year!
Happy New Year to you too, Hariod, and glad to hear we both learned something 🙂