It sounds like half the advantages, at least, are benefits of sleeping as much as you need. If I don't sleep enough I'm grumpy and apathetic, so if I only neede

It sounds like half the advantages, at least, are benefits of sleeping as much as you need. If I don't sleep enough I'm grumpy and apathetic, so if I only needed to sleep 6 hours (instead of 8+) I'm sure I would be happier. So I believe this might have done causation link.