The only part of periodization that I regularly miss is rest weeks. I always remember to relax after a hard workout, and I never forget to take it easy on the weekend after a hard Mon-Fri. But after several tough weeks, you need to take a rest week too. This is the one that I sometimes space, and once I go past it by a few weeks it catches up to me: I'm tired, overworked, and lackadaisical.
I think I forget it because with my back as up and down as it's been, my exercises have been constantly changing, so I'm never doing one thing for too long. Cross-training is a good way to avoid exhaustion, because your body uses different muscles for different things. But even so fatigue builds, and you need a rest week to shed it.
This week is my rest week, and it's been a great break. For the last month I've been doing a heavy rotation of physical therapy, extra weight training, and swimming, so it feels good to simplify. I'm not taking the whole week off, just cutting the length and the intensity of every workout. Specifically I'm only doing my physical therapy, which has some decent exercises, but nothing that leaves me completely exhausted afterward.
The last couple days have reminded me of one side-effect of rest weeks-the blues. Exercise gives you endorphins, and the more you exercise, the more endorphins flood your system. When you workout every day, you eventually get used to that high. But when you cut back, you are suddenly getting way less endorphins than usual, and some people get really depressed. The thing to remember is that eventually your body will respond to the needed break, and when you bounce back you will feel the exact opposite: a sense of energy, lightness, and enthusiasm. I've been getting that on Monday's when I come back after a day or two of rest, but I'm looking forward to that bigger payoff that follows a whole week of taking it easy.