I had a build that wasn't turned off from 2000 to 2008 except to replace parts. Over the 9 years lost 2 HDs and 1 Tape drive. Like light bulbs, TVs and many other electronic components, for most parts expected life is more related to on / off cycles than running hours.
It cant ruin it and if its downloading it will be under pretty much zero load. People do this exact thing all the time. Many people run their PCs at full load 24/7 for projects and there is no reason any properly built PC should not be able too. Really running it at load for 24 hours ought to be part of your testing after its together anyway
I'm another person who leaves his PC on 24/7. (3770K, 16GBs ram, 7950(280) GPU, 2 SSDs with 2HDDs, etc) As mentioned above unless its updating, power goes off, or I'm not at home for a long time then it's on. Relax, breathe, you'll be ok with it on.