There is a kind of tired that does not match the arithmetic. You add up the week and the hours are not indefensible. Six and a half, seven, six, seven and a half. Nothing in that column explains why Thursday feels the way it does. So the tiredness gets filed where unexplained things usually get filed, which is under something personal. Lower tolerance than you used to have. Not coping as well. Getting older, probably.

But the arithmetic might be counting the wrong thing. Not the sum. The spread.

MoMo has written before about what a short night does: to the pause between feeling and reacting, and to how other people's faces read the next day. Both of those posts are about how much sleep. This one is about a different axis, one that can hide inside a perfectly respectable weekly total: whether the sleep keeps arriving at roughly the same time.

What regularity is actually measuring

There is a metric for this, newer than you might expect. The Sleep Regularity Index, defined in 2017, scores from 0 to 100 the probability that you are in the same state, asleep or awake, at the same clock time on two consecutive days.[6] A metronome scores 100. Someone whose nights land wherever the week leaves them scores much lower. In the largest study using it, the median was 81, which tells you most people are not metronomes and were never expected to be.[1]

There is an older, blunter version of the idea. In 2006 chronobiologists named the gap between sleep timing on workdays and on free days: social jetlag.[9] The name does real work. A Friday-to-Saturday shift of two hours is, physiologically, a small westward flight, taken most weeks, in both directions.

That flight is not just a metaphor. What both measures are catching is the body being asked to keep two schedules at once, a weekday clock and a weekend clock, close enough to overlap but never quite the same, so the internal clock stays a little jet-lagged all year round. That is a different mechanism from the sleep-debt story MoMo has told before, where the cost sat in the hours themselves. Here the cost sits in the switching.

What matters about both measures is what they ignore. Neither one counts hours. A week can average a defensible seven and still score badly on both: an average has no idea when anything happened.

When the week is not yours to set

There is a version of this research that turns straight into an instruction, and the instruction lands on exactly the wrong person.

The women MoMo is for do not have an irregular week because they are undisciplined about bedtime. They have one because a child woke at 4am, because the shift pattern rotates, because the only two hours that belonged to them started at 11pm, because someone needed care and care does not keep office hours. The Saturday lie-in that shows up in the data as social jetlag is often not indulgence. It is repayment.

It is worth knowing, before the numbers arrive, who these studies watched. The cohorts skew middle-aged and general-population, around 50 to 60. There is no dedicated research here on mothers of small children, on shift workers, on carers. This is what large population studies show, not what studies of women like you show. The construct is right. The sample is not yours.

The finding worth leading with

This research reached the public through its most alarming number, and I want to step past it. The mortality studies exist, they are large, and an independent group replicated them on a separate slice of the same cohort.[1][2] I am mentioning them once and leaving them, because "your inconsistent bedtime could shorten your life" is not a useful thing to hand to a woman who is already sleeping badly about things she cannot control. It is also not the most interesting finding.

The most interesting finding is closer to a Tuesday. A 2025 study followed nearly 80,000 people with no depression or anxiety at baseline, measured a week of their sleep with accelerometers, and watched for a median of seven and a half years.[3] The regular sleepers had a 38 percent lower risk of depression appearing, and a 33 percent lower risk of anxiety, than the irregular ones.

Then the detail that makes the piece worth writing. The irregular sleepers who were getting the recommended hours still carried elevated risk: roughly 1.5 times for depression, 1.35 for anxiety.[3] The hours did not cancel the shape of the week.

This is not one lonely dataset. An independent American cohort, using commercial wearables on a different population, found irregularity tracking with the later appearance of depression and anxiety, and with weight, blood pressure and lipids.[4] Different country, different device, different research group, same direction.

A small, careful study speaks to the order of events, which is usually what you want to know and rarely what anyone measured. Forty-two people, three weeks of daily diaries and continuous heart monitoring: low regularity in one week predicted worse mood the next, independent of how long they had slept, while prior mood did not predict later regularity.[5] The sample is tiny and I hold it loosely. But it points the arrow the way most of us assume it points.

The claim I am not going to make

The headline going around says regularity beats duration. That is close to the paper, and not the same thing.

