Thanks for the question.
A sleep study is useful in identifying the following conditions;
OSA (obstructive sleep apnea)
PLMS (periodic limb movement disorder of sleep)
RBD (REM sleep behavioral disorder) etc
The sleep study, also called as polysomnography is performed in a sleep lab. During the procedure patient is hooked up with myriad electrodes so that the brain waves, breathing pattern, muscle tone, eye movements etc can be captured and interpreted.
For all the above conditions except the narcolepsy a night time study is all that is required to arrive at the diagnosis. If narcolepsy is suspected; then the following day patient needs to under go another test named as multiple sleep latency test (MSLT) during which patient will take several scheduled naps during the day time (day time test).
The sleep study test reveals the total duration the patient was on the bed, the actual duration the patient slept, awakenings from the sleep, cessation of breathing (apneas - total cessation of breathing & hypopneas- partial cessation of breathing), sleep architecture, onset & duration of REM & NREM sleep, abnormal leg movements, any behavioral changes etc and helps in arriving at the final diagnosis
Please ask if there are any further questions.