I don't want to write a long story so I'll keep it short.
- I was tired all the time and had some other issues, especially in the morning, like headaches and trouble concentrating. I would usually really wake up around noon.
- My doctor said it was stress/burnout.
- I bought a Fitbit because I wanted to exercise more and lose some weight to see if that would work (male, 39, bmi:25, 22% bodyfat).
- My Fitbit said my SPO2 fluctuated a lot during the night with almost every night high peaks.
- Asked my doctor for a sleep study
- Sleep study diagnosed me with heavy sleep apnea: 33 respiratory arrests per hour. Now I have a cpap device to help me sleep.
Normal:

First night with cpap:

Another thing that changed is my bodytemperature. It was 1.6 degrees lower than normal. Need more data to see if that's a lasting change due to the cpap. I've read that the thyroid lowers your bodytemperature for the deep sleep phase and respiratory arrest could prevent that (which makes you sweat more during sleep).