Could be as simple as increased sleep debt. 70% the maternal age correlation is swamped by the college degree relation.https://twitter.com/DegenRolf/status/789338116553400320 …
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Among SEVERAL standards applied, one was if subjects would sleep longer if given opportunity. They wouldn't. Rather watch TV.
I'm sorry, but that is an awful standard. That's exactly the kind of behaviour that increases my own sleep debt.
Of course, there were several standards of "optimal functioning", and more. They tried everything.
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