I always wonder about these types of studies too. I have diagnosed delayed sleep phase disorder, secondary to adhd. I sleep odd hours compared to the norm - approximately 3am to 10am. However, my schedule (while odd) is completely stable. I go to bed when I feel tired and I wake up feeling refreshed.

Does the fact that my natural sleep habits are so off-the-bell-curve put me in the "disrupted circadian rhythm" group or not? I never feel sleep deprived. And I've been told by specialists that trying to force myself to sleep a normal schedule could put me at a significantly increased risk of dementia, alzheimers, stroke, heart attack, and more.

So the thought that I might still be doing myself harm sucks.