Your body clock thrives on consistency. The Sleep Regularity Calculator uses one week of bedtime and wake-up times to measure how stable your sleep–wake rhythm really is. Unlike total sleep duration alone, regularity captures the daily timing of your internal clock — late nights, early mornings, and weekend sleep-ins all show up in the score. The tool calculates the average mid-sleep point for your seven-night week, then measures how far each night drifts from that average using the standard Sleep Regularity Index (SRI) formula. You can also switch the output to show the average night-to-night shift in minutes.
Who is this for? Anyone trying to improve sleep quality, shift workers tracking a rotating schedule, students resetting an irregular routine, parents monitoring a child’s sleep, or people curious about their circadian rhythm. All you need is a rough bedtime and wake-up time for each of the past seven days — a phone screen-time report or sleep tracker usually provides the data.
😴 Sleep Regularity Calculator
Enter your sleep and wake times for the last 7 nights to measure how consistent your body clock is.
How the Sleep Regularity Index Works
The Sleep Regularity Index was developed in sleep research to express timing consistency as a single number. For each night, the sleep midpoint is the halfway point between bedtime and wake time. For a person who sleeps from 23:00 to 07:00, the midpoint is 03:00. If they regularly go to bed at 01:00 and wake at 09:00, the midpoint is still 03:00 — timing is shifted later but equally regular.
To calculate the index:
SRI = 100 − 2 × average absolute deviation (in minutes) of each night's sleep midpoint from the week's average midpoint.
So a person whose midpoints are, on average, 10 minutes from their usual midpoint scores 80; an average deviation of 25 minutes scores 50. The result is closely related to circadian rhythm stability — consistent sleep schedules help anchor the body's internal clock, while irregular timing can disrupt melatonin release and affect mood and energy. If you want a different angle on the same data, the consecutive-shift metric in this calculator averages the difference between each night's midpoint and the previous night's, which captures abrupt schedule changes like a single late night.
Entering Your Sleep Times Correctly
The calculator expects exactly seven days of data, from the night before Monday through the night before Sunday. For each row, enter two times:
- Bedtime — the time you tried to fall asleep, not the time you got into bed to scroll.
- Wake time — the final time you got out of bed in the morning.
Use the actual hours slept even if they cross midnight — the tool automatically handles overnight intervals. If you work nights and sleep during the day, enter your true sleep period: bedtime at 08:00 and wake at 16:00 is perfectly valid. Naps are not included in this calculation because the SRI is designed for the main sleep period. For best results, use the same seven days you would use for a sleep diary, and avoid mixing weekday nights with last week's nights.
Interpreting Your Score
Higher scores mean a more consistent sleep–wake schedule. The table below shows general reference bands used in sleep tracking:
| SRI Score | Interpretation | Average Deviation |
|---|---|---|
| 80–100 | Excellent regularity | ≤ 10 minutes |
| 70–79 | Good regularity | 10–15 minutes |
| 60–69 | Moderate regularity | 15–20 minutes |
| < 60 | Irregular schedule | > 20 minutes |
A score in the 70s is common for people who keep a fairly fixed alarm but occasionally stay up later on weekends. A score below 60 indicates meaningful variability, which research has associated with poorer sleep quality, more fatigue, and greater metabolic risk. The night-to-night shift metric is more granular: an average shift under 20 minutes suggests a very stable anchor, while 45 minutes or more means your body clock is regularly resetting, which can feel like mild jet lag.
Worked Example: A Mixed Week
Consider someone who sleeps late on weekends but tries to stay consistent during the week:
| Night | Bedtime | Wake | Midpoint |
|---|---|---|---|
| Mon | 23:00 | 07:00 | 03:00 |
| Tue | 23:30 | 07:30 | 03:30 |
| Wed | 23:15 | 07:15 | 03:15 |
| Thu | 00:00 | 07:45 | 03:52 |
| Fri | 23:45 | 07:20 | 03:32 |
| Sat | 00:30 | 08:00 | 04:15 |
| Sun | 23:00 | 07:00 | 03:00 |
The average midpoint is about 03:29. Individual midpoints differ from this average by 29, 1, 14, 23, 3, 46, and 29 minutes, giving a mean deviation of about 20.8 minutes. The Sleep Regularity Index becomes 100 − (2 × 20.8) = 58.4, which falls in the irregular range. The average consecutive shift is (30 + 15 + 37.5 + 20 + 42.5 + 75) ÷ 6 = 36.7 minutes, confirming the schedule is noticeably disorganised even though total sleep time looks reasonable.
Tips for Improving Sleep Regularity
Regularity can be improved faster than people expect. The body clock responds to the strongest cues — light, meals, and activity — so small changes compound:
- Set a fixed wake time every day, including weekends; it anchors your midpoint more effectively than a fixed bedtime.
- Keep Friday and Saturday bedtimes within about 30 minutes of your weekday bedtime; social jet lag is the most common cause of a low SRI.
- Expose yourself to bright light within 30 minutes of waking, especially outdoors on sunny mornings.
- Move dinner and exercise earlier; late eating and vigorous training can push your sleep midpoint later.
- If you are a shift worker, stabilise the time of your main sleep block even on days off, and use bright light strategically at the start of your shift.
Limitations & Further Reading
The SRI is a timing metric, not a sleep quality score. Two people can share the same excellent score while one wakes frequently and the other sleeps deeply. It also assumes a single continuous main sleep period; split sleep or frequent naps will make the calculation less meaningful. The index is most informative over one to two weeks, and a single unusual night — travel, sickness, a newborn — can temporarily lower the score, so interpret it as a trend rather than a diagnosis.
For more background on sleep regularity and circadian rhythms, see the circadian rhythm overview on Wikipedia. For general sleep health guidance, the National Heart, Lung, and Blood Institute and the CDC Sleep and Sleep Disorders pages are reliable starting points.