That's slightly below the average amount of daily sleep advised by the National Sleep Foundation, which recommends adults get seven to nine hours of sleep each night to maintain a healthy weight and mood and improve memory and athletic performance.
It's surprising the 45-year-old entrepreneur even manages six hours, considering he splits most of his days between his ambitious aerospace and automotive companies, is a major philanthropist and the chairman of the green-energy-service company SolarCity, which has since been acquired by Tesla. He also sits on several advisory and engineering boards.
Musk has said he has slept in Tesla's factory -- even keeping a sleeping bag in a conference room next to the production line so he can better inspect vehicles as they come off it.
If you're wondering how Musk can accomplish as much as he does with little sleep, there are plenty of ways people can function at optimal levels with just four hours of sleep. Business Insider even has a step-by-step guide (with charts!) to gradually reducing your sleeping hours while getting more out of your waking hours.
This post originally appeared on Business Insider.