A sleep study is any one of a number of procedures used to diagnose problems with sleep, most often sleep apnea. The most important of these is polysomnography, which can almost definitively rule out sleep apnea. In addition, a sleep study can be performed by the patient at his home, it can consist of waking up the patient and seeing how long it takes them to get back to sleep, or it can even test if the patient can remain awake during waking hours.