Sleep occurs in recurring cycles of approximately 90 minutes, each containing several stages: light sleep (N1, N2), deep sleep (N3/slow-wave sleep), and REM (rapid eye movement) sleep. Waking naturally at the end of a complete cycle — rather than mid-cycle — significantly reduces sleep inertia (the grogginess you feel upon waking). This calculator uses that 90-minute model to suggest optimal bedtimes or wake-up times.

The National Sleep Foundation and most sleep research bodies recommend 7–9 hours for adults aged 18–64, 8–10 hours for teenagers, and 7–8 hours for seniors. These translate to 5–6 complete sleep cycles (though 4 cycles — 6 hours — may be acceptable when unavoidable).

For athletes and physically active individuals, sleep is arguably the most important recovery tool. Deep sleep (N3) is when human growth hormone is released, muscle protein synthesis occurs, and tissue repair takes place. Chronic sleep restriction below 7 hours has been shown to reduce testosterone, increase cortisol, impair reaction times, and reduce pain tolerance — all critical for sport performance.