Every rule behind your plan, and where it comes from.
ShiftDawn was not built by a sleep scientist. What it does instead is show its working. Below is every rule the calculator applies, what it does with it, and the research behind it. Two of them are our judgement rather than a published constant, and those are flagged in amber so you can weigh them yourself.
01
Your temperature minimum sits about two hours before your natural wake time
What we do We take your no-alarm wake time and subtract 120 minutes. Everything else in the plan is positioned relative to that one point.
What the research says The core body temperature minimum occurs on average around 2.3 hours before habitual wake, commonly given as a 2 to 2.5 hour range.
Light before that point delays your clock. Light after it advances your clock
What we do This is why the bright-light window sits early in your shift and the light-block window covers your commute home. Get them the wrong way round and you push your clock backwards.
What the research says The photic phase response curve: light in the late subjective day or early subjective night produces delays, light in the late subjective night or early subjective day produces advances.
What we do We move your trough later at 90 minutes per day and earlier at 60 minutes per day. That is what produces your days-to-adapt number.
What the research says Bright light alone produced a phase shift of roughly 57 minutes in one controlled comparison; bright light plus exercise produced about 80 minutes. The direction is well established. The exact rate depends on light intensity, duration and timing.
Our judgement, not a published constant Our 90 minutes per day is at the optimistic end of what light alone has been shown to achieve. If you are only getting moderate indoor light, expect adaptation to take longer than the number we show.
04
Night workers should aim to move the trough out of the shift, not fully invert
What we do We target your trough landing just after clock-out, inside your sleep, rather than trying to make you nocturnal.
What the research says Partial re-entrainment, the compromise phase position, is the standard recommendation for permanent night work. Full inversion does not survive days off.
What we do We work backwards from your sleep onset. Your first cup is timed to the start of your shift, not avoided.
What the research says Caffeine has a half-life of roughly five hours. Expert consensus for shift workers endorses strategic caffeine before and during the shift, and explicitly rejects the standard sleep-hygiene advice to avoid it.
What we do If your schedule leaves room, we place a 90 minute nap ending an hour before you clock on, and tell you to set an alarm.
What the research says Short naps of 15 to 20 minutes boost alertness; naps around 90 minutes reduce sleep debt; naps over 20 minutes carry a risk of sleep inertia on waking. Expert consensus rejects the standard advice against daytime napping for shift workers.
What we do If you choose to get your days off back, we give you a short morning sleep, daylight on waking, and an early bedtime, rather than a full daytime sleep.
What the research says Expert consensus endorses a short sleep in the morning after the last night shift, going back to bed earlier that night, with morning light helping realign to the day-night cycle.
Your crash window is the hour either side of your temperature minimum
What we do We shade two hours on the clock face and tell you not to schedule judgement calls inside it.
What the research says Alertness and reaction time fall to their lowest around the temperature minimum, and drowsy-driving crashes cluster in the pre-dawn window. The precise two-hour boundary is a display convention we chose.
Our judgement, not a published constant There is no published constant that says the risk window is exactly two hours wide. We picked a readable boundary around a real low point. Treat the edges as soft.
How we test it
The calculator runs against 15,176 generated combinations on every change: every start hour, shift lengths from four to twenty-four hours, all four roster types, all three chronotypes, commutes up to three hours, split sleep on and off, and manual trough corrections of up to three hours in both directions. Each one is checked against invariants, including that it can never schedule your sleep inside your shift.
That test caught a real failure: on long shifts with long commutes, an earlier version placed the sleep window in the middle of the working day. It is fixed, and the test exists so that class of error cannot ship again.
What this is not
It is not medical advice, a diagnosis, or a treatment.
It does not measure your sleep. No watch, no ring, no microphone, no data leaves your device.
It estimates your temperature minimum from your natural wake time. It does not measure it.
It cannot make a twelve hour night shift feel like a day shift. Nothing can.
It was not written or reviewed by a clinician.
If you fall asleep without meaning to, if you have persistent insomnia, or if you suspect a sleep disorder, see a clinician. Take your map with you.