In the mortality study, regularity did outperform duration for most causes of death, but for cardiometabolic deaths its advantage faded once duration entered the fully adjusted model.[1] Hours still mattered there. And it runs both ways: adequate duration offset cardiovascular risk for moderately irregular sleepers, but not for the most irregular.[7] The most on-point paper yet, published two weeks ago, found the weekday-to-weekend gap itself tracking with cardiovascular events, independent of duration and of cardiac genetic risk.[8]

The honest version is narrower than the headline. Timing consistency seems to carry information that hours alone do not, in addition to hours, not instead of them.

The other thing worth saying plainly: all of this is observational. Nobody has run the trial where you take irregular sleepers, regularise them, and see whether anything improves. That trial is what would license a "fix this and X follows" sentence, and it has not been run, so there is no such sentence here.

What is left, which is small

What is worth carrying out of this is not a schedule. It is a swap in the accounting.

"I only got six hours" is a discipline story, and the culprit in it is you. "My week does not let my bedtime be the same two days running" is a structural story, and it is the one the evidence points at. Not more comfortable, exactly. But more accurate, and it puts the difficulty where it belongs: in the shape of the week rather than the character of the person living it.

Noticing that a heavy Thursday connects back to a week whose edges never lined up, so the pattern reads as a rhythm rather than a verdict, is the kind of thread MoMo is built to hold across days. But that is a footnote. The essay is the spread, and how much of it the sum was hiding.

You do not have to do anything with this.

If this feels like more than a pattern (if you are in distress or crisis) please reach out: Mind Sweden 90101 | Samaritans UK 116 123 | your local crisis line.

References

Windred, D. P., Burns, A. C., Lane, J. M., Saxena, R., Rutter, M. K., Cain, S. W., & Phillips, A. J. K. (2024). Sleep regularity is a stronger predictor of mortality risk than sleep duration: A prospective cohort study. Sleep, 47(1), zsad253. PMID: 37738616.
Cribb, L., Sha, R., Yiallourou, S., Grima, N. A., Cavuoto, M., Baril, A. A., & Pase, M. P. (2023). Sleep regularity and mortality: a prospective analysis in the UK Biobank. eLife, 12, RP88359.
Li, D. R., Li, Z. X., Li, M. H., Liu, B. Q., Fang, Q., Liu, J. C., Zheng, W. R., Gong, T. T., Gao, S. Y., & Wu, Q. J. (2025). Regular sleep patterns, not just duration, critical for mental health: association of accelerometer-derived sleep regularity with incident depression and anxiety. Psychological Medicine, 55, e239. PMID: 40814280.
Zheng, N. S., Annis, J., Master, H., Han, L., Gleichauf, K., Ching, J. H., Nasser, M., Coleman, P., Desine, S., Ruderfer, D. M., Hernandez, J., Schneider, L. D., & Brittain, E. L. (2024). Sleep patterns and risk of chronic disease as measured by long-term monitoring with commercial wearable devices in the All of Us Research Program. Nature Medicine, 30(9), 2648–2656. PMID: 39030265.
Gao, L., Li, P., Hu, C., To, T., Patxot, M., Falvey, B., Wong, P. M., Scheer, F. A. J. L., Lin, C., Lo, M. T., & Hu, K. (2019). Nocturnal heart rate variability moderates the association between sleep-wake regularity and mood in young adults. Sleep, 42(5), zsz034. PMID: 30722058.
Phillips, A. J. K., Clerx, W. M., O'Brien, C. S., Sano, A., Barger, L. K., Picard, R. W., Lockley, S. W., Klerman, E. B., & Czeisler, C. A. (2017). Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing. Scientific Reports, 7, 3216. PMID: 28607474.
Chaput, J. P., Biswas, R. K., Ahmadi, M., Cistulli, P. A., Rajaratnam, S. M. W., Bian, W., St-Onge, M. P., & Stamatakis, E. (2025). Sleep regularity and major adverse cardiovascular events: a device-based prospective study in 72,269 UK adults. Journal of Epidemiology and Community Health, 79(4), 257–264. PMID: 39603689.
Kumar, N., & Krishnamurthy, S. (2026). Social jet lag is associated with incident cardiovascular disease independent of sleep duration and cardiac genetic risk. Journal of Internal Medicine, online ahead of print. PMID: 42387174.
Wittmann, M., Dinich, J., Merrow, M., & Roenneberg, T. (2006). Social jetlag: misalignment of biological and social time. Chronobiology International, 23(1–2), 497–509. PMID: 16687322.