I use an alarm clock, but mainly because I'm incredibly paranoid about oversleeping. It's incredibly rare for me to actually oversleep - I'm an early riser by nature, and if I have a set schedule my body naturally falls into it - but if I don't set an alarm my body/brain panics and instead wakes me up every hour to two hours convinced that I'm late. So I set an alarm, wake up usually a good fifteen minutes to half hour prior to said alarm, and usually only wake up a couple times during the night rather than half a dozen times.