liminfoSleep Cycle Calculator
Free web tool: Sleep Cycle Calculator
21:46
9h 0m of sleep
23:16
7h 30m of sleep
00:46
6h 0m of sleep
02:16
4h 30m of sleep
Based on 90-minute sleep cycles. Includes ~14 minutes to fall asleep.
About Sleep Cycle Calculator
The Sleep Cycle Calculator helps you find the ideal time to go to bed or wake up based on the science of 90-minute sleep cycles. Human sleep consists of repeating cycles, each lasting approximately 90 minutes, during which the brain progresses through light sleep, deep sleep, and REM sleep stages. Waking up at the end of a complete cycle — rather than in the middle of one — dramatically reduces morning grogginess and improves how rested you feel.
The tool offers two modes: "I need to wake up at" calculates the best bedtimes given your required wake-up time, and "I plan to sleep at" calculates the best wake-up times given your planned bedtime. In both modes, the tool adds 14 minutes to account for the average time it takes a person to fall asleep after lying down, ensuring the cycle counts are accurate from true sleep onset rather than the moment you get into bed.
For each mode, the calculator shows four options covering 3, 4, 5, and 6 complete sleep cycles, corresponding to 4.5, 6, 7.5, and 9 hours of actual sleep. Options with 5 or 6 cycles are highlighted in green (recommended), 4 cycles in yellow (acceptable), and 3 cycles in red (minimum). This helps you quickly identify which options align with healthy sleep recommendations of 7–9 hours for adults.
Key Features
- Two modes: calculate bedtime from wake-up time, or wake-up time from bedtime
- Based on 90-minute sleep cycle science for accurate, restorative timing
- Adds 14-minute fall-asleep latency for real-world accuracy
- Shows 4 options per query: 3, 4, 5, and 6 complete sleep cycles
- Color-coded quality indicators: green (recommended), yellow (acceptable), red (minimum)
- Displays sleep duration in hours and minutes for each option
- 100% client-side — no data is uploaded or stored
- Responsive design with dark mode, works on any device
Frequently Asked Questions
What is a sleep cycle and why does it matter?
A sleep cycle is a natural progression through four sleep stages: N1 (light sleep), N2 (deeper light sleep), N3 (deep/slow-wave sleep), and REM (rapid eye movement) sleep. Each cycle takes approximately 90 minutes. Waking up at the end of a complete cycle — when sleep is lightest — feels more natural and less groggy than waking mid-cycle during deep sleep.
Why is 14 minutes added to the calculation?
On average, it takes a person about 14 minutes to fall asleep after lying down (sleep onset latency). The calculator adds this time so that the computed bedtime or wake-up time accounts for actual sleep duration from the moment you fall asleep, not just when you get into bed.
How many sleep cycles should I aim for?
Most adults need 7–9 hours of sleep, which corresponds to 5–6 complete 90-minute cycles (7.5 hours for 5 cycles, 9 hours for 6 cycles). The calculator highlights these as the recommended options. 4 cycles (6 hours) is acceptable for occasional nights, while 3 cycles (4.5 hours) is considered minimum and not sustainable long-term.
If I set a wake-up time of 7:00, when should I go to bed?
With a 7:00 AM wake-up time, the calculator subtracts the fall-asleep time (14 min) plus each cycle option. For 6 cycles (9h + 14min): bedtime at 9:46 PM. For 5 cycles (7.5h + 14min): 11:16 PM. For 4 cycles (6h + 14min): 12:46 AM. For 3 cycles (4.5h + 14min): 2:16 AM.
What if I cannot fall asleep at the calculated bedtime?
The 14-minute average is just that — an average. If you typically take longer to fall asleep (for example, 30 minutes), you may want to go to bed earlier than the calculated time. Factors like stress, screen time, caffeine, and room temperature all affect how quickly you fall asleep.
Does this calculator work for naps?
The calculator is designed for full night sleep periods. For naps, a single 90-minute cycle (1 cycle mode) is generally recommended for a full nap, or 20 minutes for a power nap that avoids deep sleep entirely. The current interface always shows 3–6 cycle options, so it is best suited for planning overnight sleep.
Is 90 minutes accurate for everyone?
The 90-minute cycle length is an average across adults. Individual cycles can range from 80 to 110 minutes, and cycle length can change across the night — early cycles tend to be longer, later ones shorter. This calculator uses the standard 90-minute figure as a practical baseline.
How is the quality color-coding determined?
Options with 5 or 6 cycles (7.5–9 hours of sleep) are marked green as recommended, aligning with the 7–9 hour guideline for adults. 4 cycles (6 hours) is yellow as acceptable, and 3 cycles (4.5 hours) is red as minimum, indicating it is not sufficient for sustained health and performance.