For those of us who have never raised children, it can come as a shock to learn that newborns sleep up to 18 hours per day. (Unfortunately, and to the chagrin of the parents, not in one uninterrupted block.) But if you think about it, it makes sense. Most newborns triple their birth weight in their first year out of the womb, and it appears that the more infants sleep, the more likely they are to experience growth spurts. One study even quantified growth, nap by nap: newborns are 43 percent more likely to experience a growth spurt for each additional nap they took, and 20 percent more likely to grow for each extra hour of sleep they got. This is in large part because of the fact that during deep sleep, children and adults alike secrete up to 75 percent of their growth hormone, which helps to generate new tissue.

So is the amount we need to sleep dependent entirely on how much we need to grow? It’s possible. As infants become toddlers and toddlers become kindergarteners, their growth rate (in terms of increase in their body weight by percentage) declines, as do their sleep needs. Children between 1 and 3 require about 12 to 14 hours per day, and 3 to 6-year-olds need about 10 to 12 hours per day. By the time a child hits their teenage years and puberty is in full swing, something interesting happens: Their sleep requirements sit at around 9 and a half hours per night, but parents suddenly take their sleep needs less seriously, with less than 20 percent of teenagers getting more than 8 and a half hours’ sleep.

One might imagine that with the extraordinary changes that take place during adolescence, teenagers would need to sleep as much as newborns, but there’s a surprising reason this isn’t the case: At the onset of puberty, the maximum amount of growth hormone that the body can produce vastly increases, as does the amount of time spent in deep sleep. By the time adolescent development is complete, the levels of GH and deep sleep decrease to pre-puberty amounts. This lets teenagers cram in more growth and development without needing to spend too much of their time asleep. By age 18, they have achieved 90 percent of their peak skeletal mass, their bone strength is far greater, and males, with their testosterone skyrocketing all the while, will have nearly doubled their muscle mass since puberty’s onset.

But then adult life begins with a terrible misconception: That we need less sleep as we age.

On the surface, this appears to be true. From the beginning of our 20s onward, year by year, sleep tends to be just a little less satisfying, slow-wave sleep becomes a little harder to attain, and people sleep a little bit less. But this isn’t because our bodies need less sleep – it’s because we get worse at sleeping. We become more neurotic, and racing thoughts keep us awake for longer. We don’t take care of our bodies, and we have more injuries, inflammation, aches, and pains. We produce less growth hormone and less melatonin. We move a lot less and consume more caffeine and alcohol. And all of these habits do a fantastic job at damaging the length and quality of your sleep. Seniors often have the added difficulties of contending with arthritis, frequent urination, and medications that can disturb sleep.

But adults, even senior citizens, all need eight to nine hours of sleep per night, and the fact that this target gets more difficult to hit as we age is not something we should take lying down. To keep sleep cycles healthy throughout your life, remember the following tips:

1)  Get plenty of sunlight. It helps regulate the sleep cycle and keep melatonin levels optimal.

2)  Exercise every day. Working out, especially at high intensities, has been shown to improve the body’s levels of both melatonin and growth hormone, both powerful sleep aids.

3)  Maintain a healthy diet. Consume food that’s high in iron and nutrients that reduce inflammation, as well as calcium, magnesium, and vitamin D, which lower the risk of degenerative bone disease.

4)   Avoid caffeine and alcohol whenever possible. This is especially true toward the end of the day.

5)  Make sure to always take some time at the end of the day to mentally decelerate. This will help you to assuage anxiety and improve sleep.

6)   Stay flexible and foam roll regularly. This will help to reduce your risk of injury and keep you mobile as you age.

Most importantly, always see your doctor at the first sign of pain, injury, or sickness to keep it from developing into something more serious down the track. Life brings a lot of changes, but it’s nice to know that there’s one constant: sleep is great, and great for you.