This one is still on:
<a href="https:&#x2F;&#x2F;www.supermemo.com&#x2F;articles&#x2F;sleep.htm" rel="nofollow">https:&#x2F;&#x2F;www.supermemo.com&#x2F;articles&#x2F;sleep.htm</a><p>Seems interesting but I have to say it is too long to read it now.
Too bad the pdf download redirect directly to the suspended account.