My 400k 600k and 1200k times in the same year are not highly correlated or I wouldn't have remarked about how I find a 600k to be the most difficult. One factor is that on a 400k, if I'm not feeling it, I just use all the time. Like stopping at a fast food restaurant, for example. If it's the choice between that and suffering, I'll take the burger and fries, thanks. If you always get a lot of sleep on a 600k, then sleep probably seems more determinative to you than it does to me.