How many of your shifts start in the dark?

What the count does and does not tell you

A dark start is an exposure, not an outcome. It is worth counting because it is the part of the problem a roster can actually change: you cannot move sunrise, but you can move a start time, and a count of 227 dark starts makes the size of that decision visible in a way that a sunrise table does not.

What the count cannot tell you is what those dark starts cost. The closest evidence comes from a study of Arctic office workers, whose sleep began 39 minutes later in winter than in summer, with higher sleepiness, and in which more morning light was associated with earlier sleep timing. That study measured sleep and mood, not output, and it followed 32 people. It is enough to justify a local trial with real measurements. It is not enough to convert darkness into a productivity figure, and any source that offers you one has gone past its evidence. We keep the full picture in the evidence on latitude, daylight and worker energy.

There is a second, separate effect that has nothing to do with latitude: where you sit inside a time zone. Two towns either side of a time-zone border keep the same clock but see the sun at different hours, and the evidence on solar-clock misalignment is stronger than most of what gets written about winter darkness.

Questions

Why does a 06:00 shift start in the dark more often in Singapore than in Oslo?

Because the equator has no long summer evenings to compensate. Singapore’s sunrise sits within a few minutes of 07:00 every day of the year, so a 06:00 start is before sunrise on all 365 days. Oslo’s sunrise swings from about 03:55 in June to about 09:15 in December, so the same shift starts in daylight for roughly a third of the year. Seasonal swing and absolute darkness are different problems, and the tropics have the second one without the first.

Does this account for daylight saving time?

Yes. Sunrise and sunset are solved as universal time and then read back on the location’s own clock through the IANA timezone database, so the answer is correct on both sides of every clock change. This matters more than it sounds: a spring transition can move a shift from starting in daylight to starting in darkness overnight without anything about the sun changing.

What counts as dark?

The sun being below the horizon, using the standard sunrise and sunset definition that already allows for atmospheric refraction and the width of the solar disc. Civil twilight, when the sun is up to six degrees below the horizon, still gives usable outdoor light and lasts roughly 30 minutes at mid latitudes but well over an hour near the poles, so a shift counted here as starting in darkness may still start in reasonable light.

Should we move shift start times because of this?

The tool measures exposure, not harm, and the honest evidence is narrower than the popular version of it. Among Arctic office workers, winter sleep onset came 39 minutes later than in summer and sleepiness was higher, and greater morning light was associated with earlier sleep timing. That is a study of sleep timing, not of output. Treat a high dark-start count as a reason to run a local trial and measure lateness, absence and incidents, rather than as a calculated productivity loss.

Why does the night shift show daylight in some places?

Above the Arctic and Antarctic circles the sun does not set for part of the year, so a 22:00 to 06:00 shift in Tromsø in June is worked entirely in daylight. The same shift in the same city in December is worked entirely in darkness. Few scheduling problems flip that completely between seasons, and it is the clearest illustration that a shift pattern is not the same job everywhere it is run.

Which year does it calculate?

The current calendar year, computed in your browser when the page loads. Sunrise and sunset shift by only a minute or two between years, so the counts are stable, though a leap year adds a day and can move a total by one.