Rest days

Good for two reasons. One - your body absolutely has to have time to completely recover, especially when you are training hard. Muscles only get stronger during rest time, for one thing. But of course thats only one reason we need to rest. And for me personally, the other reason is that it makes me feel really lazy, uncomfortable and sluggish, and reminds me just why I train so hard in the first place, and pushes me to get right back on it the next day! God, I feel so crappy when I do nothing!