Pardon the rather explicit nature of this question, but I don't really know how to ask it without providing sufficient detail, and it seems in line with some other questions I've read here dealing with sexual topics.

I know that any involuntary feeling or physiological reaction, by its very nature, can't be sinful. But I'm wondering if there's anything sinful about actually enjoying spontaneous arousal while it lasts, without trying to prolong it, and without entertaining explicitly impure thoughts.

For example, it's the middle of the night, and as men typically do, I wake up with a full erection. As I'm falling back to sleep, I enjoy the feeling of my penis touching the sheets for as long as the erection lasts. It could be a minute, or it could be on and off for a half hour, but that's where it ends. I didn't ask for it; it's just there, and I can't flee it because I'm still barely awake and it isn't time to get up yet.

So, in that scenario, am I sinning at all? If so, is it venial or mortal? I could be splitting hairs, but I'd like to put it to rest, and it's not something I'm comfortable asking a priest or spiritual director in person. For the record, I do sleep naked, but I've experienced it with the same frequency and intensity in the rare situation where modesty forces me to sleep clothed.