Sleep Calculator
Based on 90-minute sleep cycles. A full night typically includes 5–6 cycles (7.5–9 hours).
What time should you go to sleep to wake up at this time?
How does it work?
Sleep happens in cycles of roughly 90 minutes, moving through light sleep, deep sleep, and REM stages. Waking at the end of a cycle — rather than during deep sleep — makes you feel more refreshed.
This calculator adds a 15-minute sleep onset period (average time to fall asleep) to every option.
Most adults need 5–6 cycles (7.5–9 hours) per night. If you can't hit that, 4 cycles (6 hours) is the next best option.
About the Sleep Calculator
A sleep calculator helps you find the ideal time to go to bed or wake up by aligning your schedule with your body's natural 90-minute sleep cycles. Each cycle progresses through light sleep, deep sleep, and REM sleep — and waking up at the end of a cycle, rather than in the middle of one, is what leaves you feeling alert and refreshed.
This free online sleep calculator offers two modes: enter your desired wake-up time to find the best bedtimes, or enter your planned bedtime to see when you'll naturally wake up. It also includes a Sleep Now shortcut that calculates ideal wake times starting from the current moment.
All calculations run instantly in your browser — no data is sent to any server, and the tool works offline.
How to Use the Sleep Calculator
- Choose your mode — select "I want to wake up at…" if you have a fixed wake-up time, or "I plan to sleep at…" if you have a fixed bedtime.
- Enter your time — type or select your wake-up time or bedtime using the time input.
- Read the results — the calculator shows 6 bedtime or wake-up options based on 3 to 8 complete sleep cycles. Options marked Recommended give you 7.5–9 hours of sleep.
- Use Sleep Now — click the Sleep Now button to instantly calculate wake times starting from right now, with no time input required.
When to Use a Sleep Calculator
Early Morning Commitments
Have a 6 AM flight or early meeting? Enter your wake-up time and pick a bedtime that lets you complete 5 or 6 full cycles without an alarm interrupting deep sleep.
Shift Workers
Working nights or rotating shifts? Use the "sleep at…" mode to plan your wake time around your unusual schedule and still get quality rest.
Reducing Morning Grogginess
If you wake up feeling groggy despite getting enough hours, you may be waking mid-cycle. Use this calculator to shift your alarm to the end of a cycle instead.
Power Naps
A single 90-minute cycle (3 cycles from now for a 4.5h total) can work as a structured nap when you have a fixed amount of time to rest.
Jet Lag Recovery
After travelling across time zones, use this tool to find the optimal local bedtime that aligns your new sleep schedule with full cycle boundaries.
Parents & New Babies
Maximise rest during fragmented nights by sleeping in complete 90-minute cycles where possible, rather than trying for a full 8 hours in one block.
Frequently Asked Questions
What is a sleep cycle?
A sleep cycle is a roughly 90-minute period during which your brain moves through several stages: light sleep (N1, N2), deep slow-wave sleep (N3), and REM (rapid eye movement) sleep. A full night typically includes 4–6 complete cycles.
Why does the calculator add 15 minutes?
On average, it takes about 15 minutes to fall asleep after getting into bed (this is called sleep onset latency). The calculator adds this 15-minute buffer so the suggested times reflect when you should actually get into bed — not when your first sleep cycle begins.
How many hours of sleep do I need?
Most adults need 7–9 hours of sleep per night, which corresponds to 5–6 complete 90-minute cycles. Teenagers often need 8–10 hours (up to 6–7 cycles), while older adults may feel rested with 6–7.5 hours (4–5 cycles).
Why do I feel worse after 8 hours than after 7.5 hours?
This is called sleep inertia — the grogginess caused by waking up mid-cycle, particularly during deep N3 sleep. 7.5 hours is exactly 5 complete cycles, so your alarm wakes you at the lightest point. 8 hours puts you 30 minutes into a 6th cycle, often during deep sleep.
Is the 90-minute cycle exactly 90 minutes?
Not exactly — sleep cycles vary between roughly 80–120 minutes depending on the individual, age, stress levels, and how much sleep debt you're carrying. The 90-minute figure is a well-established average used by sleep scientists and is a reliable baseline for practical planning.
What if I can't fall asleep in 15 minutes?
If you regularly take longer than 20–30 minutes to fall asleep, you may benefit from reviewing your sleep hygiene — consistent bedtimes, limiting screens before bed, and keeping your bedroom cool and dark. The 15-minute onset is an average; adjust your bedtime slightly earlier if you know it takes you longer.
Can this replace professional sleep advice?
No — this tool is for general planning purposes only. If you have persistent sleep problems, insomnia, sleep apnoea, or other sleep disorders, consult a doctor or sleep specialist. The calculator is a helpful guide, not a medical tool.