Nocturnal penile tumescence (NPT) testThe NPT test checks whether a man is having normal erections during sleep. Most men have 3 to 5 full erections during deep (rapid eye movement, or REM) sleep. Men who do not have erections because of psychological problems can still have erections during deep sleep. Occasionally, some sleep problems or serious depression can prevent these normal nighttime (nocturnal) erections.
This test can be done at home or in a special sleep lab. One of two ways may be used.
A simple ringlike device called a snap gauge made up of plastic films is fitted around the penis. The films break at certain pressures. So an erection during sleep will cause the film to snap.
An electronic monitoring device records how many, how long, and how rigid the erections are during sleep. An electronic monitoring device is more expensive than using the snap gauge, but it is more accurate and gives more detailed information about
erections during sleep.
Tests are usually done for at least two nights in a row. If good erections occur during sleep, the cause of the erection problems probably is not physical.
The NPT test may also be called the stamp test or the rigidity test.
www.webmd.com/erectile-dysfunction/tests-for-erection-problems