On 600+ km rides you will be stopping to sleep during the ride. How long you sleep for is entirely up to you, but in general riders will try to leave the sleep control no later than the closing time of the control. Even if this means only 2-4 hrs sleep, you will be amazed at how this little sleep can refresh you during a ride.
It is worth noting that fatigue through sleep deprivation can impair your riding ability. Reaction times are slowed, judgement can be compromised. Riding past mid-night when the body is normally asleep carries additional risks to the normal risks of cycling and is a time for extra caution. It is a time when motorists may not expected to see cyclists. All Audax Australia deaths have occurred on 600+ k rides.
It is typically on these rides, that there will be a designated sleep control and you will have an overnight “drop bag” at the control. The ride could be a loop back to the start, or the RO may transport your drop bag between the sleep control(s). Your “drop bag” should contain a change of cycling kit, and spare tubes/batteries to replace anything used on the ride.