Why do we need sleep?

http://www.ted.com/talks/jeff_iliff_one_more_reason_to_get_a_good_night_s_sleep#t-113294

This Ted talk is about the importance of sleep. It's my first time to watch a Ted talk completely and I am so impressed by it. 

First of all, how can he memorize this speech?! It seems to be impossible for me to memorize this kind of long speeches since my memorization is bad. But he is so confident and can expresses his idea clearly. Furthermore, I am really interested in this topic. Lacking of sleep is a common problem nowadays but not everyone has realized. Thus, this speech can detaily remind people of the importance of sleep, which everyone should notice and change their mind.