Feeling Fatigue? You May be About to be Awesome

I've heard about bio-rhythms for as long as I can remember, and as I age, I am starting to notice my own body cycles a little bit more. One particular oddity stands out in my mind and one I'm just starting to see as a pattern is in my body's ability to accommodate a hard workout. A week ago, and for the week prior, I noticed a discernible decline in my ability to get through a full-speed workout without giving out half-way or two-thirds of the way in. The workouts seemed to get progressively worse as time went by. Then a couple days ago, things went the other way - I was getting stronger and stronger, faster and faster - and on Tuesday I had one of the best workouts I've had in years. This has happened before but only now am I beginning to discern that indeed it follows a pattern. I need time to define what is happening, if I can. Is it my diet? I have had more protein lately. Is it my sleep? I've had some bad nights and some good ones. Maybe something else? Cue the Twilight Zone music.