By that token, any and all studies are correlational — unless there exist studies that determine a link at a cellular level. The lack of sleep is directly linked to a reduction in testosterone and an increase in cortisol. While the former dictates muscle/fat nutrient partitioning and the level of your “anabolicness”, the latter has been directly linked to increased appetite which can cause weight gain.

I agree with the point that sleep isn’t black or white. “Sleep 8 hours” might not apply to everyone. I personally know folks who function well with only 6 hours but they tend to be super-deep sleepers.

Also, multiple stats show how most adults are sleep-deprived. Sleeping more is generally beneficial, as long as it isn’t taken to the extremes (9, 10 or more hours).