Measuring the thing you cannot feel

A watch will cheerfully give you a dozen numbers a day. Most of them are collected because they can be, and then looked at because they are there, which is not the same as being useful.

The filter worth applying is short: does this measure something I would otherwise estimate badly, and would a different number change what I do?

Where measuring earns its place

Sleep, because self-assessment is poor and the effect on the next day is large. Steps, because the difference between a sedentary day and an active one is invisible from inside — both feel like a day.

Weight, but weekly rather than daily, because daily variation is water and the trend is the signal. Blood pressure, where it matters, because it has no symptoms until it has serious ones — this is the clearest case of all: a number you cannot feel at all.

Where it does not

Anything you would only ever look at, never act on. Anything whose number you can predict before you measure it — if you already know, the measurement is a ritual.

And anything that makes you feel judged by a machine. That is not squeamishness: a measurement you start avoiding produces a record with holes exactly where the interesting days were.

One number is almost never enough

Nine thousand steps after seven hours of sleep is a different fact from nine thousand steps after four. A heavy week and a light week produce the same average. This is why a measurement next to nothing tends to go unread: it has no meaning on its own, and people correctly stop looking at it.

So collect few things, and keep them next to each other and next to the day they belong to. Two measurements that can be compared beat six that live in separate apps.

Where a space helps

A diary here takes readings from a watch or a phone without you typing them, and puts them on the same day as everything else — what you planned, what you spent, what you did. The comparison that makes a number mean something is then available rather than theoretical.