# When should I go to sleep?

Finds bedtimes that let you wake up at the end of a 90-minute sleep cycle, or wake-up times for a given bedtime, allowing for the minutes it takes to fall asleep.

- Page: https://www.acalculator.org/health/sleep-calculator
- JSON spec: https://www.acalculator.org/health/sleep-calculator.json
- Version: 01360a27a18a

## Default answer

Example with the default inputs (I know my Wake-up time, Wake up at 07:00, Minutes to fall asleep 15): To wake up at 7:00 AM after 5 full sleep cycles, go to bed at 11:15 PM.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| mode | I know my | Whether you know the time you must wake up, or the time you go to bed. |
| wake | Wake up at | The time you want to wake up. |
| bed | Go to bed at | The time you go to bed. |
| latency | Minutes to fall asleep | How long it usually takes you to fall asleep once in bed. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| bed5 | Go to bed at | The bedtime for 5 full sleep cycles (7 hours 30 minutes of sleep). |
| bed6 | Or, for 6 cycles (9 h of sleep) | The bedtime for 6 full sleep cycles (9 hours of sleep). |
| bed4 | Or, for 4 cycles (6 h of sleep) | The bedtime for 4 full sleep cycles (6 hours of sleep). |
| wake5 | Wake up at | The wake-up time after 5 full sleep cycles (7 hours 30 minutes of sleep). |
| wake6 | Or, after 6 cycles (9 h of sleep) | The wake-up time after 6 full sleep cycles (9 hours of sleep). |
| wake4 | Or, after 4 cycles (6 h of sleep) | The wake-up time after 4 full sleep cycles (6 hours of sleep). |
| asleep | You fall asleep at about | The bedtime plus the minutes it takes to fall asleep. |
| summary | In short | The main answer as one sentence. |

## Method

Bedtime = wake-up time − minutes to fall asleep − cycles × 90 minutes; wake-up time = bedtime + minutes to fall asleep + cycles × 90 minutes; times wrap past midnight.

## Assumptions

- A sleep cycle lasts 90 minutes. Real cycles run about 90 to 110 minutes and vary through the night.
- Waking at the end of a cycle, not in the middle of deep sleep, is assumed to feel easier.
- Adults need 7 hours of sleep or more (AASM and Sleep Research Society), so 5 or 6 cycles is the main answer; 4 cycles (6 hours) is short.
- This is general guidance, not medical advice.

## Worked examples

1. mode = wake, wake = 07:00, latency = 15 gives bed5 = 23:15, bed6 = 21:45, bed4 = 00:45, summary = To wake up at 7:00 AM after 5 full sleep cycles, go to bed at 11:15 PM.. Source: NINDS (sleep cycles about 90 minutes); hand calculation in content.mdx.
2. mode = wake, wake = 06:30, latency = 0 gives bed5 = 23:00, bed6 = 21:30, bed4 = 00:30. Source: hand calculation in content.mdx.
3. mode = bed, bed = 22:30, latency = 20 gives asleep = 22:50, wake5 = 06:20, wake6 = 07:50, wake4 = 04:50. Source: NINDS (sleep cycles about 90 minutes); hand calculation in content.mdx.
4. mode = bed, bed = 01:00, latency = 15 gives asleep = 01:15, wake5 = 08:45. Source: hand calculation in content.mdx.

## FAQ

### What time should I go to bed?

Count back whole sleep cycles of 90 minutes from your wake-up time, then take off the time you need to fall asleep. To wake at 7:00 AM after 5 cycles (7 hours 30 minutes of sleep) with 15 minutes to fall asleep, go to bed at 11:15 PM. For 6 cycles (9 hours), go to bed at 9:45 PM.

### What is a sleep cycle?

During the night you pass through light sleep, deep sleep and REM (dream) sleep, then start again. One full cycle takes about 90 to 110 minutes, and a typical night has 4 or 5 cycles. Waking at the end of a cycle, when sleep is light, is thought to feel easier than waking from deep sleep.

### How much sleep do I need?

The American Academy of Sleep Medicine and the Sleep Research Society recommend 7 hours or more a night for adults aged 18 to 60. The CDC gives 7 to 9 hours for ages 61 to 64 and 7 to 8 hours for 65 and older. Five cycles (7 hours 30 minutes) or six cycles (9 hours) fit these ranges; four cycles (6 hours) is short.

### How long does it take to fall asleep?

It differs from person to person and night to night. The calculator uses 15 minutes unless you change it under More options. If you often lie awake for a long time, or feel tired after a full night, talk to a health care provider.

### What if I go to bed now?

Choose "Bedtime" and enter the time. The calculator adds the minutes to fall asleep, then 4, 5 and 6 cycles of 90 minutes, and shows when to set your alarm.

### Are sleep cycles exactly 90 minutes?

No. 90 minutes is an average; cycles run about 90 to 110 minutes and change through the night. Treat the times as a guide and move them by 10 to 15 minutes if you often wake groggy.

## Sources

- National Institute of Neurological Disorders and Stroke. Brain Basics: Understanding Sleep. https://www.ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep
- Patel AK, Reddy V, Shumway KR, Araujo JF. Physiology, Sleep Stages. In: StatPearls. StatPearls Publishing; 2024. https://www.ncbi.nlm.nih.gov/books/NBK526132/
- Watson NF, Badr MS, Belenky G, et al. Recommended amount of sleep for a healthy adult: a joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society. J Clin Sleep Med. 2015;11(6):591-592. https://doi.org/10.5664/jcsm.4758
- Centers for Disease Control and Prevention. About Sleep (recommended hours by age). https://www.cdc.gov/sleep/about/index.html