Sleep Tracking 101: HRV, REM, Recovery Scores Decoded
Sleep trackers throw a lot of numbers at you. Most people glance at "total sleep" and ignore the rest. Here's what every metric actually means and which ones matter.
Total sleep time
The number everyone looks at. Aim for 7–9 hours. Less than 6 chronically and almost every health outcome gets worse — fat loss, mood, immunity, cognitive function.
Sleep stages
- Deep sleep (slow-wave): physical recovery. Hormonal repair, muscle building, immune function. Should be ~15–25% of total sleep.
- REM sleep: mental recovery. Memory consolidation, emotional processing. Should be ~20–25%.
- Light sleep: the bulk of your sleep. Necessary but less critical individually.
- Awake: brief awakenings between cycles. Normal up to ~5%.
HRV (heart rate variability)
HRV is the variation in time between heartbeats. Higher = better recovery, generally. Each person has their own baseline — chase trends, not absolute numbers.
What lowers HRV: alcohol, late meals, hard training, illness, stress, jet lag.
What raises HRV: consistent sleep, easy aerobic exercise, sauna, cold exposure (controversial), meditation.
Resting heart rate (RHR)
Your morning RHR is one of the simplest health markers. Lower is generally better (within reason). Athletes often sit at 50–60 bpm. Sedentary adults at 65–80.
A sustained 5+ bpm increase from your baseline often signals oncoming illness, overtraining, or poor sleep.
Recovery scores
Every brand calls it something different (Whoop "Recovery", Garmin "Body Battery", Oura "Readiness", Apple Watch "Training Load"). They all blend sleep + HRV + RHR + recent training load into a single 0–100 score.
Use the score to decide intensity:
- Green/high (70+): train hard.
- Yellow/medium (40–70): moderate session.
- Red/low (<40): walk, mobility, sleep more.
What sleep tracking can't do
- Diagnose sleep apnea (need a polysomnography).
- Replace a night-time blood pressure reading.
- Tell you exact REM stages (within ±15%).
The simplest sleep upgrade you can make
Pick a wake time. Stick to it within ±30 minutes including weekends. Your circadian rhythm rewards consistency more than total hours.
Frequently asked
Do I need a wearable to track sleep?
No. Self-reported bedtime + wake time is 80% of the value. A wearable adds HRV, stages, and overnight HR — useful but not essential.
What's a 'good' HRV?
Highly individual. Adults typically range 20–80 ms (RMSSD). Track YOUR trend. A 10% drop from your 30-day average means low recovery.
Can I improve my deep sleep?
Yes — consistent bedtime, cool dark room, avoid late alcohol, last caffeine 8 hours before bed, last meal 3 hours before bed.
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