I've spent significant stretches of my life sleeping 4.5-6 hours/night on weeknights, and then 8-10 hours on weekends. It works just fine. I do drink a ton of coffee, but I'm quite alert even before my first one at 9AM, and capable of doing exercise/engaging in focused activity with zero difficulty. If I don't sleep enough on weekends though (like I have to set the alarm) I start getting jumpy and anxious though.