Working some number of hours doesn't necessarily mean working "hard". IMHO it's all about the intensity and effort level required wether it's pjysical or mental

Working some number of hours doesn't necessarily mean working "hard". IMHO it's all about the intensity and effort level required wether it's pjysical or mental. Once you repeatedly reach your boundaries and push there and even beyond are you really putting in "hard work". And that's where it gets uncomfortable. In physical exercise and work on this is felt through lactic acid physical exhaustion muscle soreness etc. office jobs and other non physical activities (for example learning math or a new language) will result in mental tiredness and exhaustion.