Body and mind doesn't really recieve any benefit from sleep beyond 5 to 6 hours. There's a variance for exactly how much each individual requires but the standard 8 hours is essentialy an indulgence of at least two hours for the brain, akin to opiates or other pleasurable stimulus that doesn't do anything short of being a treat.
Overly short sleep periods 2 or 3 hours, etc. for any sustained periods are very dangerous. The big risk is if that subject is allowed to drive or handle anything dangerous where one must be attentive. What ends up happening is the person starts to have microsleeps. The brain literally goes to sleep for a few seconds while the person is driving or doing whatever. Think the person you were talking to just wasn't paying attention? They might actually have been sleeping for a couple seconds
