> In our “Depressed” cohort we only include individuals with a (clinical) diagnosis of depression, which they report on Twitter explicitly
To the way they tried to extract significance from the control group posting more between 3AM and 6AM than the depressed group:
> Compared to the control group, depressed subjects were significantly more active from 7 PM to midnight and less active from 3 to 6 AM.
If you have a significant number of people posting at 3AM in your cohort, you might have a very non-standard cohort to begin with.
Regardless, it's important to note that circadian disruptions are extremely common in depression. It's actually a bidirectional relationship: People with depression tend to either wake up too early (short sleep duration) or too late (oversleeping) as well as either struggling to stay awake or fall asleep. But depressive behaviors can also lead people to poor self-care habits, including not putting any effort into maintaining healthy routines. The latter can further entrench the problems caused by depression.
Interestingly, modifying circadian rhythms has been shown to modulate depression in many patients. Often, advance the sleep cycle (going to bed earlier, waking up earlier) can produce notable improvements. Unfortunately, it's easy to produce these changes under doctor supervision and reporting, but much harder to get patients to commit to maintaining the schedule on their own despite the improvements. A lot of people don't like going to bed early, especially if evenings are their time to avoid their stresses by watching Netflix or playing video games or scrolling social media.
Strangely, sleep deprivation is also a surprisingly good antidepressant. Depriving people of the second half of their nightly sleep or even keeping them up all night will often produce a strong anti-depressant effect, though not in everyone. It can even trigger manic episodes in bipolar patients. Sadly, the effect disappears as soon as the person sleeps again. There has been some study into using partial sleep deprivation and circadian rhythm advancement in conjunction with antidepressant medication to accelerate the onset of antidepressant action, but it's not a sure thing and it's ridiculously hard to get patients to actually go along with it, so it barely gets any attention.
Anyone interested in trying circadian rhythm advancement out can simply make an effort to go to bed 1-3 hours earlier and wake up a corresponding amount earlier. Use large amounts of bright light (more than natural room light) in the morning to amplify the effect. Of course, if you are suffering from depression and struggling to break the cycle, it's best to engage with professionals.