Your Oura Ring shows 68 this morning. Yesterday it was 84. Nothing felt different. This guide explains exactly what wearable sensors measure during sleep, which inputs drive the score, and what actually moves the numbers — based on how the algorithms work.
Sleep trackers don't measure sleep directly. They measure physiological proxies — signals that correlate with sleep states — and infer sleep stages from those signals. The three primary sensors used by all major wearables:
Photoplethysmography (PPG): Optical sensors detect blood volume changes from which heart rate and HRV are derived. HRV patterns during sleep are the strongest signal for stage classification — parasympathetic dominance during deep sleep produces distinctive low-frequency HRV signatures.
Accelerometer: Movement detection distinguishes wakefulness from stillness. Combined with heart rate, it helps differentiate light sleep from true waking.
Temperature sensor: Skin temperature drops at sleep onset and rises before waking. Temperature deviations help detect stage transitions and, in the Oura Ring, cycle-related overnight patterns.
The most direct input. Most algorithms reward 7–9 hours of sleep for adults. Less than 7 or more than 9 typically reduces the score. Oura weights duration more heavily than Whoop; Whoop's Recovery % focuses more on sleep quality relative to duration.
Time asleep divided by time in bed. Someone who lies in bed 9 hours but sleeps 6.5 has 72% efficiency — which most algorithms penalise. Reducing time in bed before sleep onset (the CBT-I principle of sleep restriction) typically improves efficiency over 1–2 weeks.
Both stages are rewarded. Deep (slow-wave) sleep is most concentrated in the first third of the night and is suppressed by alcohol, late exercise, and elevated evening cortisol. REM sleep is concentrated in the final third and is suppressed by alcohol, cannabis, many sleep medications, and consistently waking earlier than the body's natural rhythm.
Higher overnight HRV indicates better autonomic recovery. HRV is suppressed by alcohol (dramatically — measurable even from 1–2 drinks), late meals, psychological stress, illness onset, and accumulated under-recovery. It improves with consistent aerobic exercise, lower chronic stress, and high-quality sleep.
Oura explicitly rewards going to bed and waking at consistent times — a proxy for circadian alignment. Varying bedtime by 90+ minutes across the week disrupts circadian rhythm and reduces deep sleep proportion even when total duration is maintained. The effect shows clearly in Oura trend data over 2–3 weeks of consistent vs inconsistent timing.
Alcohol is the most documented HRV and REM suppressor in wearable data. Even moderate consumption (1–2 drinks) produces measurable next-day HRV reduction and clear REM suppression in the sleep stage graph. Many users call this their most striking discovery after starting to track. Even eliminating one or two weekly drinking occasions produces visible score improvement within 2–3 weeks.
Going to bed and waking at the same time including weekends is the highest-leverage circadian intervention. The improvement typically becomes visible in Oura data within 2–3 weeks of consistent timing — most noticeably in deep sleep proportion and Readiness Score stability.
18–20°C (64–68°F) room temperature is consistently associated with better deep sleep and higher overnight HRV in tracking data. This is the behavioural basis for Eight Sleep Pod's value proposition — automating the temperature optimisation without requiring a behavioural change. For those without a smart bed: a fan, lighter bedding, or lowering a thermostat before bed achieves a similar effect.
Vigorous exercise within 2–3 hours of bedtime elevates core temperature and delays sleep onset. In tracking data, late-evening high-intensity workouts typically show as reduced deep sleep and elevated resting heart rate. Morning or early afternoon exercise is associated with better evening HRV and deeper sleep in the first sleep cycle.
Large meals within 2–3 hours of sleep activate the digestive system and elevate core temperature, competing with the parasympathetic recovery state that precedes deep sleep. In Oura data, late heavy meals typically show as elevated resting heart rate in the first few hours after sleep onset, which compresses the early deep sleep window.
Research has documented a phenomenon called orthosomnia — anxiety about achieving a good sleep score that paradoxically worsens sleep. If you're lying awake worrying about your score, the tracker is counterproductive. Use sleep data as a weekly trend tool, not a nightly target. A single low-score night in the context of a healthy weekly average is not meaningful.