I have noticed that I have gotten really, really tired lately.  And my body signals this crash by making my dick hard.  I have no idea if this is scientifically proven, but blood rushing into the penis means blood rushes out of the head, and that may be a prime setting for a body to shut down and rest.

I have listened to my body and stopped everything I was doing and gone to bed whenever my cock got hard.  Most of the time.  An instance or two I have cashed in on the erection by masturbating.  But these days, in my advanced age, I need sleep more than pleasure.  Most of the time.  So I rest.

That's all I've got to say.