Deep sleep is the most restorative stage in the sleep cycle. In this stage, our brainwaves become dramatically longer, and our daily experiences are turned into long-term memory. Deep sleep is vital to feeling rejuvenated and learning effectively; without deep sleep, it is difficult to effectively learn and process information. Deep sleep is vital, but it can be difficult to achieve.
As we sleep, we spend less and less time in Deep sleep. In the beginning of the night, it is common to spend up to 50% of the first sleep cycle in Deep sleep. By the end of the night, we only spend around 2% of the sleep cycle in Deep sleep. As we age, our bodies tend to spend less time in Deep sleep per sleep cycle. This is why sleep can feel less rejuvenating the older we get, even if we sleep for the same amount of time.
Why do we need deep sleep?
Every organism, in one way or another, sleeps. We do this for a few reasons: to save energy, to restore the body, and to adapt to what we learn during the day. During the day, the brain makes all of these connections based on our environment, but its only important to remember the most relevant things to our survival. In the past, this was information like, don’t go to that area of the forest because predators are there. But nowadays, the information we are processing, while not as pertinent to our physical survival, impacts things like social status and our financials. Basically sleep, and particularly deep sleep, functions to clear out all of the irrelevant connections that we make about our environment during the day, such that the salient things rise to the top. During dreaming, we process these salient things and encode the information into our long-term memory and personality.
In sleep science literature this theory is referred to as the “synaptic homeostasis hypothesis”, coined by the famous sleep research Giulio Tononi from the University of Wisconsin. Deep sleep is thought to be particularly important to this process of cleaning out irrelevant neural connection, where deep sleep is related to human growth hormone, cell recovery, and our ability to remember relevant information. As we get older we get less deep sleep, and researchers are beginning to make connections between quality sleep and various chronic health issues such as Alzheimer’s, hypertension, diabetes, and cardiovascular disease.