What is a sleep cycle?

A sleep cycle is the progression through different stages of sleep, including light sleep, deep sleep, and REM sleep. Each full cycle lasts approximately 90 minutes. This sleep calculator helps you wake up between cycles to avoid grogginess.

How many hours of sleep do I need? Is 8 hours best?

Most adults need 7 to 9 hours of sleep, which is 5 or 6 full 90-minute sleep cycles. While 8 hours is a common goal, our 'how many hours of sleep calculator' shows that waking up after 7.5 hours (5 cycles) or 9 hours (6 cycles) often leads to feeling more refreshed than waking up in the middle of a cycle at the 8-hour mark.

Sleep Calculator: Calculate When to Sleep & Wake Up

Sleep Calculator

Wondering **when to sleep**? This calculator finds the best time to go to bed or wake up based on 90-minute sleep cycles.

Wake-up Time:

Time it takes me to fall asleep:

minutes

Disclaimer: This tool provides suggestions based on sleep cycle theory and is for informational purposes only. It is not a substitute for professional medical advice. Please consult a doctor for any sleep-related health concerns.

When should I go to bed?

Set a time to see your ideal sleep schedule.

The Science of a Good Night's Sleep

Whether you're using this as a **when to sleep calculator** or a wake-up time planner, the answer lies in understanding your body's natural sleep cycles. This tool isn't based on guesswork; it's designed around science to help you wake up feeling energized and alert.

How Many Hours of Sleep Do You Need? The 8-Hour Myth

This tool acts as a **how many hours of sleep calculator** by showing results in 90-minute increments. While the "8 hours of sleep" rule is a common guideline, it doesn't align perfectly with sleep cycle science. Waking up after 7.5 hours (5 cycles) or 9 hours (6 cycles) often feels much better than being woken up at the 8-hour mark, which could be in the middle of deep sleep. Our **8 hours of sleep calculator** feature is implicitly included, as the optimal times fall around that duration.

The Stages of Sleep: Light, Deep, and REM

A full sleep cycle takes about 90 minutes to complete and consists of several stages, including light sleep, deep sleep, and REM (dream) sleep. You repeat this cycle multiple times each night. The key to waking up refreshed is to wake up at the *end* of a cycle, during a lighter stage of sleep.

What is Sleep Inertia and How to Beat It?

Sleep inertia is that groggy, disoriented feeling you experience immediately after waking up. It's strongest when you're forced awake from the middle of a deep sleep cycle. By using this sleep calc to align your alarm with the end of a 90-minute cycle, you increase the chances of waking from a lighter stage of sleep, which can dramatically reduce sleep inertia.

Frequently Asked Questions

Tips for Better Sleep Hygiene +

Be Consistent: Go to bed and wake up at roughly the same time every day.
Create a Restful Environment: Keep your bedroom dark, quiet, and cool.
Avoid Screens Before Bed: The blue light can interfere with sleep hormones.
Limit Caffeine and Alcohol: Avoid stimulants close to bedtime as they can disrupt sleep.
Get Some Exercise: Regular physical activity can help you fall asleep faster.

Why are 7.5 to 9 hours (not 8 hours) recommended? +

Health organizations worldwide recommend most adults get 7 to 9 hours of sleep. This corresponds to completing 5 or 6 full 90-minute sleep cycles. Our sleep calculator highlights these options because waking at the end of a cycle (like at 7.5 or 9 hours) helps you avoid sleep inertia more effectively than waking at an arbitrary time like exactly 8 hours.