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, so the honest move is to study the person directly, and the method is simple. Change one thing. Watch what happens. Bed or couch. Coffee or no coffee. Morning or afternoon.
Three things will not cooperate, and this piece is about what to do with them. Your age, your mood and your DNA cannot be assigned, so they are not levers you can pull. They are what decides how hard the other levers pull, which means anything you settle about yourself carries an expiry date.
Hypothesis Three things do not cooperate. Your age. Your mood state. Your DNA.
You cannot pick any of them for an experiment. That does not make them irrelevant. It changes their role: they are not levers you can pull, they are the things that decide how hard the other levers pull.
Suggestion Age is the clearest case. One study (Debarnot et al., 2017) 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 hold every source I could not read in full to.
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.