# Social jet lag (SJL)

The gap between mid-sleep on free days and on work days. Displayed, not scored.

Type: index · Unit: hours · Domain: Circadian Health · Accuracy: Calculated · Importance: Context

Also called: social jetlag, weekend lie-in, mid-sleep, free days, work days, catching up on sleep, weekend sleep

## In one sentence

The gap, in hours, between the midpoint of your sleep on free days and on work days: the time-zone shift your body makes every week between the schedule you keep and the one you would choose.

## Why it matters

The term was coined from 501 Munich ChronoType Questionnaires: late chronotypes showed the largest difference in sleep timing between work and free days, ran up a sleep debt on work days and repaid it on free days, and the associations of chronotype with sleep quality, wellbeing and stimulant use were strongest in people under 25; the striking correlation between late chronotype and smoking was, the authors argued, most probably a consequence of the mismatch between social and biological time rather than of chronotype itself (Wittmann 2006). In a later large-scale survey the same group found that, beyond sleep duration, social jet lag was associated with a higher body mass index (Roenneberg 2012). It is displayed and not scored, and the reason is on the record: VT's own review found no study linking social jet lag to a hard outcome, the literature being cross-sectional, small, confined to surrogate endpoints and confounded by shift work, socioeconomic position and mood, and the external reviewer's request to score it was declined on those grounds.

## What moves it

The alarm clock on work days and its absence on free days, so it is largest in late chronotypes and in anyone whose week has a hard early start, and it shrinks as chronotype moves earlier with age. A schedule kept seven days a week brings it to zero; shift work makes it undefined, and VT shows nothing rather than a number for a week with no regular work days.

## How to measure it well

Wear the device every night; the figure needs the timing of each night and which nights precede work days and free days. VT takes your work days from the chronotype questionnaire, where you say how many days a week you work, and otherwise treats Saturday and Sunday nights as the free ones; if your week is different, complete the questionnaire.

## How Vitality Tracker uses it

Shown on the sleep page beside the Sleep Regularity Index once 28 nights exist, in hours, with a chart of your mid-sleep by night that makes the weekly sawtooth visible. It feeds nothing: not the Sleep Health Index, where alignment with your own chronotype is scored instead, and not Capacity. What it is good for is explanation: a Monday that reads badly on the overnight comparison, two hours of social jet lag, and the pattern is named.

## How far to trust the number

Calculated from timing, so as reliable as the device's bedtime and wake time, and more reliable than the questionnaire estimate it was invented on. The simple difference does not allow for the catch-up sleep that lengthens free nights; the questionnaire's corrected mid-sleep does, and that corrected figure is the one the alignment component uses. A context measure by design.

## Method

Mid-sleep = sleep onset + sleep duration / 2, per night. Social jet lag = |mean mid-sleep on free days − mean mid-sleep on work days| over the window. The corrected free-day mid-sleep, MSF_sc = MSF − 0.5 × (SD_free − SD_week), where SD_week = (SD_work × work days + SD_free × free days) / 7, applied when free-day sleep is the longer, is the chronotype figure (Roenneberg 2003, 2012).

## Sources

1. Wittmann M, Dinich J, Merrow M, Roenneberg T. Social jetlag: misalignment of biological and social time. Chronobiol Int 2006;23:497–509. [doi:10.1080/07420520500545979](https://doi.org/10.1080/07420520500545979)
2. Roenneberg T, Allebrandt KV, Merrow M, Vetter C. Social jetlag and obesity. Curr Biol 2012;22:939–43. [doi:10.1016/j.cub.2012.03.038](https://doi.org/10.1016/j.cub.2012.03.038)
3. Roenneberg T, Wirz-Justice A, Merrow M. Life between clocks: daily temporal patterns of human chronotypes. J Biol Rhythms 2003;18:80–90. [doi:10.1177/0748730402239679](https://doi.org/10.1177/0748730402239679)
4. Vitality Tracker Sleep Scoring Methodology v2.0.1 (section 5.5 and the reviewer response).

## Related

- [Sleep Regularity Index](https://vitalitytracker.app/knowledge/sleep-regularity-index)
- [Chronotype and circadian alignment](https://vitalitytracker.app/knowledge/chronotype-and-circadian-alignment)
- [Bedtime and wake time](https://vitalitytracker.app/knowledge/bedtime-and-wake-time)
- [Sleep Health Index](https://vitalitytracker.app/knowledge/sleep-health-index)
- [Symptom Radar and Symptom Trend](https://vitalitytracker.app/knowledge/symptom-radar-and-symptom-trend)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/social-jet-lag