Why do i get my highest Efficiency Factor on early morning runs?

This morning I did an early morning run (straight out of bed, no hesitation). This is a rarity for me these days, but I used to do this 3-4 times per week when I was training more seriously.
I noticed this morning and it reminded me of something I noticed before, my efficiency factor is higher on early morning runs.
I normally feel pretty bad and they are often following a hard session <12h before, but the efficiency factor is typically a few points higher.
Is this something to do with circadian rhythms and my optimum time to train? It certainly doesn’t feel like it!
Your thoughts are welcome, either based on science or amusing theory.

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Not based on science (only observation in my own training), but I’d assume it’s bc additional external factors haven’t been added to your day yet that cause your HR to naturally drift upwards with whatever external stimulation - so your muscles are slow/sluggish to respond, but your heart is as well. So your EF is higher because you can train without the external factors (caffeine, stress, etc.) also affecting your hearts response.

I actually notice the opposite, my EF is lower in the morning and higher in the evening - and I assume this is because I force quite a bit of caffeine in to get moving in the morning which spikes the HR, but later in the day after I cut out caffeine around 1pm that acute response is no longer there (despite whatever frustrations and stress come from the work day).

I have no idea if any of that is based on science, but it seemed good enough in my own mind.

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Hi @Bpellegrin, external factors could be an influence. Maybe the stresses of work impact my evening run.

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Your body’s natural hormones will vary throughout the day. I read somewhere that as well as a master clock a lot our cells have their own built in clocks. The circadian rhythms are real. There’s a reason that if you measure HRV / resting HR that best practise is same time of day, and usually first thing before you do anything else that morning. Your resting HRV / HR will differ between morning and afternoon. Hence if you keep measuring at different times you won’t get a baseline.

Thus I’d imagine this all affects how your heart rate responds to the stimulus of exercise through the day. I’m sure I’ve also read somewhere that your best performances may occur at certain times of day, but when that is, is individual. We don’t all have synchronised body clocks.