Here are some excellent references about the interaction between sleep, hormones, and obesity. They were contributed by a reader (thank you very much!):
Lastly a good review paper and shows the basics (along with some fun diagrams)
These references make it very clear that inadequate sleep increases the likelihood of obesity.
The last article was interesting, but oddly lacked any discussion of culture or psychology with respect to eating behaviours or obesity.
In terms of advising a fixed, early wake time, I believe this is entirely consistent with a plan to get adequate, optimal sleep. In fact, I believe that when individuals who are struggling with insomnia have a habit of sleeping in, the overall sleep quality diminishes, the insomnia pattern is exacerbated and perpetuated, and the health problems associated with inadequate sleep are likely to worsen.
Therefore, I believe that sleep quality and the restorative health benefits of sleep are most optimal if wake times are consistent and early. Possible exceptions to this could occur in adolescents, who probably need more sleep (but even then, it would be better for them to get that additional sleep by sleeping longer hours but getting up at the same time every day, rather than by sleeping in on weekends). Another exception could be in the setting of a physical illness, in which case one might need to stay in bed longer to recover.