I don't think it's a zero sum game and I'm pretty sure other factors are involved e.g. stress, alcohol intake, volume of exercise, etc). I do know that my perceived quality of life is improved when I'm getting plenty of sleep, and for me that requires about eight hours.

As I wrote in my initial comment the occasional disturbed night is not a big problem when I'm well rested. Under those circumstances I never feel the need to get extra sleep to compensate for the lost hours.

I think it's a deficit in the sense it starts to sum up for a few weeks and you get progressively tired until it starts to harm you, your brain, whatever. You can revert your tiredness sleeping well for a few days but the damage caused by prolonged periods with insufficient sleep it's done and maybe can't be undone.