Sleep calculator

Reading the bedtime list like a decision, not a rule

The list this tool produces is most useful at exactly one moment: when it is already later than you planned and you are choosing between going to bed now or finishing something first. The cycle logic reframes that choice. If the next full-cycle bedtime is 40 minutes away, using those minutes deliberately and lying down at the cycle boundary often beats going down immediately and waking mid-cycle. The reverse discipline matters more: when a RECOMMENDED time is approaching, the episode you were about to start costs you a full 90-minute cycle, not just its own runtime, because there is no half-credit between boundaries.

Note the difference between the two times in each row. "In bed" is lights out; "fall asleep by" is 15 minutes later, when the cycles actually start counting. If you know you are a fast or slow faller-asleep, mentally slide the schedule; the buffer is an average, and a phone in hand quietly doubles it.

The bedtime mode, working forward, has an underrated use for irregular schedules: on a night you cannot control when you get to bed, enter the real lights-out time and pick the alarm from the wake times it offers, rather than keeping an alarm chosen for a bedtime that did not happen. A 6-hour cycle-aligned night often produces a better morning than 6.5 misaligned hours.

An evening, mapped out

Set the wake time to 6:30 AM. The calculator offers four bedtimes: in bed by 9:15 PM for six cycles (9 hours, recommended), 10:45 PM for five cycles (7.5 hours, recommended), 12:15 AM for four cycles (6 hours, marked OK), and 1:45 AM for three cycles (4.5 hours, flagged short). Each is built the same way: 6:30 minus the cycles at 90 minutes each, minus the 15-minute falling-asleep buffer, so the five-cycle row means asleep by 11:00 PM and lights out at 10:45. Run the other direction from an 11:00 PM bedtime and five cycles puts the wake alarm at 6:45 AM.

The arithmetic behind the rows

Everything is minute math on a 24-hour clock. In wake mode, each row subtracts a whole number of 90-minute cycles plus the fixed 15-minute buffer from your wake time; in bedtime mode it adds the buffer and then the cycles. The times wrap correctly across midnight, which is why a late wake time can produce same-day bedtimes without breaking. Four row counts are offered, six down to three cycles, with the color coding doing the editorializing: six and five earn the recommendation, four is a functional floor, three is labeled short because 4.5 hours is a debt, not a night's sleep.

What cycle math cannot promise

The model has two fixed constants, a 90-minute cycle and a 15-minute buffer, and real people match neither exactly every night. Caffeine, alcohol, stress, and screens all stretch how long falling asleep takes, and alcohol additionally disrupts the cycles themselves in ways no bedtime arithmetic can offset. The calculator also assumes unbroken sleep; a night with wake-ups resets the cycle clock unpredictably. Most importantly, alignment is a refinement, not a substitute: a consistent schedule with enough total hours does far more for how you feel than perfectly timed short nights. If mornings stay rough no matter what the list says, that pattern belongs in front of a doctor, not a calculator.