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The Science Behind the Sleep Score

Explore the research and physiological evidence that inform the factors used to calculate the Sahha Sleep Score, including sleep duration, regularity, continuity, sleep debt, circadian alignment, and stages of sleep.

Sleep health is multi-dimensional. Duration alone does not fully explain recovery, performance, or long-term outcomes — regularity, continuity, circadian alignment, and restorative sleep stages each independently contribute to physical health, cognitive performance, emotional regulation, and mortality risk. The Sahha Sleep Score synthesises these evidence-backed dimensions into a single interpretable measure.

The Scientific Model Behind the Sleep Score

The Sleep Score is based on seven validated sleep health dimensions:

  • Sleep duration: Total time asleep relative to the recommended 7–9 hour range.
  • Sleep regularity: Consistency of bed and wake times across nights.
  • Sleep continuity: How uninterrupted sleep is throughout the night.
  • Sleep debt: Cumulative shortfall in sleep relative to physiological need.
  • Circadian alignment: How closely sleep timing matches the body’s biological clock.
  • Physical recovery (deep sleep): Time spent in slow-wave sleep, the most physically restorative phase.
  • Mental recovery (REM sleep): Time spent in REM sleep, supporting memory, learning, and emotional processing.

Each dimension is supported by peer-reviewed research linking it to cardiometabolic health, cognitive performance, mental wellbeing, and long-term risk outcomes.

Sleep Duration

Why it matters Adequate sleep duration is critical for physical health, cognitive function, metabolic regulation, and emotional wellbeing. Chronic short sleep increases risk for obesity, diabetes, cardiovascular disease, and premature mortality.

What the research shows

  • Consistently sleeping 7–9 hours per night is associated with the lowest risk of death, heart disease, obesity, diabetes, and stroke.
  • Sleeping less than 6 hours per night increases risk of all-cause mortality by 20–30%, heart disease by 28%, and stroke by 15–25% compared to 7–8 hours.
  • Sleeping more than 9 hours per night is also associated with elevated risks, showing a U-shaped relationship.

Why it’s included in the Sleep Score Sleep duration is one of the strongest and most consistent predictors of health and recovery outcomes.

Sleep Regularity

Why it matters Regular sleep patterns support stable circadian rhythms, which are crucial for hormonal regulation, metabolic health, and sleep quality. Irregular sleep schedules can disrupt circadian rhythm and affect metabolism, heart health, mental wellbeing, and cognitive function.

What the research shows

  • People with irregular sleep patterns (varying bed and wake times by more than 90 minutes) have about double the risk of developing cardiovascular disease compared to those with consistent schedules.
  • Decreased sleep regularity is linked to a 20–30% higher risk of all-cause mortality.
  • Irregular sleep timing is also linked to increased risk of depression and poorer cognitive outcomes, even after controlling for total sleep time.

Why it’s included in the Sleep Score Sleep timing consistency predicts health outcomes independently of total sleep duration.

Sleep Continuity

Why it matters Frequent awakenings or fragmented sleep disrupt restorative processes, impair memory, weaken immunity, and increase risk for chronic disease, regardless of total sleep time.

What the research shows

  • High sleep fragmentation is associated with higher all-cause mortality and higher cardiovascular disease risk.
  • Each additional nighttime awakening is associated with higher risk of developing hypertension and higher odds of metabolic syndrome.
  • Greater sleep continuity is linked to improved memory, cognitive function, and mood.

Why it’s included in the Sleep Score Uninterrupted sleep is critical for recovery quality, not just sleep quantity.

Sleep Debt

Why it matters Sleep debt is the cumulative shortfall in sleep relative to physiological need. Even modest nightly deficits compound into measurable impairments in cognition, mood, metabolism, and immune response.

What the research shows

  • A sleep deficit of 1–2 hours per night for one week can result in 20–32% slower reaction times, reduced alertness, and impaired learning.
  • Partial sleep restriction (6 hours per night for two weeks) produces cognitive deficits comparable to 1–2 nights of total sleep deprivation.
  • Habitual sleep debt is associated with increased risk of obesity, diabetes, hypertension, and depression.

Why it’s included in the Sleep Score Cumulative sleep loss is a strong driver of fatigue and impaired readiness, even when short-term effects aren’t obvious.

Circadian Alignment

Why it matters Circadian alignment reflects how closely sleep timing matches the body’s biological clock. Poor alignment can disrupt hormones, metabolism, mood, and overall health even if total sleep duration is adequate.

What the research shows

  • Each one-hour shift in sleep midpoint is associated with higher metabolic syndrome risk and higher obesity risk.
  • Social jetlag (mismatch between internal clock and actual sleep times) increases risks of type 2 diabetes, depression, unhealthy dietary habits, and cardiovascular disease.
  • Chronic circadian misalignment (e.g., shift work) is associated with higher risk of heart disease, stroke, and certain cancers.

Why it’s included in the Sleep Score When sleep occurs influences recovery and health outcomes, not only how long sleep lasts.

Physical Recovery (Deep Sleep)

Why it matters Deep (slow-wave) sleep is the most physically restorative phase, supporting muscle repair, immune function, and growth hormone release. Insufficient deep sleep impairs physical recovery and increases susceptibility to illness.

What the research shows

  • Adults typically spend 13–23% of sleep time in deep sleep, decreasing with age.
  • Each 10% decrease in deep sleep is associated with increased cardiovascular disease risk and increased all-cause mortality risk.
  • Very low deep sleep is associated with higher hypertension risk.
  • Greater deep sleep is linked to better physical recovery after exercise.

Why it’s included in the Sleep Score Deep sleep is a key marker of physical recovery quality.

Mental Recovery (REM Sleep)

Why it matters REM sleep supports memory consolidation, emotional processing, learning, and mood regulation. Insufficient REM sleep is linked to depression, anxiety, impaired cognition, and reduced stress resilience.

What the research shows

  • Adults typically spend 20–25% of sleep time in REM sleep.
  • Each 5% decrease in REM sleep is associated with higher all-cause mortality risk and substantially higher dementia risk.
  • Low REM proportion is associated with higher depression risk and higher Parkinson’s disease risk.
  • Reduced REM sleep is also linked to impaired memory, learning, and emotional regulation.

Why it’s included in the Sleep Score Mental recovery is as essential as physical recovery for overall wellbeing.

Why a Multi-Factor Sleep Score Matters

Sleep health cannot be captured by a single metric. Two individuals with the same sleep duration may experience very different recovery outcomes depending on regularity, fragmentation, circadian alignment, and sleep stage balance. The Sleep Score integrates these validated dimensions into a single interpretable measure that reflects real-world sleep health and recovery.

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