> How
You need a significant period of time without any time-bound obligation (or even any obligation at all). Any need for an alarm or anything that you'd possibly materialise in a calendar needs to be removed.
You need a room which is pitch black at night (e.g blocks street lights, no leds), yet still allows a tiny bit of sunlight (rationale: without cues the circadian cycle extends up to 48h, there was an experiment about that)
Then, once you have that, start freewheeling, just when you feel tired go rest and when you wake up get up. It'll be a mess at first, for two reasons.
First you may be unable to properly recognise the "I need to take some rest" signal: we're trained by life to largely ignore it.
Second, you need to pay off any debt, sleep, physical, mind, that would play a role in altering your base cycle. Have fun, work out, meditate, go see a therapist even. In a nutshell, find your balance.
Once you have recovered from everything, once all biasing sources have been removed, then sleep will converge to some rhythm, which carries some error margin, so you can only observe it statistically over time. There's your baseline.
It could take months, which more often than not isn't practical to have, so from the above ideal scenario one could devise a protocol that would try to stick as close as possible to it.
Source: first principles+anecdata, sample size of 1, double-non-blind protocol; a.k.a myself digging out of a hole.
Protocol: 3 week sick leave for burnout x anxiety depression, coincidental breakup (so no SO), went from office to remote working, workout plan, 6 month therapy, statistically reliable sleep/health/performance tracking via watch allowing for outlier identification/retrospective deviation root cause analysis.
Result: 0130-0930~30min sleep, 7h45min~5min sleep duration made of two ~4h-ish blocks, 25h~30min cycle. When bound to 24h rhythm, occasional sleepless night leading to 48h day and three aforementioned blocks of sleep (~11h-ish total).
From the data it became painfully obvious that I experience delayed sleep phase disorder, took me 6 months to figure the baseline out, and 6 more to confirm, which is quite hard when you don't even know that it is a thing.