Wake up refreshed by timing your alarm to the end of a natural 90-minute sleep cycle — not the middle of one.
We add 15 minutes for you to fall asleep, then calculate complete 90-minute cycles.
Minimum — not recommended for sustained use
Fair — functional but below optimal
Good — meets most adults' needs
Ideal — optimal for most adults
Generous — great for recovery or active days
Extra — best for illness or heavy training weeks
A sleep cycle is a recurring sequence of sleep stages your brain passes through during the night. Each cycle moves through light sleep (N1, N2), deep slow-wave sleep (N3), and REM (rapid eye movement) sleep. REM sleep is associated with dreaming, memory consolidation, and emotional processing. Your brain repeats this cycle roughly 4–6 times per night.
The average sleep cycle lasts approximately 90 minutes, though it can range from 80 to 110 minutes depending on the individual and the time of night. Early cycles tend to have more deep (N3) sleep, while later cycles contain proportionally more REM. This 90-minute average is the basis for this calculator.
This is called sleep inertia — the disorientation and grogginess you feel when woken mid-cycle, particularly from deep N3 sleep. Even if you slept 8 hours, if your alarm cuts off a sleep cycle in the middle, you can wake up feeling worse than if you had woken 30 minutes earlier at the natural end of a cycle. Aligning your wake time with cycle boundaries can significantly improve how refreshed you feel.
Most adults need 5–6 complete sleep cycles per night, which corresponds to 7.5–9 hours of sleep. Four cycles (6 hours) is often the minimum for functional performance but is not adequate for long-term health. Three cycles (4.5 hours) is insufficient for most people and should only be a short-term emergency measure. The National Sleep Foundation recommends 7–9 hours for adults aged 18–64.
HC Webhook sends your sleep stages, duration, and quality data from Apple Health to any endpoint — so you can analyze and optimize your rest over time.
