As you know I'm obsessed with power laws in biology, which is a biological consequence of fundamental principles, like energy conservation from the first law of thermodynamics. Geoffrey West showed how highly optimized biological networks—think blood vessels or respiratory systems—lead to allometric scaling. Specifically, the energy production per unit of body mass (mass-specific metabolic rate) scales as body mass (M) to the power of -0.25. This is part of what's known as Kleiber's law (or as we've dubbed it in our research, the Kleiber-West law), where whole-body basal metabolic rate scales as M^{0.75}. It's why elephants burn energy more efficiently per gram than mice, but mice live fast and die young. What's interesting, is that this same scaling pops up in something as everyday as sleep. Across mammals, daily sleep duration follows a similar power law: it decreases with body size as roughly M^{-0.25}. Smaller animals like shrews might snooze 15+ hours a day, while giants like whales get by on just a few. This is a clue that sleep is deeply tied to metabolism. Nervous systems are energy hogs, guzzling up to 20% of our body's oxygen despite making up only 2% of our mass. In smaller creatures, those fractal-like distribution networks deliver more oxygen per cell, letting their brains run "hotter" with faster firing rates and higher energy demands. But this revved-up metabolism exhausts resources quicker, creating energy deficits that sleep likely evolved to fix. Essentially, tinier mammals burn through their neural fuel faster and need more downtime to replenish. In this view, sleep isn't just rest—it's an ancient fix for the energy trade-offs imposed by Kleiber-West scaling, ensuring that high-metabolism critters don't fry their circuits. Sure, sleep does fancy stuff today. In humans and other mammals, it consolidates memories by pruning unnecessary synapses during REM phases and clears brain toxins via the glymphatic system, which ramps up during non-REM sleep to flush out waste like beta-amyloid. The relation of sleep and metabolism may have evidence from evolutionary history. The emergence of anaerobic metabolism could be tied the Great oxygenation event, 2B years ago. The next oxidation event (Neoproterozoic Oxygenation Event , 750M years ago) set the stage for Cambrian explosion leading to emergence of neural systems across species. And we had never had enough oxygen ever since. The link to a great Nature paper by @RafSarnataro et al, and some practical implication of that study are in the next comment. As usual, please like and repost - this is cool science (thank you @Alexey_Kadet for bringing this up) 1/2