Tests for Erection Problems and Penile Tumescence

When erection problems occur, it is important to identify the source of the impotence. One possible evaluation technique is an overnight study of penile tumescence during sleep. What is penile tumescence, how does it measure the vigor of erections in REM sleep, and how might assessment of it identify a cause of impotence?

A man looks to his erection during nocturnal penile tumescence testing
Stephanie Phillips / E+ / Getty Images

What Is Penile Tumescence?

Penile tumescence refers to the hardening and expansion of the penis into an erection. It often occurs in association with REM sleep and may be noted first thing in the morning. Therefore, an overnight sleep study called a polysomnogram may be used to assess the function of the penis during sleep.

How Does a Penile Tumescence Test Help Identify the Cause of Impotence?

There are various causes of erection problems. Sometimes impotence is due to problems with blood flow or neurologic function. It may also be due to psychological problems, such as may occur when related to stress or performance anxiety. In order to distinguish these various causes, it can be useful to test the "equipment" during sleep, when erections often occur.

Penile tumescence testing monitors the frequency, rigidity, and change in the circumference of the penis during erections that occur during REM sleep. An absence of erections may indicate a probable physical cause of the dysfunction.

How Nocturnal Penile Tumescence Testing Is Performed

When assessment is made during sleep, the measurement is added to a standard sleep study called a polysomnogram. Strain gauges, most often consisting of metal bands or springy loops, are placed over the base and tip of the flaccid (soft) penis.

The frequency of these erections can be observed. In addition, pulsations in the penis can be measured. This results in transient increases in circumference that may last up to 1 second. If there is a low frequency of erections, or penile pulsations are not observed, this may reveal a physical cause to the impotence.

It is also possible to assess the strength, vigor, or hardness of an erection. Longitudinal rigidity is measured with something called the buckling pressure. Historically, the strength of the erection was somewhat skillfully tested by smacking it down to gauge its vigor. (For obvious reasons, this is no longer done.) As the name indicates, buckling pressure is the amount of pressure that needs to be applied by a technician in order for the erection to bend or buckle. This usually requires forces that range from 1,000 to 1,500 grams (g).

In order to assess the rigidity of the penis, the patient is awakened (most often from REM sleep) and the penis is uncovered and stabilized between the index finger and the thumb. A force gauge is applied to the penis tip and the amount of force applied is gradually increased. In general, this force is increased until the shaft buckles or until the meter reaches 1,000 g. This measurement can usually be completed by a skilled technician in 30 seconds so that the erection can usually be maintained.

At home, there is also a so-called “stamp test,” in which a small roll of postage stamps is secured around a flaccid penis to see if the erections that normally occur at night are able to bust apart the restriction. If the stamps remain in place, the erections either didn’t occur or were not sufficiently vigorous.

Normal Findings and Morning Wood

In normal healthy men, the rigidity of an erect penis will range from 750 to 1200 g. It is estimated that rigidity from 500 to 740 g is sufficient for penetration while less than 500 g would rarely be adequate for sexual performance.

In some cases, a photograph may be used to assess the percentage of a full erection that was achieved as part of testing.

Morning wood or erections upon awakening normally occur in men, but if they do not, further testing may be warranted to identify possible causes.

A Word From Verywell

If you are concerned about the underlying cause of your impotence, speak with your physician about the testing options available to you. In some cases, nocturnal penile tumescence testing may reveal the nature of the dysfunction and lead to optimal treatment options.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Sexual Medicine Society of North America. Erectile dysfunction.

  2. International Society for Sexual Medicine. What is the nocturnal penile tumescence (NPT) test?

  3. Qin F, Gao L, Qian S, Fu F, Yang Y, Yuan J. Advantages and limitations of sleep-related erection and rigidity monitoring: a review. Int J Impot Res. 2018;30(4):192-201. doi:10.1038/s41443-018-0032-8

  4. Partin AW. Evaluation and management of erectile dysfunction. In: Campbell-Walsh-Wein Urology. Elsevier; 2021.

By Brandon Peters, MD
Brandon Peters, MD, is a board-certified neurologist and sleep medicine specialist.