SleepCalc

Sleep-cycle calculator — find the best bedtime or wake-up time, and log how you actually slept.

The cycle model is that sleep runs in roughly 90-minute rounds, and waking at the end of one feels better than waking mid-cycle from deep sleep. Timing backwards in 90-minute steps from when you must be up is the practical application.

Worth holding loosely, though: 90 minutes is an average and real cycles vary between about 70 and 120 minutes, changing across the night. Which is why logging how you actually slept is the more useful half — your own pattern beats the population average, and total sleep time still matters far more than which minute you wake.