I believe we all have different sleep requirements based on our fitness, amount of work, stress levels etc. Some people thrive with 7 hours and some need 9 hours. I believe anythign under 6 hours can be prolematic. However, there might be some exceptions.

I only use biphasic sleep durign the emergencies and travel times by having a 20 minute power nap taking a caffeine and N-Acetyl Tyrosine tablet before hand so I wake up fresh. It might work for some people but biphasic sleep does not work for me.