BMI Calculator
Calculate your Body Mass Index and healthy weight range.
Open Tool →Sleep Cycle Calculator
Stop waking up groggy. Enter your planned wake-up time or bedtime to find the optimal times to fall asleep or rise, perfectly aligned to your natural 90-minute sleep cycles.
⏱ Includes a 15-minute average for falling asleep.
Each sleep cycle lasts approximately 90 minutes and includes light sleep, deep (slow-wave) sleep, and REM sleep. Waking mid-cycle — especially from deep sleep — causes sleep inertia, the groggy feeling that can last over an hour.
⭐ = Recommended ✨ = Best choice for most adults (7–9 hours)
Sleep Cycle Calculator — Why Completing 90-Minute Cycles Changes Everything
Most people focus exclusively on how many hours they sleep — but emerging sleep science suggests that when within a cycle you wake up matters just as much. A full sleep cycle lasts approximately 90 minutes and progresses through three distinct stages: N1 (light sleep), N2 (true sleep), and N3 (slow-wave deep sleep), followed by REM (rapid eye movement) sleep. Most adults cycle through 4–6 of these 90-minute blocks per night.
What Happens Inside a Sleep Cycle
During the N3 deep sleep stage, your brain consolidates motor memories, your body releases growth hormone, and tissue repair is at its peak. This stage predominates in the early cycles of the night. Later cycles contain progressively more REM sleep, which is critical for emotional processing, creative thinking, and declarative memory consolidation. Interrupting either stage — especially N3 — triggers sleep inertia, the heavy, disoriented grogginess that can persist for 30–90 minutes after an ill-timed alarm.
The 15-Minute Fall-Asleep Allowance
This calculator adds a 15-minute buffer to account for the average sleep onset latency — the time it typically takes to fall asleep after lights out. Sleep onset latency varies considerably between individuals (10–20 minutes is considered normal). If you tend to fall asleep unusually quickly or slowly, adjust your bedtime or wake-up time slightly to compensate.
How Many Cycles Do You Need?
The NHS and National Sleep Foundation recommend 7–9 hours of sleep per night for adults aged 18–64, which translates to 5–6 complete 90-minute cycles. Consistently getting fewer than 5 cycles (under 7.5 hours) is associated with impaired immune function, increased cardiovascular risk, weight gain, and reduced cognitive performance. While individual sleep requirements vary — some naturally function well on 6 hours while others need 9 — most people benefit from aiming for at least 5 complete cycles on workdays and prioritising 6 on rest days.
Tips to Improve Sleep Quality
Timing is just one component of good sleep hygiene. Consistent bed and wake times, even on weekends, help entrench a stable circadian rhythm. Avoiding blue-light-emitting screens for 60–90 minutes before bed, keeping the bedroom cool (16–19°C is optimal for most people), and limiting caffeine after 2 pm significantly improve sleep architecture. Even one night of adequate, well-timed sleep can noticeably improve mood, memory, and reaction time the following day.