I think Kahneman has a good explanation for this stuff in his book "Thinking, Fast & Slow." (I've talked about the book in past threads. Kahneman is a psychologist who was awarded a Nobel Prize for ground-breaking work on how our brain works and makes decisions, etc.)
To oversimplify:
Kahneman divides up the brain into two distinct functions: System 1 and System 2. (Corresponds roughly to "Right brain" and "Left brain" in common parlance.)
System 1 is automatic & emotional, works on the basis of associative memory, "continually constructs a coherent interpretation of what is going on in our world at any instant," consists of automatic and often unconscious processes. (p. 13) It's impulsive and intuitive. (p 48) It's your self-chatter and your spatial and situational awareness and your instinctual reactions to the world around you.
System 2 is controlled, slower, more deliberative, more logical. Capable of reasoning and cautious, but also lazy. The center for language and rational thought. Also the center for your executive functions: Attention and focus.
Here's the big difference between the two:
--System 1 is an auto-function that requires little or no energy. It's always cranking along at full speed as long as you're conscious, and it never tires or takes a break.
--By comparison, System 2 is a massive energy hog. When you use it, it burns down energy fast. And once it becomes depleted, it shuts down. Once it becomes depleted, you feel exhausted, burnt out, and spent. You find it difficult to focus on anything.
So the ideal is to rely on System 1 as much as possible, and use System 2 sparingly. For example, trust your emotions and gut instincts for routine work and functions during the day. But when System 1 starts going awry (for example, your emotions start spinning out of control and you get paranoid, or you require some heavy intellectual work), then jump in with System 2 and use it to bring your emotions back under control or for focus and attention. Once that's done, stop using System 2 and go back to relying on System 1 for routine work.
That's the healthy way to do things: Use System 1 for routine stuff, because it's essentially a "free ride" in terms of energy usage. And then use System 2 sparingly and mainly for making course corrections, because it's an energy hog; if you overuse System 2, you'll burn out and get tired.
The problem is that people start overusing System 2 in their daily functioning. (Since System 2 contains more of your conscious functions, your ego tends to associate itself with System 2 and overuse it.) That works for a while, but stores of energy available for System 2 are limited. When System 2 starts crashing, your executive functions start breaking down. Meantime, System 1 is still up and operative just like usual, but it starts spinning out of control without System 2 to act as a corrective.
From my notes on Kahneman's book:
--System 2 self-control becomes depleted over time or repeated applications. "If you have had to force yourself to do something, you are less willing or less able to exert self-control when the next challenge comes around. The phenomenon has been named ego depletion." (p. 41-2) Basically this is why healthy habits break down under stress. When working hard on a task, people lose self-control as a result of mental exhaustion, resulting in everything from breaking diets to reacting aggressively to others to giving in on other types of temptations. (p. 42)
Kahneman talks at length about exhaustion of self-control (ego-depletion) and its relation to blood glucose levels. High levels of self-control burn down glucose levels, resulting in poor self-control; and self-control can be restored by intaking sugar. Hence a lot of sugar consumption in those who do a lot of intellectual work. But of course that's not good for your health over the long-term. Meantime, tests show that people tend to make poorer decisions when they are hungry and tired, in the sense that there is little System 2 self-control left and thus an inability to maintain System 2 cognitive load and monitoring, so they revert to their System 1 defaults (p. 43-4).
To sum up: The healthy way to do things is to use System 1 for routine stuff, because it's essentially free in terms of energy usage. And then use System 2 sparingly and mainly for making course corrections, because it's an energy hog; if you overuse it, you'll burn out and get tired. Meantime, if you're going to do tasks that require a lot of System 2 thought, then do them in the morning when your energy levels are highest. Save the routine stuff (System 1 stuff) for the afternoon, when you're tired. Also, when you're using System 2 on a regular or heavy basis, then do things that replenish System 2 in sensible ways: Give System 2 frequent breaks via diversions (get up and do a few jumping jacks or take a short walk at good stopping points), take rest breaks, do a routine System 1 task or errand to give System 2 a break, eat healthy food at lunch to get your blood sugar back up a bit, enjoy some entertainment or chat with a friend as a break, etc.
That's the psychological concept of "flow." The concept of "flow" is an exception to the rule about energy depletion in the use of System 2. But that basically happens when you learn an intellectual subject so well that it essentially becomes part of System 1. For example: A long-time chess-player who is playing an intellectually demanding but also entertaining chess game often enters a state of "flow" as part of the game. At such times, he's basically working "from the gut," using System 1 instincts.
System 1 doesn't tax your energy supplies (it's a "free ride" in terms of energy usage), so "flow" is experienced as pleasurable: You're productive at some kind of intellectual endeavor, but there's no strain or depletion of energy occurring. Best of both worlds.
