What time should you go to bed?
Caffeine in your system
Projected: one filter coffee at 18:07. Add what you've actually had to replace it.
Cycle-aligned bedtimes
Counting back from your alarm in 90-minute cycles, so you wake between cycles rather than mid-cycle. These are windows, not appointments — real cycles run 70 to 120 minutes and vary night to night.
- 23:55–00:35 4 cycles · 6 h of sleep
- 22:20–23:10 5 cycles · 7 h 30 min of sleep
- 20:45–21:45 6 cycles · 9 h of sleep
When do you need to be up? It counts back in sleep cycles to bedtime windows.
Ninety minutes is an average, not a measurement of you. That's why these are windows.
Why sleep cycles matter, and how much
Sleep runs in repeating cycles through light sleep, deep sleep and REM. Waking at the end of a cycle, in light sleep, generally feels easier than being pulled out of deep sleep partway through one. That is the whole idea behind cycle-aligned bedtimes.
The usual figure is 90 minutes a cycle. It is a population average, not a property of you: real cycles run from about 70 to 120 minutes, they are shorter earlier in the night and longer towards morning, and they vary night to night in the same person.
This is why every option here is a window rather than a time. A calculator that tells you to go to bed at 22:47 is presenting an average as if it were a measurement of you.
Falling asleep is part of the sum
The time between lights-out and actually being asleep is sleep latency, and it typically runs 10 to 20 minutes in adults who are not sleep deprived. It has to be added on: if you need six cycles and you take 15 minutes to drop off, bed is 9 hours 15 minutes before your alarm, not 9 hours.
Latency is also the part caffeine hits hardest. Enough caffeine on board mostly does not stop you sleeping — it stretches the gap before you do, which quietly eats a cycle.
What the windows here mean
Each window is centred on a cycle-aligned bedtime and widened by five minutes per cycle, because timing drift compounds across the night. A four-cycle option carries a twenty-minute window; a six-cycle option carries thirty.
Fewer cycles is not automatically worse. Five solid cycles beats six broken ones, and going to bed before you are sleepy tends to produce the second.
A worked example
You need to be awake at 06:30 and you usually take about 15 minutes to fall asleep.
Five cycles is 450 minutes of sleep. Add 15 minutes of latency and count back from 06:30, and bed lands at 22:45 — presented as 22:20 to 23:10, because five cycles carries a 25-minute window.
Four cycles is 360 minutes. The same arithmetic puts bed at 00:15, on the other side of midnight, with a 20-minute window either side.
Common questions
What time should I go to bed to wake up at 6:30?
Counting back in 90-minute cycles from 06:30 with 15 minutes to fall asleep: 22:45 for five cycles, 21:15 for six, or 00:15 for four. Treat each as a window of twenty to thirty minutes rather than an exact time.
Is 6 hours of sleep enough?
Six hours is four cycles, which is below the seven to nine hours most adults need. It is a reasonable target for a single short night, not a sustainable pattern.
Are sleep cycles really 90 minutes?
Ninety minutes is the average. Individual cycles run roughly 70 to 120 minutes, get longer towards morning, and vary between nights, which is why cycle-aligned times should be read as approximate.
Should I go to bed earlier if I have had caffeine?
Caffeine mostly lengthens the time it takes to fall asleep, so an earlier lights-out can offset it. Adding your drinks to the calculator shows how much is still on board at each of the bedtime options.
Does waking mid-cycle actually make you groggy?
Waking from deep sleep produces sleep inertia — that heavy, disoriented feeling — and it can last twenty to thirty minutes. Aligning wake-up with the end of a cycle is an attempt to avoid it, though the alignment can only ever be approximate.
What this cannot tell you
- Your own half-life. The 3-to-7-hour range for healthy adults is wide, and where you fall in it is genuinely unknown without testing. Every number here inherits that uncertainty.
- Your own sleep cycles. Ninety minutes is a population average; real cycles run 70 to 120 minutes and vary night to night, which is why bedtimes are shown as windows.
- The caffeine in the cup in front of you. Brew strength and cup size move filter coffee from 70 to 140 mg. The presets are central values from a range, not measurements.
- The effect of medication or liver impairment. Some drugs change caffeine clearance far more than anything modelled here. This tool does not attempt them.
- Anything about safety. It estimates sleep impact, and nothing it says bears on driving, operating machinery, or a cardiac question.
An educational estimate, not medical advice. Talk to a clinician about sleep or cardiac concerns.
Put this on your own site
The calculator embeds as a single iframe. It carries no ads, no analytics and no cookies inside the frame — it will not add anything to your page that you have not chosen — and it works on a phone at any width.
<iframe src="https://caffeinecurfew.com/embed" width="100%" height="1650" style="border:1px solid #e1e0d9;border-radius:12px" loading="lazy" title="Caffeine cutoff calculator"></iframe>
<p><a href="https://caffeinecurfew.com/">Caffeine Curfew</a></p> Set a starting point with the same options the page uses — for example a cold brew drinker with an eleven o'clock bedtime: /embed?bed=23:00&usual=cold-brew
The frame is tall enough for the chart at the narrowest width a phone will give it, so a wide column gets some slack at the bottom. The credit line under the frame is the only thing we ask for; please keep it.
References
Written by Toni Ristola — Built the calculator and wrote the explainers. Not a clinician.
Last updated 2026-08-30