Ancestral correlations and internal proxies

Our bodies need to spend energy to do things in the world. They also need to heal and build up. Evolution compelled them to budget for both programs. But there's a tradeoff: both could run well enough, but not at the same time. So the lives of our early, unthinking ancestors were already split by a bipolar rhythm: waking and sleeping.

Sleep is not the only rhythm in the life of animals. There's also all that breathing, thinking, moving, and eating we do while we're awake. The timescales of the different rhythms may vary, but they're all coupled together to serve survival and reward.

Rewards aren't just those immediate, external signals an organism reacts to, and learns from. It learns and exists as it does, because of the ancient history of reward. The contracts that keep time – and many of the signals, too – are internal, written long ago as the structure of the bodies of its ancestors.

Whenever an animal moves a significant distance through the world, exploring, it arrives somewhere else, a place where there might be fresh opportunities. The further the animal moves at once, the more it should prepare to pay attention – shift towards exploring on a smaller scale – for local resources to exploit. And the longer it moves the more it switches, internally, to an exercised state: its motor cortex oscillates for a while, sending signals to its muscles. Its muscles spend more energy than usual, and send out messengers to complain. Its brain can sense these things; it hardly needs to see the world moving past, to know it should be preparing to arrive somewhere else.

Evolution took advantage of reliable correlations between our bodies and our world. For our ancestors, moving at distance through the environment was strongly correlated with their tissues being in a cardio-like state. So, that state was a good proxy for "maybe there will be new local opportunities soon"; if the brain notices that the body internally appears to be cardio-ing, it should start biasing attention away from broadly moving and towards narrowly consuming. But we don't intrinsically want to "consume", or whatever you want to call it. We want the experience of reward. We want to enjoy. And that's what we see: our hedonic baseline and sensory appreciation increase when we do cardio.

There's an attentional drive which makes us slow down and consume, by making us more sensitive to local enjoyment. Something is there to flip the rhythm back around.

And because movement → enjoyment is built-in to your body as the proxy signal cardio-state → enjoyment, you still benefit from it even if you violate the ancestral correlation that bound the cardio-state as a proxy. A stationary bike is stationary, it definitely doesn't bring you anywhere new, but it still works as an antidepressant. (Could it be the immediate deficit of calories that's responsible? I doubt that's the whole reason. You can pre-load with ten times as many calories as you'll burn, and still experience the mood boost.)

Evolution cannot easily go back and change its decisions: new patterns tend to ride on top of the old, not simply replace them. Rational thought is a recent discovery. It's great, it's powerful, but you can't use it to force your sleep rhythms to be independent of how much blue light you're exposed to, or how much you eat before bed.

Have you tried just not being depressed? It isn't so easy. Still, you can trick a hamster with a wheel.