About this calculator
Enter when you need to wake up to see bedtimes, or when you will go to bed to see good wake-up times.
How to use it
- Choose wake-up or bedtime.
- Enter the time.
- Adjust how long you take to fall asleep.
The formula
Bedtime = wake-up time − time to fall asleep − cycles × 90 minutes
- cycle
- about 90 minutes
- time to fall asleep
- about 15 minutes for most people
Worked example
Waking at 6:30 am
- For six cycles (9 hours), go to bed at 9:15 pm.
- For five cycles (7½ hours), go to bed at 10:45 pm.
What the result means
Aim for 7 to 9 hours on most nights; regular timing matters as much as length.
Assumptions
- Cycles of about 90 minutes.
Limitations
- Cycle length varies between people and through the night.
- Not medical advice for sleep disorders.
Frequently asked questions
How many hours of sleep do I need?
Most adults need 7 to 9 hours.
Why 90 minutes?
It is the average length of one sleep cycle.
What if I still feel tired?
Long-term tiredness can have medical causes; see a doctor.
Not medical advice. This calculator is a general screening tool and cannot diagnose any condition. Results are estimates. Please talk to a doctor or a qualified health professional about your health and before making changes to diet, exercise or treatment.
Last reviewed on 3 October 2026. Found a mistake? Tell us.