How to use the sleep calculator
- Choose whether you know when you want to wake up, when you will go to bed, or whether you are going to bed now.
- Enter the time, and how long you usually take to fall asleep – about 15 minutes for most people.
- Pick your age group so the result matches the hours of sleep recommended for you.
- Press Calculate for the best bedtime or alarm time, a picture of your night in cycles, and the other good options.
How sleep cycles work
Through the night you pass through repeating cycles of light sleep, deep sleep and REM (dream) sleep. One cycle lasts about 90 minutes on average – between roughly 70 and 120 minutes in different people and at different times of night. Deep sleep is longest in the early cycles and REM sleep in the later ones.
Waking from deep sleep often leaves you groggy (sleep inertia), while waking at the end of a cycle, in light sleep, feels easier. That is why the calculator times bedtimes and alarms in whole cycles:
\begin{aligned} \text{bedtime} = {} & \text{wake time} - \text{time to fall asleep} \\ & - n \times 90 \text{ min} \end{aligned}- 6 cycles (9 hours): go to bed at 9:15 PM
- 5 cycles (7.5 hours): go to bed at 10:45 PM
- 4 cycles (6 hours): go to bed at 12:15 AM
For an adult, 5 cycles (7.5 hours) or 6 cycles (9 hours) are within the recommended 7–9 hours.
How much sleep do you need?
| Age | Recommended | May be appropriate |
|---|---|---|
| Newborns (0–3 months) | 14–17 hours | 11–19 hours |
| Infants (4–11 months) | 12–15 hours | 10–18 hours |
| Toddlers (1–2 years) | 11–14 hours | 9–16 hours |
| Preschool (3–5 years) | 10–13 hours | 8–14 hours |
| School age (6–13 years) | 9–11 hours | 7–12 hours |
| Teenagers (14–17 years) | 8–10 hours | 7–11 hours |
| Adults (18–64 years) | 7–9 hours | 6–11 hours |
| Older adults (65+) | 7–8 hours | 5–9 hours |
Source: US National Sleep Foundation (Hirshkowitz and others, 2015). Babies and young children have shorter cycles (about 50–60 minutes in infants), so the 90-minute timing is meant for school-age children and adults.
Tips for better sleep
- Keep the same wake-up time every day, including weekends – it anchors your body clock.
- Get daylight in the morning and dim the lights in the hour before bed.
- Avoid caffeine for about 8 hours before bed, and large meals and alcohol late in the evening.
- Keep the bedroom cool, dark and quiet, and use the bed for sleep.
- If you can't sleep after about 20 minutes, get up and do something calm until you feel sleepy.
Cycle timing is a rule of thumb, not a precise measurement – cycle length varies through the night. If you regularly feel unrested after enough hours, snore loudly or stop breathing in your sleep, talk to a doctor; conditions such as sleep apnoea are common and treatable. To count the hours between two times, use the hours calculator.
Frequently asked questions
What time should I go to bed to wake up at 6 AM?
Allowing 15 minutes to fall asleep: 8:45 PM (6 cycles, 9 hours), 10:15 PM (5 cycles, 7.5 hours) or 11:45 PM (4 cycles, 6 hours). Adults need 7–9 hours, so 8:45 PM or 10:15 PM.
How long is a sleep cycle?
About 90 minutes on average, ranging from roughly 70 to 120 minutes. Most adults go through 4–6 cycles a night.
Is it better to sleep 6 hours or 7.5 hours?
7.5 hours. Adults need 7–9 hours, and regularly sleeping 6 hours or less is linked to poorer concentration, mood and long-term health.
Why do I wake up tired after 8 hours of sleep?
You may be waking in the middle of a deep-sleep cycle, sleeping poorly (for example with sleep apnoea), or simply need more sleep. Try a wake time at the end of a cycle and a regular schedule.
How much sleep does a teenager need?
8–10 hours a night, according to the US National Sleep Foundation.