When you have been working flat out at work and reach that point where you think, I need coffee, I need something with sugar to keep me going, this is the time your brain is sending you a message based on ultradian rhythms.
We know about the circadian rhythm, the biological system that controls your sleep/wake cycle over 24 hours, but you have biological cycles that are shorter than 24 hours, too. These are ultradian rhythms.
Utradian rhythms have been made famous primarily through sleep study. The "father of sleep", sleep researcher Nathaniel Kleitman, figured out that people go through ultradian cycles whenever they get some shuteye. It was Kleitman who discovered rapid eye movement (REM) and proposed that sleep included active brain processes. But Kleitman also discovered that a Basic Rest Activity Cycle (BRAC) is present when people are awake, too. Generally, these daily ultradian cycles involve alternating periods of high-frequency brain activity (about 90 minutes) followed by lower-frequency brain activity (about 20 minutes). Scientists think that it's a delicate balance of potassium and sodium that ultimately controls these cycles. They also know that brain cells use sodium and potassium ions for electrical signals, and that your sodium and potassium levels are involved in the osmosis process that transports other chemicals in and out of your brain cells.
The brain is kind of a resource hog. It uses more energy than any other organ in the body, draining up to 20 percent of the fuel you have available. Most of that energy (about two thirds) is spent getting nerve cells to fire, with the rest spent in cell maintenance. When you work really hard, are alert and your brain is functioning in high brain wave states, you eventually disrupt the sodium/potassium balance. The brain detects this and downshifts, moving toward lower brain wave frequencies for a break. You perceive this as a general fogginess, fatigue or inability to concentrate. Once your brain has had time to get itself together, restoring the sodium-potassium ratio back to normal in the Theta state, the haze passes and you're ready to charge at your work again.
But what happens if you try to push through the rest phase of your ultradian rhythm? You trigger your body's fight-or-flight (stress) response. That's bad news because, as part of your natural survival mechanisms, the fight-or-flight response causes the parts of your brain that handle logic to become less active and you lose the ability to focus, and you're hyper alert and anxious.
It might be difficult to fight the cultural constructs that push you and others to ignore the physical signals associated with your ultradian rhythm but ideally, when you start to feel your focus draining due to your ultradian rhythm, take a break, shift your attention to something less demanding. Make a lap through your building, kick back in your chair with some relaxing music you like, turn away from your desk with a snack or do anything else you enjoy that winds you down. Just pay attention to your body and work with it, the goal is to work for 90 minutes and then take time out before returning to your next 90 minute focused period of work. Try your best to put this into practice, the rewards might surprise you.