How do sleep cycles work?
During the night you move through repeating sleep cycles of light sleep, deep sleep and REM (dreaming) sleep. Each cycle lasts roughly 90 minutes (anywhere from 70 to 120 minutes), and most adults need 4–6 cycles a night. Waking at the end of a cycle, when sleep is lighter, usually feels easier than being woken from deep sleep — which can leave you groggy for up to an hour (“sleep inertia”). This free sleep calculator plans your night around full cycles.
How the sleep calculator works
- Bedtime = wake-up time − 15 minutes (to fall asleep) − number of cycles × 90 minutes
- Wake-up time = bedtime + 15 minutes + number of cycles × 90 minutes
Example: to wake at 7:00 AM after 5 full cycles (7.5 hours), go to bed at 11:15 PM; for 6 cycles (9 hours), at 9:45 PM.
How much sleep do you need?
| Age | Recommended per night |
|---|---|
| Toddlers 1–2 years | 11 – 14 hours (including naps) |
| Pre-school 3–5 years | 10 – 13 hours (including naps) |
| Children 6–12 years | 9 – 12 hours |
| Teenagers 13–18 years | 8 – 10 hours |
| Adults 18–64 years | 7 – 9 hours |
| Older adults 65+ | 7 – 8 hours |
Adult sleep: too little, healthy and long
| Average sleep | Level | What it means |
|---|---|---|
| Under 7 hours | Too little | Linked to weight gain, insulin resistance, high blood pressure, low mood and poor concentration |
| 7 – 9 hours | Healthy | Best for health, appetite control and performance |
| Regularly over 9–10 hours | Long | May point to poor sleep quality, depression or another health issue |
Why good sleep matters for your weight and health
- Appetite hormones: short sleep raises ghrelin (hunger) and lowers leptin (fullness), so you crave sugary, high-calorie food.
- Blood sugar: even a few nights of poor sleep reduce insulin sensitivity.
- Weight loss: dieters who sleep well lose more fat and less muscle.
- Heart, mood and immunity all benefit from regular, sufficient sleep.
Health warnings
- Cycle length varies between people and nights — treat suggested times as a guide and adjust by 15–30 minutes if needed.
- See a doctor for loud snoring with pauses in breathing or gasping (possible sleep apnoea, common with excess weight), insomnia lasting more than 3 months, or severe daytime sleepiness.
- Do not drive or operate machinery when very sleepy.
- Avoid relying on sleeping pills without medical advice.
Food and drink tips for better sleep
Stop caffeine at least 6–8 hours before bed (check your daily total with the caffeine calculator), avoid large, spicy or very fatty meals within 2–3 hours of bedtime, and keep evening fluids moderate. A light snack such as warm milk, yoghurt, a banana or a few nuts can help if you are hungry. During Ramadan, plan sleep around suhoor with our fasting planner. Read more about nutrition and stress.
Frequently asked questions
What time should I go to bed to wake up at 6 AM?
For 5 sleep cycles (7.5 hours) go to bed at about 10:15 PM; for 6 cycles (9 hours) at about 8:45 PM. Both include 15 minutes to fall asleep.
How long is one sleep cycle?
About 90 minutes on average, though it can range from 70 to 120 minutes and changes through the night.
Is 6 hours of sleep enough?
For most adults, no. Experts recommend 7–9 hours. Regularly sleeping 6 hours or less is linked with weight gain, poorer blood sugar control and lower concentration.
Why do I wake up tired after 8 hours?
You may be waking during deep sleep, have poor sleep quality (for example from caffeine, late meals or sleep apnoea), or keep an irregular schedule. Try timing your sleep in full cycles and keeping a regular wake-up time.
Does sleep affect weight loss?
Yes. Short sleep increases hunger hormones and cravings and makes the body hold on to fat. Getting 7–9 hours supports weight loss.
Sources
- Hirshkowitz M, et al. National Sleep Foundation’s sleep time duration recommendations. Sleep Health 2015;1:40–43.
- Watson NF, et al. Recommended amount of sleep for a healthy adult (AASM/SRS consensus). Sleep 2015;38:843–844.
- Spiegel K, et al. Sleep curtailment in healthy young men is associated with decreased leptin, elevated ghrelin and increased hunger. Ann Intern Med 2004;141:846–850.