My kids have each had one. They will put lots of electrodes on you to monitor breathing, o2 saturation, leg movement, 2 around your chest to monitor heart rate and respiration, 2 under your nose for the o2 info, between 10-20 electrodes (not as extensive as an eeg usually but can pick up some abnormal activity if it is there.)
you are told to sleep "normally" with all that stuff on. It videos and has a mic in the room to pick up if you snore etc. The tech will watch from another room all night. They can assess the info to see if you reach all stages of sleep, have any apnea events, etc....We were sent home around 6 am....