One of the things that I constantly point out is that erections have everything to do with arterial plaque (atherosclerosis). There are many lines of evidence for this, something I discuss in my page on Erectile Dysfunction and Arterial Plaque. Further evidence of this was shown in a study I just came across that used Cialis (tadalafil) to show some remarkable things about daytime erections, morning erections, nitric oxide and the role of atherosclerosis in each. The study involved was from 2005 and they took middle-aged and senior men with erectile dysfunction and compared them to younger men with psychogenic (psychological) erectile dysfunction for comparison purposes.
Why did they use men psychogenic erectile dysfunction as the control group? The reason is that this group had nothing physically, if you will, with their ability to get an erection. In other words, the plumbing was just fine at least, and one sign of this is the fact that men with psychogenic erectile dysfunction usually still have reasonable morning erections. This is not necessarily the case generally with middle-aged and older men, who generally have a cardiovascular / arterial origin for their E.D. The study authors decided to study a different aspect of this phenomenon and hypothesized that the loss of nighttime erections was primarily due to arterial plaque.
All men in the study were given 20 mg of Cialis, which is a huge dose, every other day. Let's look at some of the significant findings of this study and the conclusions that one might come to based on the results: 
1. No Arterial Plaque = Restoration of Morning Erections. The authors actually used a penile "cuff" if you will to monitor morning erections as the patients were sleeping. What they found was that Cialis restored morning erections only in the men with no arterial plaque. Of course, the reason for this is that you "can't squeeze blood out of a turnip." Basically, nitric oxide triggers (downstream) cGMP which relaxes the artery, including those feeding the penis and within the penis. Cialis protects the degradation of this process and thus preserves this relaxing effect. However, if there is not enough nitric oxide to go around, then there is nothing to preserve: Cialis cannot do its job. (I cover this in my page on Viagra Failure.)
2. No Arterial Plaque = Improvement in Daytime Erections. The authors also gave a standard quesitonnaire to monitor improvement in erectile dysfunction, which of course has to do with the ability to have an erection during the waking hours. You can read about it on this page if you are interested: IIEF Scores and Erectile Dysfunction. What they found was that only the men with no arterial plaque had a significant improvement in their IIEF scores.
3. Morning Erections Were Correlated with the Amount of Arterial Plaque. The authors stated that "NPTRM [nocturnal erectile strength] and P-CDU [doppler score] parameters were inversely related to different degrees of carotid wall alteration ." In other words, the more the plaque, the worse the nighttime erections. Yes, plaque lowers your ability to produce nitric oxide, even with huge doses of Cialis!
CONCLUSION: Why is arterial plaque so hard on the arteries and your ability to get an erection? Caldwell Esselstyn, one of the Doctors Reversing Arterial Plaque, illustrates this nicely. He points out that young, healthy men have 6-8 tennis courts of endothelial cells lining their arteries pumping out nitric oxide. However, the typical senior man, due to the Western lifestyle, will have 1-2 tennis courts of endothelial cells. This occurs because the Western lifestyle is so hard on the lining of the arteries that it literally destroys the endothelial tissue. Of course, part of this process is the buildup of atherosclerotic plaques that are a root cause in heart attacks, angina and stroke.
So is all hope lost for men in the U.S. and other Westernized countries? Of course not. Remember that step 1 is to get your plaque measured somehow. You can have significant plaque with fairly minimal symptoms, so it's important to get it measured early. (If you have erectile dysfunction with no morning erections, then is a strong sign that you have an arterial issue.) Measuring your plaque can be done with either a Heart Scan / Calcium Score or an IMT. The former is the most accurate assessment of your heart attack risk but does involve some radiation. Both of these are under $100 generally. An IMT is an ultrasound of your neck artery and can be obtained through Lifeline for a cost of $70.
Step 2 is to beging regressing any plaque if you have any. I have many pages discussing some of the important information surrounding this here: Summary Page of Arterial Plaque Regression. You may want to start out with my page on How to Clear Your Arteries.
1) European Urology, Aug 2005, 48(2):326 332, "Resumption of Spontaneous Erections in Selected Patients Affected by Erectile Dysfunction and Various Degrees of Carotid Wall Alteration: Role of Tadalafil"