A habit I've acquired and developed, which might be worth considering...

A few years ago, I was in a sleep study, in which I had to keep a record of my sleeping habits. While the study's long over, I kept on keeping track, and started throwing in more things. These days, a typical day's entry looks something like this. (Ignore the ...'s, Weasyl isn't letting me add whitespace at the start of a line.)

---

Sat, Apr 4: 8:45am: Wake
... 9am: [morning meds]
... 9:30: Exercise: Stretches, 10 burpees
... 10am: Brush, floss, mouthwash. Shower. 183 lbs.
... 10:30: [more meds], Supplements
...... Oatmeal, 120 Cal
... 11:30: Cookies, 160 Cal
... 2pm: Papaya, dried, ~100 Cal
... 6:30: Pasta + cheese + ham, carrots: ~500 Cal
...... Ginger Ale, ~200 Cal
... 7pm: [evening meds]
...... 5040 steps, 1080 Cal
6/5: 12:30am: Sleep

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The horizontal lines make it easy to tell one sleep-cycle from the next; I keep the dates in the leftmost part of the page, the times all in their own column, and whatever I'm actually describing to the right of that. I don't worry about pinning the times more accurately than the nearest half-hour, and usually write any entry down within a half-hour of doing whatever I'm writing about, though I've occasionally gone most of a day before bothering. (I started keeping track of the daily hygiene line as a proxy for how badly I was depressed that day.)

While not a regular thing, I also jot down any symptoms I notice, from sniffles due to allergies to headaches to headaches or back pain or the time I sprained an ankle. If I get any other interesting data, such as blood-test results or getting my blood pressure taken or seeing an interesting stutter in my pulse in my oximeter's display, that gets written down, too.

Every so often, I take the three most trackable numbers - weight, steps, and calories - and add them to a spreadsheet, so I can graph out how they're changing over time. (For steps and calories, the numbers vary so wildly that I use Google Sheets' ability to graph a multiple-day running average.)

I started out using some el cheapo mini-composition notebooks and ballpoint pens, keeping one of each in a pocket. I eventually decided I might as well go for some acid-free pens and moleskine mini-notepads. An important detail I learned early on was to make sure each journal (and pen) would fit in my pocket without any unsightly bulges, so that it was always ready to hand. (A minor detail I started doing was to keep a second, backup pen in my coat pocket, in case my main pen ran out of ink or got dropped off a bridge or something.) To keep track of steps, I clip a pedometer to my belt every day. I also now regularly wear a digital watch; my wrist adornment of choice is the astonishingly inexpensive, and ridiculously rugged, Casio F-91W.

On the first page of each journal, I start out with the current year, in case I ever have enough of them that I risk losing track of which one was written when. On the last page of each journal, I jot down my own contact info (in case I lose a journal), my emergency contacts (in case I go down for the count and someone checks my pockets instead of my phone), and a list of my meds and supplements* (the latter so I don't have to write out a half-dozen different things every day). These days, I usually also keep a small spiral-bound-on-top notepad in the same pocket, for whatever note-taking, doodling, or other pen-and-paperwork happens to crop up; the journal is for info I want to keep a record of indefinitely, the notepad is for scratchwork I'll probably rip the pages out of once I'm done with - such as to pin onto my corkboard to-do list.

I've tried a few other ideas that haven't stuck. For example, getting a bunch of highlighters of different colours, and highlighting all the food lines in yellow, sleep/wake times in blue, meds in peach, exercise in pink, and mood/hygiene in green; all to better tell apart which is which at a glance. But I didn't see any actual benefit arising from that that was worth the effort, so stopped bothering. (Though that one would be easy enough to go back and add, if it ever seems worthwhile.)

As a whole, I found this habit easy to get into, especially since I started small, with just the sleep and wake times. And once I had the basic habit in place, it was easy to expand the sets of data I kept track of, step by step, until I was keeping track of everything a medical professional might be interested in. (The folk working at the weight-loss clinic I've recently been referred to seemed pleasantly surprised I was already writing down and spreadsheeting everything they wanted me to keep track of, except for specific calorie counts, which only took a bit of effort to start paying attention to.)

It's not exactly a bullet-point journal; those are usually pre-planned so that each day takes up a specific amount of space, while my records may go from a half-page to one-and-a-half pages per day. I also don't bother jotting down inspirational quotes I come across, my dreams, story ideas, meandering thoughts, philosophy, or other such things; I've got the notepad if I ever feel the urge to actually write any of that, but haven't. The journal is a tool, a way for me to collect and collate a specific set of data that my future self is likely to find useful, and early on I decided that I didn't want to risk making too much work for myself, getting annoyed with the process, and abandoning the habit of record-keeping.

*: I don't feel like sharing my meds, but supplements aren't so personal. You should probably start by taking a look at the bubble-chart at https://informationisbeautiful.net/visualizations/snake-oil-scientific-evidence-for-nutritional-supplements-vizsweet/ , which shows the strength of evidence for most supplements, before you decide to start taking any. My current list is: a multivitamin (on general principles); Vitamin D (4000 IU per day, because I live up in Canada); fish oil/omega 3; calcium/magnesium; St. John's Wort (for depression); and zinc (25 mg/day, recently added in hopes of helping fend off respiratory virus infections). Things I've tried in the past, but no longer keep up with, included daily green tea and dark chocolate, coconut oil, and creatine.