Over the years I have met many people who say that 5 hours is enough sleep for them. That is definitely NOT the case for me. I need my 8 hours, I would be fine with 7 but I prefer 8.
Here's a tip for you that I heard in the past. The hours of sleep that you get before midnight are better quality sleep than those you get after midnight.
So, if you are limited to a certain amount of hours of sleep, try getting most of them in before midnight.