I think it's a different sort of exhaustion once the baby arrives. You need to feed it at least every three hours so if you are breastfeeding and so can't delegate feeding then your sleep all comes in chunks of two hours tops if that and days and nights run into each other in a relentless sort of way. However you don't have the physical exhaustion of pregnancy and after the first week or so I've always been able to drop off instantly, plus it all feels much more worth it.. A lot will depend on what your baby is like too in terms of sleep etc it seems to vary quite a lot. I know it is a cliche but not sure anything can really prepare you though.