Your overnight heart rate follows a predictable curve — the "hammock" — that sleep researchers use to assess autonomic recovery quality. Deviations from this shape are diagnostic: each pattern maps to a specific cause you can act on tonight.
Key Takeaways
- Alcohol is the most consistent cause of a flattened overnight HR curve — even moderate amounts eliminate the normal nocturnal dip
- Sleep apnea prevents the parasympathetic dominance needed for the dip — consistently elevated overnight HR is a key warning sign
- A rising trend in overnight minimum HR over 7+ days reliably signals overtraining, illness onset, or accumulated sleep debt
- Bedroom temperature above 70°F raises cardiac workload for heat dissipation — directly elevating overnight HR
- Regular aerobic fitness progressively lowers overnight HR baseline — the long-term adaptation is significant and measurable
Reading and Responding to Your HR Curve
- Check the shape first: Open your wearable's overnight HR graph. Does it show a clear U-shape? Flat curves need investigation.
- Flat curve — audit in order: alcohol prior evening → bedroom temperature → psychological stress → possibility of sleep apnea
- Frequent spikes during the night: Arousal events — check for noise, SpO2 drops (apnea), or periodic limb movement
- Rising 7-day trend in overnight minimum: Reduce training load, check for illness onset, audit recovery behaviors
- Consistently poor dip (<5% below daytime): Chronic stress, cardiovascular deconditioning, or unaddressed apnea — worth a clinical conversation
What Each Device Reports
Oura
Reports nightly minimum HR prominently. Rising minimum trend over 7 days = reliable early-warning signal for illness or overtraining.
Whoop
Overnight HR graph in Sleep Coach. RHR incorporated into Recovery. Visual curve shape tells more than the summary number.
Garmin
Sleep HR widget on newer models. Health Snapshot measures RHR, HRV, and SpO2 simultaneously for a composite recovery check.
Apple Watch
Overnight HR graph in Health app → Heart Rate → sleep period. The shape is more informative than the average value alone.