I did an hour on the turbo this morning but unlike most times when I have plent of energy, this time I felt really weird, here is what happened.
06:00 - A little bit of porridge to fuel my workout and gave it 30min to 'settle' same as most mornings before a workout. 06:30 - Started workout, I did my usual 10min warmup, then the plan was to do 50min in Z3. 06:50 - So about 20min into the workout I started feeling flushed, light headed and really weak, so I backed right off. 07:05 - I started feeling better so gradually upped the pace again and I ended up finishing really strong doing Z4 work as I felt like I had a lot of energy.
WTF happened, was that the porridge kicking in, or maybe my body switching to fat? I'm inclinded to think it wasn't either as from what I have read the body holds about 2000kal of energy in the muscles, this would have depleted over night but not so much to not allow me to work out...?
No idea what happened, but you did not deplete glycogen stores overnight nor switch to burning fats. Neither of those things happen during sleeping or training.
Yeah a typical adult male can store around 1400 to 1800 Calories in muscle glycogen but very, very little of that is burned during sleep. The vast majority of our energy burned during sleep comes from fats at the very low intensity related to sleep, some blood and liver glucose is burned to fuel the brain but we don't deplete our sugar stores while sleeping.
And we don't switch fuel substrates to fat mid workout, sure we burn some fats up through mid to high Tempo pacing and little to none as we approach Threshold intensity but even when we back off towards Endurance pace there isn't a discrete switching point that you'd feel in the way you've described.
Really hard to say what you experienced, perhaps it was just warming up sufficiently but it does sound very odd. No real ideas but the couple of ideas you threw out don't really fly IMO.
