My first job out of college was a start-up in the financial industry; strolling in at 11am was highly looked down upon, and so ended my budding career as a finance whore.
Added: One of my responsibilities was to make sure my brothers got up in time for school. I can tell you from experience, that if you have someone to help, ice water is a great motivator to get out of bed.
He was mad, as he pulled a double shift because of that, but hey...beauty sleep :D
There's also some past health problems I've had which sometimes leaves me really light-headed when I stand up in the morning, so my options upon hitting the alarm are to lose vision and nearly pass out, or hurry to lay back down.
Also, keep in mind that due to the strange sleep schedule, 8am classes commonly mean I get less than 5 hours of sleep. Try getting 3-4 hours of sleep per night (or less, if I've had a project) for a week, and see how well you wake up to your alarm clock. Now sleep in on the weekend to try to make up for the deficit, and continue this for a few years. Alarm clocks just don't cut it.
Waking up with the sun is a slow process, whereas an audible alarm is abrupt.
Yesterday I woke up much earlier than I'd like (via alarm clock) in the hope of being tired enough to get to sleep before midnight last night. I struggled to concentrate and keep my eyes focused all day, but I was still awake until 3am.
I hate spending 12 hours in bed and get up feeling like I've had 6 hours of actual sleep. It's hard to understand if you don't have this problem yourself.