My wake up HR was 50 this morning. When I left on my run the temperature was 31F with a 10 mph wind (17F wind chill). I had a nice run but felt tired at the end of the run. I ran in shorts again, but because of the wind, I kept my breaker on for the full run.
I can think of two reasons why I've felt tired at the end of my last two runs. It could be the increase in distance that I did on the 24th, or it could be that I didn't get quite as much sleep this week as I need. Additional distance (or speed) and lack of sleep put a greater stress on my body, and this reduces my endurance. If the lack of sleep is up to half an hour, it may not affect my waking HR much, but if it is more than that, it does increase my HR.