# Sleep Calculator

> Free sleep calculator. Enter when you need to wake up (or when you are going to bed) and get bedtimes that line up with 90-minute sleep cycles, so you wake between cycles.

Waking in the middle of deep sleep is what makes an alarm feel brutal. Pick your wake-up time and this calculator works backwards in full sleep cycles, so the alarm lands at the end of a cycle instead of in the middle of one.

*Interactive tool: enter your values at https://toollantern.win/sleep-calculator/ and results update instantly in your browser.*

## How sleep cycles work

Sleep is not one long block. Through the night you move through repeating cycles of light sleep, deep (slow-wave) sleep and REM sleep, each lasting roughly 90 minutes on average. Waking at the end of a cycle, when sleep is lightest, usually feels far easier than waking from deep sleep, even if the total hours are similar.

## How the calculator works

- **Bedtime** = wake-up time − (cycles × cycle length) − minutes to fall asleep.

- **Wake-up time** = bedtime + minutes to fall asleep + (cycles × cycle length).

It lists options from 6 cycles (9 hours of sleep) down to 3 cycles (4.5 hours), so you can pick the latest bedtime that still fits your night.

## Worked example

To wake at **7:00** with 15 minutes to fall asleep: 6 cycles is 9 hours, so lights out at **9:45 pm**. Five cycles (7.5 hours) means bed at **11:15 pm**. Four cycles (6 hours) means **12:45 am**.

## Limitations

Cycle length varies from person to person and through the night: early cycles carry more deep sleep, later ones more REM. Treat the times as a starting point and adjust the cycle length if you still wake groggy. This tool is not medical advice; persistent insomnia, loud snoring or daytime sleepiness are worth discussing with a doctor.

## Frequently asked questions
### What time should I go to bed to wake up at 7 am?

With 15 minutes to fall asleep and 90-minute cycles, aim for 9:45 pm (6 cycles, 9 hours) or 11:15 pm (5 cycles, 7.5 hours). Both put your alarm at the end of a cycle.

### How long is a sleep cycle?

About 90 minutes on average, though individual cycles range from roughly 80 to 110 minutes. You can change the cycle length in the calculator to match how you feel.

### Is 6 hours of sleep enough?

Six hours is four full cycles, and waking at the end of one will feel better than a random 6.5 hours. But most adults need 7 to 9 hours regularly, so 6 hours is a short-term compromise, not a target.

### Why do I feel worse after more sleep sometimes?

Waking in the middle of deep sleep causes sleep inertia, that heavy, groggy feeling. A slightly shorter night that ends between cycles can feel better than a longer one that ends mid-cycle.

### Does this sleep calculator work for naps?

For naps, either keep it under about 20 minutes to avoid deep sleep, or take one full 90-minute cycle. Use the 'go to sleep right now' mode to see when one cycle ends.

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Interactive version: https://toollantern.win/sleep-calculator/