It is most beneficial if you can sleep, even if just for four hours. Inside of four hours, you will cycle through all the stages of sleep, including REM, at least twice...ideally.

The tech, who must monitor all the rooms all night, awakened me about an hour after I fell asleep to re-attach one of the chest leads. I fell asleep again minutes later, thank goodness, and managed about five hours. It was long enough that he was able to diagnose me over about three hours, and then run a CPAP mask I was already wearing and he 'titrated' me to find what level of air volume was best to be delivered over time. In some cases, some labs, you will have to do this over two nights, maybe a week or more apart, but the idea is to titrate you if possible...and that would ideally be on the second night if that is the protocol.

You'll get leads on your chest, your back, your scalp, your arms, and on your legs. They need a whole bunch of information. They'll scrub your scalp in about six/eight places to clean it of oils, and then affix pads there. You're literally covered in wires and pads. Still, I was able to fall asleep after finding a comfortable position. The leg and arm leads are to detect periodic limb movement. Some people have 'restless leg syndrome', which at times does disrupt their sleep.