What are the benefits of sleeping nine hours rather than seven or eight? what are the costs

Recent studies have actually shown that it is overall healthier for you to get less than nine hours of sleep.  The ideal amount of sleep is 6.5 hours. Anyways, as far as the answer goes the benefits would be that you would think you had more energy, therefore you would act that way, and beautyrest can give you clearer and softer skin+ reduce dark bags under eyes. The costs would be not actually having more energy(I know it sounds absurd, but believe me it's true), becoming dehydrated during your sleep, wasting away the day, waking up feeling rushed instead of refreshed, and less health benefits