The longest stretch I've gone as an adult without setting an alarm clock was three years. At first it was great, but after awhile I started sleeping 9-10 hours a night. I didn't notice it was a problem until everywhere I went people started telling me I looked tired, and I noticed that I had perpetual dark circles under my eyes. I think I was oversleeping. When I went back to only letting myself sleep 7-8 hours a night, the dark circles went away and I started having more energy. Of course, this is all completely anecdotal, but it seems to me that there are a number of things about modern life aside from artificial light and alarm clocks that disrupt natural sleep and waking up cues. It's important to have both, waking up cues included, for healthy sleep.