Sleep Cycle Calculator
Tell the tool when you need to wake up — or when you plan to fall asleep — plus how long it takes you to drift off, and it lists bed/wake times based on 3–7 complete 90-minute sleep cycles so you can avoid being yanked out of deep sleep.
Please enter a valid number
Suggested bedtimes
Going to sleep at one of these times lets you wake at the end of a cycle — not yanked out of deep sleep.
90 minutes is the average adult sleep cycle (light → deep → REM). Waking at the end of a cycle, in light sleep, leaves you far more alert than being pulled out of deep sleep by an alarm.
Formula
Bedtime = Wake time − N × 90 min − minutes to fall asleep Wake time = Bedtime + minutes to fall asleep + N × 90 min (N = number of full sleep cycles; 5–6 is the adult sweet spot, i.e. 7.5–9 h.)
- · A typical adult sleep cycle averages 90 min (NREM light → deep → REM); individuals range 70–110 min. This tool uses 90 min as the standard average.
- · Healthy adults usually need 5–6 full cycles per night (7.5–9 h). 3–4 cycles (4.5–6 h) is short-term only — chronic short sleep impairs cognition and metabolic health.
- · Average time to fall asleep ("sleep latency") is 10–20 min. The tool defaults to 15 min; adjust to match your usual pattern.
- · Setting an alarm aligned with cycle boundaries means you wake in light sleep instead of deep sleep — sharply reducing "sleep inertia" (groggy wake-ups).
- · Teens (14–17) typically need 8–10 h; children younger still. Stage breakdowns follow the National Sleep Foundation guidelines.
- · Pure arithmetic — does not model caffeine, alcohol, shift work, new-parent fragmentation, or sleep disorders. Sleep quality also depends on regularity and environment.
Frequently asked
Does the 90-minute cycle apply to everyone?
Ninety minutes is the population average for adults; individuals range from about 70 to 110 minutes. Cycle length varies with age, health, and stress, and your cycles tend to lengthen later in the night (more REM, less deep sleep). For a personalised average you would need a wearable or a sleep-lab study (polysomnography). For everyday planning, the 90-minute rule is a much better target than a random alarm time.
Will I feel better with fewer cycles aligned to the 90-minute rule?
For one or two nights, 4–5 cycles (6–7.5 h) aligned to the 90-minute rule can feel sharper than a random 5–6 h. But over time adults still need 7–9 h. That is why the tool flags 5–6 cycles (7.5–9 h) as "Recommended" — don't shrink your total sleep just to hit a cycle boundary on a regular basis.
Why include 'minutes to fall asleep' if I'm already drowsy?
There is a measurable gap between "lights out" and the first sleep cycle — sleep latency averages 10–20 minutes. Ignore it and the alarm fires mid-cycle (deep sleep or REM). If you drop off in seconds, lower the value to ~5 min; if you often lie awake, raise it.
Is 7.5 hours or 9 hours better?
It depends on you, but adults are advised 7–9 h by the National Sleep Foundation, CDC and WHO. Try a one-week experiment: 7.5 h (5 cycles) one week, 9 h (6 cycles) the next, and track alertness. Use whichever lets you wake naturally without an alarm. Don't routinely sleep over 9–10 h — chronically over-long sleep is associated with higher cardiovascular risk in epidemiological studies.
Related tools
BMI (Body Mass Index) Calculator
Enter height and weight to get your BMI and category (underweight / normal / overweight / obese).
BMR & TDEE Calculator
Estimate daily BMR and TDEE using Mifflin-St Jeor or Harris-Benedict, with calorie targets for cutting and bulking.
Body Fat Calculator (US Navy method)
Estimate body fat percentage using the US Navy circumference formula (neck, waist, plus hip for women).
Daily Water Intake Calculator
Estimate daily water intake from body weight, exercise duration and climate so you stay hydrated without overdoing it.
Running Pace Calculator
Compute pace per km / per mile from distance and time, or project finish time from a target pace — covers 5K, 10K, half- and full-marathon distances.
Heart Rate Zone Calculator
Compute 5 training heart rate zones from age and optional resting HR using the Karvonen formula (Z1 warm-up to Z5 max effort).
Lean Body Mass (LBM) Calculator
Estimate lean body mass (LBM), fat mass and body fat % from height, weight and sex using the Boer, Hume and James formulas.
Daily Macros Calculator (carbs / protein / fat)
Split a daily calorie target into grams of carbs, protein and fat at any macro ratio.
One-Rep Max (1RM) Calculator
Estimate one-rep max from a sub-maximal lift (weight × reps) with Epley, Brzycki, Lombardi and Lander formulas, plus a 50–95% intensity training table.
Body Surface Area (BSA) Calculator
Estimate body surface area from height and weight using the Mosteller, Du Bois, Haycock and Boyd formulas — commonly used in drug-dosing, chemotherapy and cardiac-index calculations.
Race Time Predictor (Riegel Formula)
Project a finish time at one distance from a known time at another distance using the classic Riegel formula T₂ = T₁ × (D₂/D₁)^1.06.
Mean Arterial Pressure (MAP) Calculator
Compute mean arterial pressure from systolic and diastolic blood pressure with clinical bands and pulse-pressure context: MAP = DBP + (SBP − DBP)/3.
Pack-Year (Smoking History) Calculator
Compute cumulative smoking exposure in pack-years from cigarettes per day and years smoked, with lung-cancer-screening thresholds.
Waist-to-Hip Ratio (WHR) Calculator
Divide waist by hip circumference and check the WHO risk bands — a quick screen for central obesity and cardiovascular risk.
Exercise Calorie Burn (MET) Calculator
Use METs, body weight and duration to estimate calories burned for common activities (running, hiking, swimming, cycling and more).
Caffeine Half-Life Calculator
Pick the dose (mg) and the time you drank it; the calculator uses the standard ~5-hour caffeine half-life to project how many milligrams remain hour-by-hour, including the level at bedtime.
Body Adiposity Index (BAI) Calculator
Bergman’s BAI estimates body-fat percentage from hip circumference and height alone (BAI = hip ÷ height^1.5 − 18) — no scale required.
Blood Alcohol Concentration (BAC) Calculator
Estimate blood alcohol concentration (BAC %) with the Widmark formula from drinks consumed, body weight, sex and hours elapsed — and compare against the Hong Kong 0.05% drink-driving limit.
VO₂ Max Calculator (Cooper 12-Minute Run Test)
Enter your distance covered in a 12-minute run to estimate VO₂ max and read off the age/sex fitness category.