It's essential to sleep in most multiday races. The body cannot go much more than 36 hours without losing a serious amount of mental clarity. The art of multiday running is discovering how much sleep you can get away with. There's a fine line beyond which the body needs a break. If you are uninjured yet averaging less than 3 miles an hour in a session, one should contemplate a break. Under extreme exhaustion its not always easy to make that decision for oneself. Hence one of the advantages of having a handler who is able to assess your situation from a clearer perspective.
How Much Sleep
The amount of sleep necessary will of course be different for everybody. Some people may prefer 2 or even 3 sessions spread throughout the day. Generally there is a trend of thinking that sees a minimum of three hours and with another REM cycle thrown in, maybe four and a half hours is ideal. For races longer than 10 days perhaps a little more would help recoup energy levels and aid recovery. The runners in the Self-Transcendence 3100 mile race rest between midnight and six in the morning as the course is closed at that time. That leaves about five hours of actual sleep.