What does deep sleep do that other states of sleep do not? That is a revealing question.
During the deepest stages of sleep our brain and nervous system going into a state of massive recuperation. And, perhaps just as importantly many vital hormones are released. Human growth hormone, often purported to be the fountain of youth is secreted in deep sleep. So what happens if we don't reach deep sleep, or stage 4 sleep as it's often called? Well, quite simply these hormones are not fully secreted. And, we age. Have you ever looked at someone who was 35 and they looked 50? Have you ever looked at someone that was 50 and they looked 35? If we take the time to stop and look around we will see that chronological age and the body's appearance often don't "synch up". Could deep sleep be a contributing factor? Almost undoubtedly so.
Now, let's discuss some ways to increase our chances achieving deep sleep on a daily basis and becoming that person, that at 70 or 50 or 89 just doesn't look or act their age. This is possible for all of us if we apply ourselves.
One of the first methods to use is exercise. Daily physical exercise triggers deeper sleep states at night, this has been proven many times in studies. So walk, run, do strength training, swim, whatever, but do something frequently.
Eating much of your daily calories during the morning and daytime hours will help you sleep deeper. Eating a large dinner or eating late at night is not recommended. Think of it this way, the more digestion the body has to do at night the less the body and brain will be able to rest.
Meditation and things like yoga will put us on a much easier path to achieving deep sleep, both of these activities burn out stress hormones (like cortisol) and excess mental chatter, allowing for deeper sleep.
And lastly, the occasional short fast (going for a few days without solid food) massively increases deep sleep and growth hormone production. From a purely evolutionary and genetic stand point our bodies were designed to go through short periods of fasting, it has been suggested that the fact that food is always (perhaps unnaturally) available these days throws our bodies "off". Don't overlook deep sleep, it's critical.