By a previous patient at Royal Brompton Hospital
If you have been diagnosed with sleep apnoea and have had continuous positive airway Pressure (CPAP) treatment for about six months you will have a second sleep study (sometimes called CPAP retitration – pronounced RE-TIE-TRATION).
This study is to see that your CPAP machine has been set right for you and prevents the oxygen level in the blood dropping during the night. You need to bring your own CPAP machine with you to the hospital sleep study.
The procedure 
For this study all you have is the finger oxygen monitor clip taped to a finger and a wire to a small data storage unit (less than cigarette packet in size) that is on a strap on the wrist.
The procedure can actually be done at home after a demonstration of the procedure (and you were hopefully offered that option), but if, like me, you have a long journey from home to the Brompton you may not find it convenient to come to the hospital on two separate days to collect and return the monitor.
Overnight you wear a hospital CPAP machine that is set like your home CPAP machine. When the monitor is taken from you in the morning a technician will read the results and if needed re-set your own CPAP machine. The doctor will see you with the results, your CPAP machine and discuss any problems with the treatment.
Living with CPAP
Hopefully, like me, your life has changed for the better with CPAP. In my case, the Brompton’s advice was against the “wonder laser treatment”. I have heard of lack of success in that approach more often than I have heard of benefit (and at less much cost). I left the hospital for home at around 10 am.