I was thinking along the same lines, but bigger. Mitochondria don't "shut down" when we sleep. If they did, we would die very quickly. If anything, they produce quite a bit of energy during things like REM sleep and digestion. I'm sure I'm missing some subtle details about HOW they "rest", but from a 30000 ft view, it's puzzling.

The paper's core idea isn’t that all cells that use mitochondria need sleep, but rather:

> In a specific subset of sleep-inducing neurons, mitochondrial electron leak builds up when energy is available but underused during neuronal inactivity. That mismatch acts as a sleep signal.

The heart doesn’t fall into that subset.