# Work Out How Much Sleep You Really Need With This Maths Trick

Most people think the ideal amount of sleep is eight hours per night. As it turns out, that number is just a myth. Some people need more sleep to function optimally during the day, while others require less. Thankfully, it’s possible to work out your own magic number with a little experimentation. Here’s how it’s done.

Dr. Michael Breus, sleep specialist and author of The Power of When, says that the average person only needs 7.5 hours of sleep a night.

“I’ve been a 6.5 hour sleeper almost my entire life,” Breus says. “But my wife needs 8.5. Everybody is different.”

### What’s a sleep drive?

The reason for that is that sleep — specifically your sleep drive — is genetically predetermined, according to Breus.

Your sleep drive is a lot like your hunger drive. Just like your hunger drive makes you want to eat when you’re feeling hungry, your sleep drive makes you want to sleep when you’re feeling tired.

Your sleep drive also determines the speed at which you have a sleep cycle.

As a general reference, most people go through five 90-minute sleep cycles per night, Breus says. That’s why the average person needs 7.5 hours of sleep. Five cycles of 90 minutes each works out to be 450 minutes in total, which is the equivalent of 7.5 hours. However, some people’s cycles are longer or shorter.

When figuring out the ideal amount of sleep you need per night, Breus suggests starting with 7.5 hours as an approximation.

### How to figure out your sleep drive:

Take the time that you usually wake up and count back 7.5 hours. So if you normally wake up at 7 a.m., you would need to go to bed at 11:30 p.m. to get 7.5 hours of sleep. Make it a point to go to bed at 11:30 p.m. for seven to 10 days.

Breus says that if by the end of those seven to 10 days, you’re waking up around five minutes before your alarm goes off, 7.5 hours is your ideal amount of sleep.

If, however, your alarm is still waking you after those seven to 10 days, try going to bed half an hour earlier. Keep moving your bedtime up by 30 minutes until you wake up just before your alarm. The number of hours you end up with is how much your body needs.

Unfortunately this method won’t work for everyone. According to Breus, about 50% of the population has what’s called a chronotype. A chronotype refers to everyone’s own biological clock — so when people are early birds or night owls, or just have trouble sleeping in general.

### Night owls and early birds need to fit their lives to their chronotype:

Breus says that while people with a chronotype still need around 7.5 hours of sleep, they need to adjust the window of time in which they get that sleep. These people need to fit their lives to their chronotype, and not the other way around.

Breus uses himself as an example. He’s a night owl and never goes to bed before midnight. Because of this, his whole day shifts later. He’s found that he’s way more productive when he does things later in the day, so he’s adjusted accordingly.

Sleep is an important part of life, so be aware of your needs and respect them.

This story originally appeared on Business Insider.

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