The Sleep Regularity Index: Why Consistency Beats Duration

You slept 8 hours last night. But did you sleep at the same time as the night before? The Sleep Regularity Index says that when you sleep may matter as much as how long.

What Is the SRI?

The Sleep Regularity Index was developed by Andrew Phillips at Harvard (2017). It measures the day-to-day consistency of your sleep-wake timing by calculating the probability that you are in the same sleep state (asleep or awake) at each minute of the day compared to the same time yesterday.

Result: a score from 0 (perfectly random) to 100 (perfectly consistent same timing every day).

Population Norms

From the UK Biobank study (n=88,975 adults):

• Population median SRI: ~60
• SRI ≥75: good regularity
• SRI <41: significantly elevated mortality risk

The Landmark Finding

Windred et al. (eLife, 2024) analyzed sleep regularity in 88,975 UK Biobank participants over 7.8 years. Key finding: people with SRI <41 had a 53% higher all-cause mortality risk compared to those with SRI ≥75, independent of sleep duration, age, sex, BMI, smoking, and other confounders.

Why Does Regularity Matter?

The circadian system regulates hundreds of biological processes — hormone secretion, metabolism, immune function, cell repair — through 24-hour timing. When your sleep timing shifts repeatedly, the circadian clock desynchronizes from the external environment. This "internal misalignment" disrupts downstream biological processes even when total sleep time is adequate.

Practical Strategies to Improve SRI

1. Fix your wake time first. A consistent wake time anchors your circadian clock better than a consistent bedtime.
2. Limit weekend sleep drift. Even 1 extra hour on weekends reduces your SRI noticeably.
3. Use morning light. Bright light before 9am is the most powerful circadian anchor available.
4. Treat your sleep schedule like a training schedule. Athletes don't take "schedule days off" — consistency is the performance variable.