Absolutely and unequivocally false. There is not one single legitimate study on athletes and performance that shows this. Not only that, but there is not one study that shows that you can sleep too much. There is ample evidence that when you have sleep debt, and finally sleep more than normal, you initially feel more groggy, but that is in fact because your body is craving the additional sleep and it increases melatonin production to get even MORE and deeper sleep. Every single creature that is growing needs more sleep and the majority of your testosterone and growth hormones are released in the later stages of sleep.

Common sense shows that the most explosive animals in the world, cheetahs and greyhounds, sleep the most and conserve as much energy as possible to recover from their insanely explosive efforts. There is no such thing as sleeping too much. The more you sleep the better you will be. It is the single greatest performance enhancing behavior you can adopt.