I can't do it justice, but sleeping 8 hours > sleeping 6 and napping 2 later.
So sleeping 5 hours 5 days a week and 12 hours on the weekends just doesn't cut it.
As to sleeping less during the week, the three major ideas are polyphasic sleep (doesn’t work), cocaine (works, may kill you), and “rethinking your priorities”.
The simple matter is, if you're sleeping extra on the weekend, you aren't saving any time by sleeping less during the week.
The first wake-up call was when I moved from Belgium to the US. In Belgium, one enjoys much more paid vacation than in the US. This discrepancy showed me that I did not truly recover in the weekends - I needed my vacation days to recover.
Though I consider the work-life balance a misnomer (you have one life; part of the time you 'work,' part of the time you do other things), it is still something you need to regulate. First of all, assuming you are what I would like to call a 'knowledge' worker, you need to do other stuff to keep your mind fresh.
Secondly, as a rule of thumb, I would suggest that if your job requires you to work 10+ hours a day, you are doing your job wrong. Though it might sound odd, this is a useful rule of thumb. Whenever you find yourself in such a situation - STOP. STOP and reconsider.
EDIT: changed to 10+ hours... (btw: this includes transit)
Other things that help are not eating just before bed, and ensuring you have enough time in the morning to actually wake up! Oh, and get the best mattress you can afford
More seriously, sleep is physiologically essential - but so are other things. So make sure you optimize the hell out of the other things, starting with diet & exercise. (I'm guessing that "optimal" exercise under sleep deprivation is less than with adequate sleep but more than zero.)
Try to take naps. If you have enough downtime take a chance to nap rather than e.g. surf the web or futz with your phone.
Spend time on weekends preparing so you can sleep more during the week. E.g prepare food and have it as close to ready to eat as possible. Minimize the errands or cleaning you have to do on weekdays by doing it on weekends and spend the time saved on weekdays (if any) sleeping.
Also try to figure out how to get out of this unhealthy situation.