Yes, I have fatigue and other symptoms (brain fog, weakness, feeling faint if I exercise or stand still for long periods, feeling worse after a shower) that start in the morning and then clear up at the same time every day.
Once the symptoms clear up, I feel normal. I can exercise fairly vigorously (considering I spend the rest of the day resting), do chores, concentrate better, etc. etc.
I've actually had this for 15 years. It used to go away by about 10am, but it's steadily extended longer and longer into the day until now it goes away about 10pm or later.
I have been trying for a year to sort it out. Many of my symptoms align with POTS, but I don't have tachycardia, my blood pressure seems normal, and the symptoms don't manifest immediately on rising in the morning, so it's still a mystery.
Sleep disorder doesn't seem likely either because I don't feel excessively sleepy, and symptoms don't get better or worse with better or worse sleep quality or time of year.
I am seeing an endocrinologist (second one – first one wasn't up to much). I have to be retested for cortisol levels and function in six weeks because I think the first endo stuffed up the testing. (Have to wait six weeks because I have to discontinue a medication to see if it is contributing and also in order for the tests to be unaffected.)
I have been a night owl since my 20s, but I don't know whether this is related because the symptoms used to resolve at 10am when I was younger!
If I find out anything of use I will report back here.
P.S. My symptoms do feel a lot like low blood sugar, so I think I am actually going to purchase a blood sugar monitor just to rule this out. (I have had standard blood sugar tests ordered by GP that showed nothing wrong, but I felt they didn't relate to my normal life. I tried to get the first endo to investigate postprandial or reactive hypoglcemia but he brushed me off, so I feel like it is worth spending the money just to rule it out for my own satisfaction.)