I heard that sleeping long hours can damage brain functions and harm the heart. Is that true?
Although this question is hard to research - you're dependent on self-reported data on sleeping hours, and there are many confounders that influence this relation (why do people sleep longer? maybe people who sleep longer are more likely to be without a job, and being without a job is also associated with higher risk of heart disease etc.).
However, the consensus seems to be that: Yes, it has been found that longer sleep (which is usually defined as >8 hours or 9 hours or longer) is associated with a higher risk of heart disease and also of diabetes. I'm not sure about damage to the brain functions, though I'm sure you could find this out easily too. This review gives an overview of studies that have investigated the association between sleep duration and cardiovascular disease.