I have found that the amount of activity I get in during a day is directly related to the amount of sleep I got the night before! If I get very little sleep for whatever reason...puppy woke me up several times, couldn't fall asleep, woke up too early and couldn't get back to sleep...I feel it all day. Not only am I sleepy, but I can't find the motivation to get out of my chair to go do something. Then, when I finally do get out there and do something, I have less energy even to do what I do every day. I can't run as far, I don't feel up to riding, etc. By contrast, when I'm well rested, I can go all day.
Today, I woke up with a crick in my neck. I'd had plenty of sleep, but now I have this horrible sharp pain in my neck. Great! I decided to go for my run anyway, prepared to walk the whole way if I had to, but at least I'd still burn some calories. I did manage to run, but it hurt...and I got tired faster. I think I wasn't breathing as well because of the pain, plus it is warm out. I had to stop running several times just because of the pain...but if I walked for a minute and stretched, I could run a bit more. I did get through my whole circuit, albeit in a little more time than usual. I probably didn't run quite as much, but I did run some and worked up a good sweat. Better than nothing!