Sometimes, what our brain wants is not always good for our bodies. Donuts are a good example. It is early morning and you're driving to work after a nice breakfast of black coffee and two eggs, easy-over, with bacon. Yet, you're still hungry and having difficulty paying attention to the traffic. Why? Your brain is not cooperating because it is not satisfied with that breakfast because it lacked one critical ingredient that your brain urgently needs, sugar. You have been fasting since dinner last night and your blood levels of sugar have fallen to very low levels. From your brain's perspective, sugar is indispensable. It will do whatever is necessary to convince you to eat sugar as often as possible. Why? Your brain needs sugar (usually in the form of glucose) to function normally. The billions and billions of neurons in your brain require a constant supply of sugar to maintain their ability to produce energy and communicate with other neurons. Your neurons can only tolerate a total deprivation of sugar for a few minutes before they begin to die. Therefore, as blood levels of sugar decrease with the passage of time since your last meal, you begin to experience a craving for food, preferably something sweet. Essentially, the presence of sugar in your brain is considered normal, and its absence leads to the feeling of craving and the initiation of hunting or foraging behaviors, such as seeking out a vending machine for some cupcakes or a candy bar.
Once inside the brain, sugar is also used to produce a very important neurotransmitter chemical call acetylcholine. Acetylcholine allows you to learn and remember, to regulate your attention and mood, and to control how well you can move. Your brain makes acetylcholine from choline, which is obtained from the diet, and from acetyl groups that originate from the metabolism of sugar. We frequently obtain choline in our diet by eating lecithin. Lecithin can be found in many different bakery goods such as donuts and cupcakes and is commonly added to chocolate. Thus a tasty chocolate covered donut first thing in the morning is going to provide your brain with everything it needs to pay attention and learn new things. Sadly, those eggs and bacon that you had for breakfast were completely insufficient for the task of preparing your acetylcholine neurons to function.
As the day progresses your acetylcholine neurons are busy consuming choline and sugar as you spend your day thinking and learning. However, as evening arrives you cannot help but notice that you're having trouble paying attention and you're experiencing a little mental slowing. What's happening to your acetylcholine neurons and what can you do about it? The cure for our mental slowing: coffee. While we were busy thinking and learning all day another neurotransmitter chemical was increasing in concentration and it has slowly and powerfully begun to turn off our acetylcholine neurons. This chemical is called adenosine. Adenosine inhibits the function of acetylcholine neurons our brains and the longer we are awake the more persuasive is its influence. The caffeine in our coffee is able to prevent the actions of adenosine and release our acetylcholine neurons from their chemical shackles; our attentiveness improves and we are ready for anything - at least until the caffeine effect wears off.
So, tomorrow morning do not fight the urge to stop for coffee and donuts. It's what your brain wants you to do. Now please go back and read the first sentence again.