Health / Sleep Calculator
😴 Sleep 🆓 Free 90-min Cycles

😴 Sleep Calculator

Enter your target wake-up time to find the best bedtimes based on completing 4, 5, or 6 full 90-minute sleep cycles - so you wake during light sleep feeling refreshed, not groggy. Or flip it: enter your bedtime and find the optimal wake times. Includes sleep debt tracker, age-based recommendations, chronotype guide, and sleep hygiene tips.

😴 Sleep Calculator

😓 Sleep Debt Tracker

Enter how many hours you slept each night this week. See your cumulative sleep debt.

🌊 Sleep Stages Explained

🔄 The 90-Minute Sleep Cycle
Each complete sleep cycle lasts approximately 90 minutes and includes all four stages. In early cycles, deep sleep (N3) dominates. In later cycles (toward morning), REM sleep increases. Waking mid-cycle - especially during deep sleep - causes sleep inertia (grogginess). The calculator times wake-ups to fall at the end of a cycle when you are naturally in lighter sleep.

📐 How Sleep Timing Works

The 90-Minute Cycle Rule

Each sleep cycle = approximately 90 minutes Optimal sleep = 5–6 complete cycles If wake time is 6:30 AM and latency is 14 min: Actual sleep start = 6:30 AM − 14 min = effective sleep onset reference point Working backwards in 90-min increments: 6 cycles: 6:30 − (6×90 min) − 14 min = 11:16 PM bedtime 5 cycles: 6:30 − (5×90 min) − 14 min = 12:46 AM bedtime 4 cycles: 6:30 − (4×90 min) − 14 min = 2:16 AM bedtime

Age-Based Recommendations (NSF / AAP 2024)

Newborns (0–3 mo): 14–17 hours/day Infants (4–11 mo): 12–15 hours/day Toddlers (1–2 yrs): 11–14 hours/day Preschool (3–5 yrs): 10–13 hours/day School age (6–13 yrs): 9–11 hours/day Teens (14–17 yrs): 8–10 hours/day Adults (18–64 yrs): 7–9 hours/day Older adults (65+): 7–8 hours/day

Sleep Debt Formula

Sleep Debt = Sum of nightly deficit over time Daily deficit = Target sleep − Actual sleep Example (8-hr target): Mon: slept 6h → deficit 2h Tue: slept 7h → deficit 1h Wed: slept 5h → deficit 3h Weekly sleep debt = 6 hours Recovery: +1-2 extra hours per night on weekends Full recovery from major sleep debt takes 1-3 weeks. Cannot fully recover large debt in one long sleep.

Sleep Hygiene Tips for Better Sleep Quality

Consistent schedule: Same bed/wake time every day (even weekends) keeps circadian rhythm stable Temperature: 16–19°C (61–67°F) optimal Body temp must drop for sleep onset Light: Block blue light 1-2hr before bed Bright light in morning anchors circadian clock Caffeine cutoff: 14:00 (2 PM) for most people Half-life: 5–6 hours Alcohol: Disrupts REM sleep - avoid near bedtime Screen-free wind-down: 30-60 min before bed Exercise: Finish 3-4 hrs before sleep (boosts SWS)

❓ Frequently Asked Questions

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AI-powered personalised sleep schedule, circadian rhythm optimisation and sleep hygiene recommendations.
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😴 Sleep Quick Facts

Cycle length~90 min
Cycles per night4–6
Adults need7–9 hrs
Avg fall-asleep time7–20 min
Deep sleep %15–20%
REM sleep %20–25%
Optimal bedroom temp16–19°C

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Sleep Calculator - Sleep Cycles, Sleep Debt and Why Timing Matters

Health note: This calculator provides general sleep timing guidance based on average sleep cycle research. Individual sleep needs vary. If you have ongoing sleep difficulties, excessive daytime sleepiness, or suspect a sleep disorder, consult a healthcare professional or sleep specialist.

Sleep isn't just about quantity - it's about timing and quality. Eight hours of fragmented sleep does not produce the same restoration as eight hours of uninterrupted cycles. Understanding how sleep cycles work explains why setting an alarm for exactly 8 hours sometimes leaves you more tired than 7.5 hours - and why waking at the right point in your cycle makes all the difference.

The 14-minute factor: This calculator adds 14 minutes (the average time to fall asleep) before calculating your sleep cycle windows. If you want to wake at 7:00 AM after 5 complete cycles (7.5 hours of sleep): ideal bedtime = 7:00 AM − 7.5h − 14 min = 11:16 PM. Setting your alarm for 7:00 AM and getting into bed at 11:30 PM means you actually sleep until approximately 7:14 AM - slightly overshooting your cycle. Small adjustments matter.

The Four Stages of a Sleep Cycle

NREM Sleep (Non-Rapid Eye Movement)

  • N1 (~5 min): Light sleep, transition between wake and sleep. Hypnic jerks common. Easily woken. Body temperature starts to drop.
  • N2 (~25 min): Light sleep. Heart rate slows, muscles relax. Sleep spindles and K-complexes - memory consolidation begins. Hard to wake but not the deepest.
  • N3 (~20–40 min, more in early cycles): Deep sleep / slow-wave sleep. Physical restoration, immune function, growth hormone release. Hardest to wake from - waking here causes sleep inertia (grogginess).

REM Sleep (Rapid Eye Movement)

  • ~20–25 min per cycle (more in later cycles - 6th cycle can be 50+ min REM)
  • Brain highly active. Dreams occur here.
  • Critical for: emotional regulation, memory consolidation, learning, creativity
  • Muscles are temporarily paralysed (sleep paralysis - prevents acting out dreams)
  • REM-deprived sleep leads to emotional dysregulation and memory deficits
  • Alcohol, cannabis, and many sleeping pills significantly suppress REM

Sleep Debt - What It Costs and How to Repay It

Sleep debt accumulates when you sleep less than your body requires. Unlike popular belief, you cannot fully "catch up" on sleep in one or two nights - some effects of chronic sleep deprivation (particularly on metabolic and immune function) are not fully reversible by recovery sleep.

What sleep debt costs: one night of 6 hours instead of 8 reduces cognitive performance by approximately 25%. Two weeks of 6-hour sleep causes performance equivalent to 48 hours without sleep - but subjects no longer feel sleepy (impaired self-assessment is one of the most dangerous effects of sleep deprivation). Research links chronic short sleep to increased risk of type 2 diabetes, cardiovascular disease, obesity, depression, and impaired immune function.

Evidence-Based Sleep Hygiene - What Actually Works

  • Consistent wake time: Wake at the same time every day, including weekends. This is the single most powerful anchor for your circadian rhythm - more important than consistent bedtime.
  • Cool bedroom (16–19°C / 61–67°F): Your core body temperature must drop to initiate sleep. A warm room prevents this. Cool environments support deeper, more consolidated sleep.
  • No screens 30–60 minutes before bed: Blue light from phones and laptops suppresses melatonin production. Dim, warm lighting in the evening accelerates melatonin onset.
  • No caffeine after 2 PM: Caffeine has a half-life of approximately 5–7 hours. A 3 PM coffee still has 50% of its caffeine active at 8–10 PM, reducing sleep quality even if you fall asleep normally.
  • No alcohol within 2 hours of bed: Alcohol sedates but severely suppresses REM sleep, causing rebound lighter sleep in the second half of the night. Two drinks can reduce total REM by 20–25%.