While everyone strives to get enough sleep as possible, the actual number of hours of sleep that people need per night actually changes as you age. For example, you probably needed more shut-eye during your childhood years when growing up than you did as you began to get older and reached adulthood. That’s why babies and children tend to go to bed earlier at night and sleep for more hours during the day than busy, working adults.

There are some variables that come into play, but there’s a general guideline as to the number of hours (estimated) you’d need based on your age group. Here’s how and why the number changes with age, as well as how you can figure out what’s the best number for your own body’s needs.

That way you know what your ideal “magical” number really is and can tweak your lifestyle accordingly to secure those zzz’s, without overdoing it too.

How the number of hours changes

For starters, as you get older, there’s generally a decrease in the amount of sleep needed to function throughout the day. There are nine age groups, and within each group, there is a range in needed hours based on a few factors, like gender, lifestyle and health, for example.

“The younger a person is, the more sleep they need,” says Ryan Fiorenzi, certified sleep coach and founder of StartSleeping.org. According to Fiorenzi, newborns should sleep 14-17 hours per day, infants will need between 12-15 hours, teenagers should aim for 8-10 hours, adults should sleep 7-9 hours, and seniors require the fewest number of hours—just 7-8 hours per night.

Why does it change?

The reason behind the difference in sleep needs with age is still somewhat a mystery. “Many would say that reasons we need less sleep as we age are still up for debate,” says Fiorenzi. “One of the most interesting theories is that as we age there is a loss of neuronal connections in the brain that receive the chemical message our brains send that tell us we are tired.

“The interesting thing is that experiments found that the chemical message is present in the same levels in both young and old,” he says. So, there’s still a need for more research to really discover what’s going on. What we can gather is that the receptors that receive and translate the chemical signal appear to decline with age.

What’s more, sleep becomes harder to attain naturally and more disruptive with age. “Nighttime awakenings become more frequent because older people spend less time in the deep sleep stage,” says Alex Savy, a certified sleep science coach and the founder of SleepingOcean.com.

It can also come from growth hormone secretion during sleep, as shown in research. “As we grow older, the levels of growth hormone drop, and the body starts to produce less and less of it, and that’s when the amount of deep sleep we get decreases too,” says Savy.

How to find your dream number

You can use a variety of apps and calculators to find your dream number. You can also track your sleep using a sleep tracking app. Sleep Cycle is a great option, as it doesn’t require a wearable, it remains silent at night, and it will collect and record data on your sleep cycle and patterns—pretty accurately too!

“Sleep Cycle is both a smart alarm and a sleep tracker in one, and you can calculate how much sleep you need exactly to go through a full sleep cycle before waking up,” says Savy. And that full cycle is how you get your “dream” hour requirement of sleep each night, with a full cycle of snoozing to help you wake up feeling refreshed come morning.

“The app will prevent you from waking up in the middle of a sleep cycle feeling groggy and tired,” says Savy.

If you have trouble falling asleep you can unwind with soothing apps, like Calm and Headspace, which offer meditations, music and other features that may promote drowsiness and relaxation. You can also use a sound machine, or a smart alarm clock like Loftie ($139), to create white noise in order to induce slumber and keep you sound asleep throughout the night.

The best sleep calculators

You can use a classic, the Sleep Foundation’s online Sleep Calculator, which has been designed to take into account not only a person’s age but, also, their desired wake-up time and bedtime. And it’s pretty simple to use, too.

Savy’s calculator, the SleepingOcean calculator, helps you figure out not just the number of hours you need as well as the precise time of day and night you should sleep and wake for optimal sleep.

Fiorenzi’s calculator, StartSleeping sleep calculator, allows you to enter your age for precision and greater insight. The calculator will estimate the best time for you to wake up and go to bed, with tips based on your age as well as your goal wake time and bedtime. “The results are broken down to optimize your REM and non-REM sleep cycles,” says Fiorenzi.

With the tool, you shouldn’t wake up in the middle of a sleep cycle. “Because sleep cycles and recommended sleep times vary by age, we took that into account when approximating the best times for you,” says Fiorenzi. It pulls data from a few areas.

The average person takes 15 minutes to fall asleep, and this factor is included as a data point in order to calculate that magical number.

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