“Lack of activity destroys the good condition of every human being, while movement and methodical physical exercise save it and preserve it.” (Plato)
Exercise triggers chemical responses in the brain.
To counteract the cortisol and adrenaline, which is running about far too freely in most of us, it's essential you undertake a little regular exercise to release some of the more restorative neurotransmitters.
How's about some endorphins, norepinephrine, dopamine, serotonin?
If you're interested in how these amazing brain chemicals work, this poster gives you a brief summary, which helps explain why exercising makes you feel better, both short-term and longer-term.
It may be the last thing you feel like doing when tired or depressed or stressed out, but if you can find something you enjoy, it's well worth the effort.