Oh, I forgot, I think sleep deprivation does increase their rates as well. Since you said that you come home knackered, it could very well be due to your tiredness. Or, it could be that because you are knackered, and because you have had a hard days work, you are feeling stressed - and the stress is what may increase the jerks. Could be either way really. Or it could be a mixture of both. We can't be sure. But like I said, try and be strong and not like me… let it drive you crazy!