Free sleep calculator.
Find the best bedtime or wake-up time using 90-minute sleep cycles — so your alarm lands at the end of a cycle, not the middle, and you wake up refreshed instead of groggy.
Tell it when you have to wake up, or when you're heading to bed — it does the rest. 100% free, no sign-up, and it all runs locally in your browser.
The science, briefly
How sleep cycles work.
You don't sleep in one flat block. Across the night your brain runs the same loop over and over: light sleep, down into deep slow-wave sleep, then up into REM (where most dreaming happens), and back toward light sleep again. That whole loop is one sleep cycle, and it takes roughly 90 minutes.
A normal night is four to six of these cycles stacked back to back. The mix shifts as the night goes on — deep sleep dominates your first couple of cycles, while REM stretches longer toward morning. That's why the last few hours of sleep feel dream-heavy and lighter.
The practical takeaway: the moment your alarm fires matters as much as how long you slept. Catch the light-sleep gap at the end of a cycle and waking is easy. Land in the deep-sleep trough in the middle and it's a fight.
Sleep inertia
Why waking mid-cycle wrecks you.
Ever slept eight hours and still felt destroyed — yet another morning got up after six and felt fine? That gap is usually sleep inertia: the groggy, heavy-limbed, foggy-headed state you get when an alarm yanks you out of deep slow-wave sleep instead of letting you surface naturally in light sleep.
In deep sleep your brain is running its slowest waves, your body temperature and heart rate are at their lowest, and waking is hardest. Forced awake there, your alertness can stay impaired for 15 to 30 minutes — sometimes longer. You hit snooze, you stumble, the coffee barely helps.
Wake at the end of a cycle instead and you're already in light sleep, half a step from awake. Same total hours, a completely different morning. That single timing difference is what this calculator is built to find.
How much is enough
How many cycles do you actually need?
For most adults the sweet spot is five to six cycles — about 7.5 to 9 hours. That's why those two options are highlighted in the calculator: they cover the standard "seven-to-nine hours" recommendation while still landing cleanly at the end of a cycle.
Four cycles (6 hours) is the floor a lot of people can run on for a day or two without falling apart — useful for an early flight, not a lifestyle. Three cycles (4.5 hours) is a survival option for the occasional brutal night; you'll get through tomorrow, but you're borrowing against a sleep debt you'll have to repay.
Stack too many short nights and that debt compounds — slower reactions, worse mood, weaker focus and immunity. If you're regularly forced under five cycles, the real fix is an earlier bedtime, not a cleverer alarm. The calculator will happily show you what time that means.
Honest limits
A helpful heuristic, not a sleep lab.
Let's be straight about what this is. The 90-minute cycle is an average. Real cycles vary from roughly 70 to 120 minutes — between people, and even within the same person across a single night. So the times here are an educated estimate, not a measurement of your brain.
The 15-minute fall-asleep buffer is an average too. If you're out cold in two minutes, or you lie there for forty, the timing drifts. Caffeine, alcohol, stress, age, screens and a warm bright bedroom all shift your cycles around as well.
Use this the way it's meant: a smart starting point that beats picking a bedtime at random. The only way to know your actual cycles is a sleep study or a tracker reading your real brain activity. Everything here runs on the 90-minute heuristic — genuinely useful, and honestly imperfect.
Sleep calculator — FAQ
How do sleep cycles work? +
A night of sleep is not one continuous block — it runs in repeating cycles. Each cycle moves through light sleep, then deep (slow-wave) sleep, then REM, before lifting back toward light sleep. One full cycle takes roughly 90 minutes, and a typical night is four to six of them back to back. You feel best when your alarm goes off near the end of a cycle, in light sleep, rather than mid-cycle in deep sleep.
Why 90 minutes per cycle? +
Ninety minutes is the commonly cited average length of one full sleep cycle for adults. It is an approximation, not a fixed law — real cycles range from about 70 to 120 minutes and even change across the night (deep sleep dominates early cycles, REM dominates later ones). This calculator uses 90 minutes because it is a reasonable, well-known midpoint that makes the math useful for planning.
Why should I wake up between cycles instead of mid-cycle? +
At the end of a cycle you are in light sleep, so your brain and body are already close to waking — getting up then feels relatively easy. If your alarm interrupts deep slow-wave sleep in the middle of a cycle, you wake with "sleep inertia": that heavy, foggy, disoriented feeling that can linger for 15 to 30 minutes or more. Timing your alarm to the end of a cycle is the whole point of this tool.
How many sleep cycles do I actually need? +
Most adults do best on five to six cycles, which is about 7.5 to 9 hours of sleep — that is why those two options are highlighted as ideal. Four cycles (6 hours) is the minimum many people can function on for a short stretch, and three cycles (4.5 hours) is a get-through-it option for the occasional rough night, not a habit. Consistently sleeping fewer than five cycles tends to build up sleep debt.
Is this scientific? How accurate is it? +
It is grounded in real sleep science — cycles, stages and sleep inertia are well established — but the exact 90-minute figure is an average, so the suggested times are an educated estimate, not a personalized measurement. Your true cycle length varies by person, by night, and by factors like age, stress, alcohol and caffeine. Treat the output as a helpful starting point, not a guarantee. Only a sleep lab or a tracker measuring your actual brain activity can pinpoint your real cycles.
Does this work for naps too? +
Partly. The same idea applies: a 90-minute nap lets you complete one full cycle and wake in light sleep, which usually feels good. The other sweet spot is a short 20-minute "power nap" that ends before you fall into deep sleep — that one this calculator does not target. The times to avoid are roughly 30 to 80 minutes, where you are likely to wake mid-deep-sleep and feel worse than before.
Does the 15-minute fall-asleep buffer really matter? +
It helps. The buffer assumes it takes about 15 minutes to actually drift off after your head hits the pillow (sleep researchers call this sleep-onset latency, and 10 to 20 minutes is normal). Without it, your cycles would start at lights-out rather than at actual sleep, throwing the timing off by a chunk of a cycle. If you fall asleep much faster or slower than 15 minutes, nudge the suggested times accordingly.
What if I still wake up tired even after using this? +
Cycle timing is only one lever. If you wake tired despite hitting a clean cycle, the usual suspects are total sleep amount (too few cycles), an inconsistent schedule, caffeine or alcohol too late, a warm or bright bedroom, screens before bed, or an underlying issue like sleep apnea. This tool fixes when you wake within the night; it cannot fix not getting enough hours, or a sleep environment working against you. If tiredness persists despite good habits, it is worth talking to a doctor.