Spending energy creates energy. A short run and I’m more alert. A slightly harder run and (once I recover) I have energy for days. I take a push-up break from the computer instead of a cookie break, and I feel more energised. And the same seems to apply to most of us.

Physics still applies: there are no perpetual motion machines and the laws of thermodynamics remain unbroken.

But we are enormously complex machines made of mechanical systems that grow and change and run on hormones, directed by neurotransmitters. Inputting energy in the form of a processed snack has many effects beyond the extra blood sugar. Moving my muscles and joints and starting to strain my cardiovascular system has impacts far beyond using up some of that blood sugar and some of that energy.

I seek to be an empiricist – one who deals with the reality of the world even when it disagrees with ideology – and when I exercise I feel more awake and alive. Times when I’ve taken up effortful exercise after a long break, the benefits have been dramatic.

Your mileage may vary, of course – if you’re already working out hard, if you’re not getting a full restful sleep each night, if you  have chronic fatigue or are recovering from illness, then collect your data and observe your body’s needs with kindness.

For many of us (and for the typical reader of this blog) our default response to lethargy and tiredness is to sit or lounge and reduce our energy output. The result is less energy, in a self-reinforcing cycle.

Spend energy to create energy.