> the much more likely explanation is that the brain uses contrastive learning somewhere

This doesn’t explain sleep in virtually brainless creatures. A metabolic root cause seems more likely.

Why would neurons be exempt from training just because they weren't clustered into a brain? By comparison, transport-related phenomena are almost completely trivialized in the simple creatures. The existence of sleep in simple critters seems strong evidence in favor of contrastive learning over cleaning explanations.

Could you make the case for metabolic explanations? Are there a bunch of coupled mechanisms that all require downtime and couldn't possibly be phased?