Your age, your DNA. What do you do about the things you cannot change?
Findings about people on average do not transfer cleanly to any particular person. That gap is what this whole site sits in. So the alternative is to study the person, and the method is simple. Change one thing. Watch what happens. Bed or couch. Coffee or no coffee. Morning or afternoon.
Hypothesis Three things do not cooperate. Your age. Your mood state. Your DNA.
None of them can be assigned. That does not put them outside the model. It changes what they are doing in it. They are not levers. They are the things that decide how hard the other levers pull.
Suggestion Age is the clearest case. One study found the sleep-and-insight effect appeared significantly less often in older adults, and within that group sleeping was no better than staying awake for the same hours. So a finding about how a person works is not permanent. Whatever you settle about yourself at 30 may not be true at 50, and the only honest way to know is to run it again.
Observation Mood state is the one I know from the inside. Sleep and idea generation both move when my state moves. Neither is causing the other. Anything I concluded by watching only those two would have been wrong.
This is the limit of the method and I would rather say it than have it found. A self-experiment can tell you what one change did, for one person, during one stretch of a life. That is smaller than a law. It is also more than anyone has told you about yourself so far.
Where this stops. The age result is Debarnot and colleagues 2017. I could reach the abstract and an author manuscript but not the full text, so it is graded Suggestion until I read the paper, the same standard I used for Brodt in the draft above.
Untested. Whether findings about a person really do expire, and how fast. Nobody has run the same self-experiment on the same person 20 years apart, as far as I can tell.