There is no such thing as getting too much sleep. As some have already said, waking up during a deep phase in your natural sleep cycle will make you feel more groggy. However, your body will only sleep as much as it needs (unless some outside "influences" are involved). I took an entire class on sleep in college, and it's one of the few that I still feel like I remember.
There is a concept called "sleep debt" that is not without some controversy about the particulars. The basic concept is that your body needs a certain amount of sleep each night. Whatever hours you are deprived of will continually accumulate as your debt, weighing down on you and causing daytime drowsiness. The only way to repay this debt is to get extra sleep. Your debt can actually be measured by something called sleep onset latency. This essentially measures how easily one falls asleep (easier = more debt).
There was actually a study done by the Navy (I believe) that proved the sleep debt theory. In the study, subjects were allowed to sleep as much as they wanted to each night without interruption. In the beginning days, the subjects slept "excessive" amounts. However, as their debts were repaid, the amount eventually shortened and leveled off . This final amount was their personal required sleep, and the subjects could not physically sleep more than this.
Personally, I can verify this study. Last summer, I quit my job and took a few months off. In the beginning weeks, I slept 10-12 hours a night. But eventually, as in the study, I reached a point where I could not sleep anymore than about 8 hours a night. I felt much better during the day and had more energy. Of course, I had to get another job, and I'm now back to feeling like crap
