Yes I find my patience wearing thin when I'm tired, and have felt myself getting shouty, DP also. I hate it, it's not how I want to parent (or ever imagined I would!) I was a CM for years and never once shouted at the DCs. I don't want to shout at mine!
The last thing I want to be is a shouty mum though, it's not fair on DS. I've got this book specifically in an attempt to tackle this: Calmer Happier Easier Parenting
It's written by someone who actually is a childcare expert (relevant qualifications, decades of experience, a background as a teacher and a mum) and was recommend to me by mumsnetters.
There's a technique per chapter, you learn it, have a go at implementing it and then when you feel comfortable you move on to the next chapter. DP and I have read the intro and chapter 1 and are just starting on doing the first technique, hope it works! It's for children aged 3-13.
Another book which has been recommended to me here, I haven't got it yet but will do (might order it now actually!): How To Talk So Kids Will Listen and Listen So Kids Will Talk
It's so hard when you're tired, but I'm ashamed to say I've left it too long to deal with this I think. (DS is 4). I get the feeling he just zones me now out if I shout now, it doen't make him more cooperative! It's up to me to find a way make this change I think. Muddling through simply wasn't working.