That's a very good point. There are lots of potential confounders here. But I also want to point out that this study isn't a shot in the dark for random correla

That's a very good point. There are lots of potential confounders here. But I also want to point out that this study isn't a shot in the dark for random correlations -- there are theoretical reasons to believe that poor sleep hygiene would increase the risk of diabetes. Cortisol is tightly coupled to the sleep-wake cycle and to blood sugar levels. It is known to be causative of diabetes in certain rare disease (Cushing disease, for example) and strongly associated in garden variety type 2 diabetes. The article passingly references this (but it uses the more general term glucocorticoid).