How much sleep do we need?

This video teaches us to listen to our body, so we can get the sleep hours that we deserve. In average a human should sleep 7-8 hrs. But it really depends inch one of us and and our genetics. Sometimes we don't even bother to sleep but we need to, so we can be healthy and function properly. A word I didn't understand was groggy. It means daze and weakened, as from lack of sleep. And they used this word to describe how you feel when you don't get enough sleep. 

Dictionary: dictionary.reference.com/browse/groggy