I actually cope really well missing out on sleep having had two children 13 months apart. However I have always advocated a work/life balance and believe that work is not the be all and end all... With graphic design however, sometimes you stare at the page for hours with not much to show for it and then wham all of a sudden a light bulb goes off and you start creating.. Next thing you know, 4 hours have past, the house is silent and it's 1am! So whilst I do love my sleep I also feel great completing something. I also work much better under pressure. Are you a slow and steady worker or a frantic just get it done person? I think I fall somewhere in between.
Hope you have found your balance.
Have a great day