Batteries Dying

There is some scientific basis to support what people often say when they feel worn out - I need to recharge my batteries. 

"...neurons involved in anxiety-related behaviors showed abnormalities: Their mitochondria, the organelles often described as cellular power plants, didn’t work well — they produced curiously low levels of energy.

Those results suggested that mitochondria might be involved in stress-related symptoms in the animals. But that idea ran contrary to the “synapto-centric” vision of the brain held by many neuroscientists at the time. "

That's basically our power supply running out of steam as the author explains the scientist's rationale:

"Sandi’s work sprang from an intuition that mitochondria might alter the operation of select brain pathways. Our brains eat up 20% of the oxygen our bodies take in, even though the brain accounts for only 2% of our weight. A deficit of cellular energy production in critical neural circuits, she hypothesized, might explain an overall lack of motivation and self-esteem seen in anxiety-prone people."

The remedy is exercise which seems to work for a lot of people once they can make themselves get out of their slump and say go for a run.