{
  "slug": "sleep-cycle",
  "title": "Sleep Cycle Calculator",
  "category": "Timing & Pacing",
  "category_slug": "timing-pacing",
  "tier": 1,
  "summary": "Calculate bedtimes for complete 90-minute sleep cycles based on your desired wake time.",
  "description": "This calculator computes recommended bedtimes based on Kleitman's 90-minute ultradian sleep cycle model to help align wake time with the end of a complete sleep cycle. It accepts a desired wake hour and minute, then returns suggested bedtimes for 3, 4, 5, and 6 full cycles, accounting for a fixed 15-minute sleep-onset latency. The output reflects population-average cycle timing and may not match individual sleep architecture variations.",
  "page_url": "https://fitmetriclab.com/en/tools/timing-pacing/sleep-cycle/",
  "api_url": "https://fitmetriclab.com/api/tools/sleep-cycle.json",
  "formula": {
    "id": "sleep_cycle",
    "expression": "T_{sleep} = T_{wake} - 90n\\,\\text{min}, \\quad n \\in \\{3, 4, 5, 6\\}",
    "variables": [
      {
        "symbol": "H",
        "description": "Wake hour"
      },
      {
        "symbol": "M",
        "description": "Wake minute"
      }
    ]
  },
  "inputs": [
    {
      "id": "wake_hour",
      "label": "Wake Hour",
      "type": "number",
      "unit": "h",
      "default": 7,
      "min": 0,
      "max": 23,
      "step": 1
    },
    {
      "id": "wake_minute",
      "label": "Wake Minute",
      "type": "number",
      "unit": "m",
      "default": 0,
      "min": 0,
      "max": 59,
      "step": 5
    }
  ],
  "worked_example": {
    "inputs": {
      "wake_hour": 7,
      "wake_minute": 0
    },
    "expected_output": "21:45"
  },
  "result_summary_template": "For a wake time of {wake_hour}:{wake_minute}, working backward through 90-minute cycles with a 15-minute sleep-onset buffer, the calculator suggests {result}.",
  "methodology": "Bedtime = (wake_time_seconds − cycles × 5400 − 900) mod 86400. Uses the 90-minute ultradian sleep cycle model with a fixed 15-minute sleep-onset latency. Returns targets for 3, 4, 5, and 6 complete cycles. Based on Kleitman's Basic Rest-Activity Cycle (BRAC) hypothesis.",
  "sources": [
    {
      "label": "Carskadon MA, Dement WC. Normal Human Sleep: An Overview. In: Kryger M, Roth T, Dement WC, eds. Principles and Practice of Sleep Medicine. 6th ed. Elsevier; 2017:15-24.",
      "url": "https://www.elsevier.com/books/principles-and-practice-of-sleep-medicine/kryger/978-0-323-24288-2"
    },
    {
      "label": "Kleitman N. Sleep and Wakefulness. Revised ed. University of Chicago Press; 1963.",
      "url": "https://press.uchicago.edu/ucp/books/book/chicago/S/bo3683507.html"
    },
    {
      "label": "Ohayon MM, Carskadon MA, Guilleminault C, Vitiello MV. Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals. Sleep. 2004;27(7):1255-1273.",
      "url": "https://pubmed.ncbi.nlm.nih.gov/15586779/"
    }
  ],
  "faqs": [
    {
      "question": "Why does the calculator use 90 minutes per cycle?",
      "answer": "Ninety minutes is the average duration of one complete progression through non-REM and REM sleep stages in adults. Individual cycle lengths vary from approximately 70 to 120 minutes and tend to shift across the night, with early cycles containing more deep sleep and later cycles containing more REM sleep."
    },
    {
      "question": "What is the 15-minute buffer for?",
      "answer": "The calculator subtracts 15 minutes from each bedtime to account for sleep-onset latency—the time between lying down and falling asleep. This value is a population average; actual latency ranges from under 5 minutes to over 30 minutes depending on sleep drive, circadian timing, and individual differences."
    },
    {
      "question": "Should I always aim for six cycles?",
      "answer": "Six cycles (9 hours) may exceed the sleep requirement for many adults. The tool offers four options (3, 4, 5, and 6 cycles) because optimal sleep duration varies by individual, age, activity level, and recovery needs. Most adults function well on 4–6 cycles per night."
    },
    {
      "question": "Does waking between cycles always feel better?",
      "answer": "Waking at the end of a cycle may reduce sleep inertia compared to waking from deep slow-wave sleep, but individual responses vary. Sleep quality, total sleep time, circadian alignment, and sleep debt all influence how rested someone feels upon waking, independent of cycle phase."
    },
    {
      "question": "Can this tool help with shift work or jet lag?",
      "answer": "The calculator does not adjust for circadian misalignment, which is central to shift work and jet lag. It assumes a stable sleep-wake schedule. Light exposure timing, melatonin secretion patterns, and gradual schedule shifts are more relevant interventions for circadian disruption than cycle-phase targeting alone."
    }
  ],
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    {
      "slug": "emom-calculator",
      "title": "EMOM Round Calculator",
      "api_url": "https://fitmetriclab.com/api/tools/emom-calculator.json",
      "page_url": "https://fitmetriclab.com/en/tools/timing-pacing/emom-calculator/"
    },
    {
      "slug": "hiit-ratio",
      "title": "HIIT Work-Rest Ratio Calculator",
      "api_url": "https://fitmetriclab.com/api/tools/hiit-ratio.json",
      "page_url": "https://fitmetriclab.com/en/tools/timing-pacing/hiit-ratio/"
    },
    {
      "slug": "rest-time",
      "title": "Rest Time Between Sets Calculator",
      "api_url": "https://fitmetriclab.com/api/tools/rest-time.json",
      "page_url": "https://fitmetriclab.com/en/tools/timing-pacing/rest-time/"
    }
  ],
  "last_updated": "2026-04-22",
  "price": {
    "value": 0,
    "currency": "USD"
  },
  "runs_in_browser": true,
  "disclaimer": "Educational estimate produced from a published formula. Not medical, nutritional, or training advice. Calculations are deterministic — apply the formula to the inputs to reproduce the result."
}