Disclosure: I make an iOS sleep app (linked at the end). The code below is free to use and does not need it.
A lot of "sleep calculators" boil down to one idea: sleep comes in cycles of roughly 90 minutes, so count backwards from your alarm in whole cycles. That's a rough model, not a medical fact, because real cycles vary between people and across the night. But it is a handy rule of thumb, and it takes about 15 lines of JavaScript.
The code
// Bedtime calculator: count back whole 90-minute cycles from a wake time.
const CYCLE = 90; // minutes, an average, not a constant
const FALL_ASLEEP = 15; // minutes, a typical assumption
function bedtimes(wake, cycles = [6, 5, 4]) {
const [h, m] = wake.split(":").map(Number);
const wakeMin = h * 60 + m;
return cycles.map((c) => {
const t = (((wakeMin - c * CYCLE - FALL_ASLEEP) % 1440) + 1440) % 1440;
const hh = String(Math.floor(t / 60)).padStart(2, "0");
const mm = String(t % 60).padStart(2, "0");
return { cycles: c, sleepHours: (c * CYCLE) / 60, bedtime: `${hh}:${mm}` };
});
}
console.log(bedtimes("06:30"));
Running it for a 06:30 alarm (I ran this with Node before posting):
[
{ cycles: 6, sleepHours: 9, bedtime: '21:15' },
{ cycles: 5, sleepHours: 7.5, bedtime: '22:45' },
{ cycles: 4, sleepHours: 6, bedtime: '00:15' }
]
Two details that bite
-
Wrap past midnight.
(x % 1440 + 1440) % 1440keeps the result in 0 to 1439 even when the subtraction goes negative. Without the double modulo, JavaScript returns negative remainders and you print times like-45:00. - The fall-asleep buffer. The 15 minutes is an assumption. If you take longer to drop off, the "cycle-aligned" result is already off before you start. Treat the output as a target to head to bed, not as a guarantee of how you will wake.
Where this model stops
- The 90-minute figure is an average. Individual cycles run shorter or longer.
- It says nothing about regularity. Going to bed and waking at the same time most days usually matters more than hitting an exact cycle boundary.
- If you have persistent insomnia or daytime sleepiness, a calculator is not a substitute for talking to a clinician.
Where to take it
Easy extensions: read the wake time from an <input type="time">, show the next three alarms for a given bedtime instead, or let the user set their own cycle length and buffer.
If you would rather not build it yourself: I make Sleep Reset for iPhone. It has a free sleep reset score; the personalized plan and bedtime timing need a paid subscription.
