What that workout actually cost you.

The watch said 45 minutes. Your legs say otherwise.

You went out for an easy run. The summary screen shows a tidy box: distance, average heart rate, calories. Tomorrow you wake up flattened. Both readings are honest — and neither one explains the other.

A workout isn't its duration. It's a distribution of effort across zones, a recovery cost that lands hours later, and — if you wear a CGM — a glucose curve that shapes how the next half-day feels. Apple Watch records most of it. Most workout apps don't translate any of it.

Body Insights does. After a session you get one sentence: "That drifted from easy into tempo for twenty minutes — plan a lighter day tomorrow." The numbers are there if you want them. The recommendation is in words.

The part nobody else does: your glucose response.

If you wear a CGM — Stelo, Dexcom, Libre — Body Insights reads the glucose curve before, during, and for the six to ten hours after a session. Most workout apps stop at heart rate. The metabolic half of what the workout actually cost gets ignored.

You see whether your glucose stayed steady through the effort, climbed (a glycolytic pattern that points to a hard session your liver had to fuel), or fell into the kind of post-exercise dip that arrives long after the watch has stopped recording. That last one — the late-evening crash after a midday workout — is a pattern most people don't know to watch for.

If you don't wear a CGM, this layer drops out cleanly and the zone, recovery, and load reads still work.

Recovery is the workout's other half.

Most apps tell you what you did. Body Insights tells you how long it'll take to put yourself back together — and what to do until you have.

The recovery read isn't a fixed timer. It reads your average heart rate, the duration of the session, and the zones you spent your time in. A 40-minute walk and a 40-minute interval block produce very different numbers. The output is hours, not a score: "Recovered around dinner tomorrow."

For people with chronic illness, perimenopause, or post-illness fitness rebuilds, this matters more than zone charts. The cost of yesterday's effort is what determines what today can hold.

How accurate is the Apple Watch for workouts?

For continuous heart rate during steady cardio, very good. For interval work where heart rate moves fast, expect a brief lag at the rising edge of each spike — the optical sensor is averaging across pulses.

For distance and pace outdoors with GPS, good. For calories, treat them as a relative trend, not a fixed truth — the model is generalized, your physiology isn't. Body Insights reads calories as one signal among several, never as the headline.

The most useful information lives in the shape: which zones, for how long, and what came afterward. A session's identity is in its distribution, not its average.

Who this is for.

Three people, mostly.

Apple Fitness users who've outgrown the rings. The Move/Exercise/Stand frame is a great starting point and a low ceiling. If you want to know which zone your minutes lived in, or why an easy session left you wrecked, the rings can't tell you. This can.

Hybrid athletes. Lifting plus cardio plus the occasional long ride. The training load math built for one sport doesn't track what you're doing. A five-zone read across activity types does.

Return-to-fitness after illness. Long COVID, post-viral, perimenopause, an autoimmune flare that's finally settled. The rule isn't "push harder" — it's "stay under the line that triggers a crash." A recovery-hours read that updates per session is the single most useful number for this.