I know exactly what you mean. I have days where I feel sluggish and tired, and just drained! With me, I don't know if its from my depression or my heart procedure I had a few months ago, making me so tired, but yes one day at a time. And yes, it can be a lot worse in the heat. It's winter here, I will send you over some of our cool weather :)
And you have a good day too.