there are connections between getting proper amount of sleep and how your body manages all kinds of stuff including cortisol. plenty of credible studies tie lack of proper amount of sleep to weight gain
Like I said, these are observational studies only relating sleep duration and obesity. While observational studies give some idea to researcher to possible cause of problem but no way we can deduce any authentic results from them.
Example: Framingham studies are the largest longitudinal studies started decades ago based on observation. There is a link to cigarette smoking and obstructive lung disease but a causal relation cannot be determined between the two.
If you take all smokers, obstructive lung disease is very high among them BUT if you take all people with obstructive lung disease, only 15% people had ever smoked. *Hence no causal relation.
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People who are obese tend to have sleep disorder including sleep apnea.
They sleep less at night but stay groggy and even sleep during the day. (Narcolepsy).
These studies which are being sensationalized on media are NOT active studies meaning not prospective studies and a causal relation between sleeping for X- number of hours and obesity cannot be made.
Scientific studies on health have many flaws and even medical community acknowledges and criticizes a seemingly great well designed study.
There is a large topic of what a medical research requires to make it valid.