In case anyone is depressed and reads this, I'll add an anecdote that isn't as heavy as the article. When I was depressed, I would wake up at 4am every night and not be able to fall back asleep. So I'd get up for a few hours, work, then go back to bed for a sleep cycle before actual work. Over the course of a few months of recovering, this completed stopped. Now I can tell how depressed I am by how many times a week this happens, but generally only every few weeks compared to every day no exceptions. For me, they are very closely related.