There is no denying that sleep is beneficial and I agree with that assertion. I was specifically addressing his point about the drawbacks of sleep - "There are several survival drawbacks from sleep, as it leaves one vulnerable.".

Sleep wasn't a liability to our survival, especially our ancestors. It actually aided in our survival. Human beings who were active at night would have been the vulnerable ones as they would attract predators and were more likely to perish as a result. Especially considering out vision would be limited in the dark.

In short, sleep kept us away from the dangers at night.