The Sleep Time Calculator

Methodology

How the calculator actually works

No black-box "AI" estimates. Just well-established sleep science applied to a simple formula. Here's exactly what's happening when you tap Calculate.

The core formula

The calculator assumes:

  • A standard sleep cycle of 90 minutes (adjustable 60–120 min in Advanced).
  • A sleep latency (time to fall asleep) of 15 minutes (adjustable 0–60 min).
  • Optimal waking occurs at the end of a complete cycle, when sleep is lightest.

Wake-up mode

bedtime = wake_time − (cycles × 90 min) − latency

Bedtime mode

wake_time = bedtime + latency + (cycles × 90 min)

"Sleep now" mode

wake_time = now + latency + (cycles × 90 min)

Age-band guidelines

We use the duration ranges from the National Sleep Foundation's consensus panel, an 18-member multidisciplinary group that reviewed hundreds of studies using the RAND/UCLA Appropriateness Method. The panel published its findings in Sleep Health in 2015 (Hirshkowitz et al., 1(4):233–243). The "recommended" cycle count for each band is the midpoint of the panel's range, rounded to the nearest full cycle.

Why 90 minutes?

The 90-minute figure popularized by Nathaniel Kleitman in the 1960s is a population average. Real cycles range roughly 70–120 minutes and typically lengthen through the night, earlier cycles are richer in deep sleep, later cycles in REM. If you've tracked your own cycles (with a wearable or sleep study), use the Advanced slider to match your actual average.

Sleep stages within a cycle

One complete cycle moves through:

  1. N1 (light sleep), 1–7 minutes. Easy to wake from.
  2. N2 (stable sleep), 10–25 minutes per cycle. Roughly 45–55% of total night.
  3. N3 (deep / slow-wave), 20–40 minutes early in the night, less later.
  4. REM (dream sleep), 10 minutes in cycle 1, growing to 60+ minutes in cycle 4–5.

Sleep inertia, that groggy, mentally foggy feeling after waking, is strongest when you're pulled out of N3 deep sleep. Catching the cycle's end (when you're back in N1 or REM) minimizes it.

Sources

  • Aserinsky, E., & Kleitman, N. (1953). Regularly occurring periods of eye motility, and concomitant phenomena, during sleep. Science, 118(3062), 273–274.
  • Dement, W., & Kleitman, N. (1957). Cyclic variations in EEG during sleep and their relation to eye movements, body motility, and dreaming. Electroencephalography and Clinical Neurophysiology, 9(4), 673–690.
  • Hirshkowitz, M., et al. (2015). National Sleep Foundation's sleep time duration recommendations. Sleep Health, 1(4), 233–243.
  • Ohayon, M., et al. (2017). National Sleep Foundation's sleep quality recommendations. Sleep Health, 3(1), 6–19.
  • American Academy of Sleep Medicine (AASM). Clinical practice guidelines. aasm.org/clinical-resources.