Sleep glossary
Sleep regularity index
The sleep regularity index scores how repeatable your nights are on a 0 to 100 scale, and it captures something the hours you slept never will.
In short
The sleep regularity index, or SRI, is a score from 0 to 100 for how likely you are to be in the same state, asleep or awake, at the same clock time on two different days. 100 is a perfectly repeated schedule.
Sleep regularity index is also called SRI or sleep regularity score.
Most sleep metrics answer how much or how well. The sleep regularity index answers how repeatable, and it does that without caring what time you actually sleep. Take every minute of the day, compare your state at that minute today against your state at the same minute yesterday, count how often the two agree, and scale the agreement onto a range from 0 to 100.
A score near 100 means a schedule that repeats: same lights out, same rise time, every day. A score near 0 means your state at a given clock time tells you nothing about the next day, which is roughly what a rotating shift pattern looks like. Most people sit somewhere in between, and the score falls fastest from weekends, because two nights on a shifted schedule disagree with five nights on the weekday one. That gap is social jetlag, and the SRI is the cleanest way to see it as a single number rather than a feeling.
What moves the sleep regularity index
Wake time moves it more than bedtime does, because rise time anchors the circadian rhythm and it is the one you can still control after a late night. One 3am Saturday costs you across two day pairs, not one, which is why a single big night has an outsized effect on a weekly score. Long naps count too, since an afternoon asleep on one day and awake on the next is a disagreement at exactly those minutes.
It is not a score to optimise for its own sake. A late but consistent night owl chronotype can rate very well, and that is correct rather than a bug. What the index does is separate two people who both average seven hours, one of whom takes them at the same time every night and one of whom does not. Tock computes an SRI from your tracked nights and shows it beside bedtime consistency and sleep debt in its insights. If yours is low and you want it higher, how to fix your sleep schedule is the practical version, and it starts with the wake time rather than the bedtime.