Running In The Sun | 80/20 Endurance

Running In The Sun

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    I generally do almost all of my running early, while it’s still dark. Today, I waited until my lunch break. It was about the same temperature as a lot of my morning runs. Comparable effort and heart rate. The only difference was having the sun directly overhead. For some reason, it just felt more difficult than usual and I felt more drained afterward.

    All other variables the same, is there something about running in the sun that drains your batteries more?

    Matt Fitzgerald

    Could be coincidence, or could be a matter of what you are and aren’t accustomed to. In general, running at any given intensity is actually harder in the dark, or so says science.


    Even though the temperature of the air is the same, the direct sunlight will heat up your body more, causing you to use more energy to cool it down.

    Also, through a lot of HRV (heart rate variability) tracking, I’ve noticed that my HRV decreases immediately (body is stressed) when I’m in direct sunlight. It doesn’t take long nor does it require that much intensity for this to happen. Once I go back indoors, my HRV score quickly increases again.


    That’s interesting. I wish I had thought of this sooner, but I’m running in the sun a lot during the 3 weeks leading up to my Thanksgiving Day race. Consequently, my new (started using 3 days ago) Garmin Forerunner 245 seems to hate me for it. My VO2 max keeps dropping and it’s calling everything I do unproductive!

    Matt Fitzgerald

    I’m trained in mathematical statistics and so inclined to examine numbers before making conclusions.

    My own experience is that the Garmin numbers are an accurate reflection of the work I am doing. They will track with good fidelity where I am in an 80/29 training segment. For example, I am currently in the race specific segment of the Level III 5k plan – my training load and fitness have improved throughout the segment and my VO2 estimates have jumped three points during the segment. I have seen the same in prior training segments.

    Not long ago I contracted an bronchial infection and Garmin picked it up with it’s pulse oxygen capabilities. I compared the watch reading with the the one reported doing my annual physical exam and they were spot on. As the infection cured the watch showed the improvement, and that was reflected in my training. Similarly, Garmin picked up on a sleepless night and I had a note in the recommended workout that I should rest because of poor sleep (I ignored it and just did the 80/20 scheduled workout, maybe to my disadvantage).

    I’m looking forward to Matt’s discussion of Artificial Intelligence and training. There are some interesting challenges in applying AI, but I really believe we are on the cusp of something great…

    WRT running in the sun, the standard should be wet-bulb temperature, and perhaps some solar loading. Moving to a lunch time run vs morning run I’d look first at any pre-run rituals and where you are at in your digestive cycle as possibly greater impacts on training performance. Garmin also reports a Body Battery reflecting your energy state, usually at 100% in the morning you can expect that by noon it will be somewhat lower from your morning activities whatever they may be.

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