Heart rate and heart rate variability: the strong suit
Optical heart-rate sensors (the green or infrared light on the back of a ring or watch) have matured to the point of reasonable accuracy for resting heart rate and heart rate variability (HRV) — a measure of the tiny variation in time between heartbeats that reflects nervous system balance between "fight or flight" and "rest and digest" states. Rising HRV over time is a genuinely useful, well-validated signal of improving recovery and reduced physiological stress, which is why most wearable "readiness" scores lean on it heavily.
Sleep staging: good at the big picture, shaky on the details
Consumer wearables estimate sleep stages (light, deep, REM) using a combination of movement and heart rate patterns rather than the brain-wave monitoring a clinical sleep lab uses. This makes them reasonably reliable for total sleep time and rough sleep quality trends over weeks, but genuinely unreliable for precise stage-by-stage breakdown on any single night — treat the trend line, not any one night's exact numbers, as the useful signal.
Continuous glucose monitoring leaves the diabetes clinic
Continuous glucose monitors (CGMs), originally developed for diabetes management, are increasingly marketed to people without diabetes for general metabolic insight — showing in real time how specific meals, exercise, sleep, and stress affect blood sugar. The underlying data is legitimate, but interpreting "normal" glucose variation in someone without diabetes is a genuinely newer area of research, and the marketing sometimes outpaces the science on what a given glucose spike actually means for a non-diabetic person's long-term health.
Where the data gets genuinely shaky
Metrics like "recovery score," "readiness," "biological age," and stress percentages are typically proprietary composite scores — the company's own weighted formula combining several underlying measurements — not independently validated clinical metrics. They can be useful for spotting your own personal trends over time, but comparing your score against someone else's, or treating the exact number as medically meaningful, reads more precision into the number than the underlying measurement actually supports.
The practical takeaway
Wearables are best used the way a good coach uses data: watching trends over weeks, not obsessing over a single day's number, and treating any wearable metric as one input alongside how you actually feel — not a replacement for it. See our Body Hacks hub for the underlying sleep, energy, and focus science these devices are trying to measure.