Sleep has baffled humans since time immemorial. From ancient traditions and beliefs that we still have and believe in, to early philosophers, scientists and modern high-tech EEG equipment, the study of sleep was at the heart of human culture. Most of our studies concentrated mainly on the spiritual aspect of sleep, but starting with the middle of the 19th century, technology and medical advances allowed us to have a more scientific approach to sleep research.

Thanks to modern-day technology we know much more about sleep than our ancestors. Sleep is  divided in what scientists call stages. First discovered in the 1930s by a team led by Alfred Loomis, these stages reflect how we sleep and the nature of our sleep. The five stages, initially labeled A to E, represent the five separate types of sleep on the spectrum, from complete wakefulness to deep sleep. Further discoveries were made by William Dement and Nathaniel Kleitman in the 1950s, when they discovered REM sleep as a distinct stage. Additional studies were done on the electrical, chemical and biological aspects of sleep, sleep patterns and what are the effects of sleep deprivation on people.

While most of the research was done on REM sleep, mainly because that's the time when dreaming occurs, scientists also studied deep sleep as a separate stage. Deep sleep is known as delta sleep, slow wave sleep or N3, according to the newest naming system. During deep sleep our brains emit a lot of high-amplitude,
low-frequency delta waves, which suggest that the brain activity is at its lowest point. Traditionally, deep sleep was further divided into two distinct sub-stages – stage 3 and 4. Stage 4 has more delta wave activity, suggesting that it is a state of even deeper sleep. However, there are no relevant benefits to stage 4 vs. stage 3 deep sleep.

What happens during deep sleep?

Deep sleep is a stage of sleep characterized by a complete detachment from the outer world. Arguably, it's very difficult to wake up a person during deep sleep; for instance, some parents complain that they are simply unable to wake up their sleeping infants. This is because during deep sleep both the brain activity and the motor (muscle) activity have the lowest levels. An interesting phenomenon which occurs only during deep sleep is sleepwalking, which is an electro-chemical error in the brain's motor center.

Other important physiological processes occur during deep sleep. For instance, several hormones are released in massive amounts during the deep sleep stage. Human growth hormone (HGH) is one of the hormones which is released in pulses during this period. An interruption of deep sleep ceases the release of HGH, a well known fact among athletes and bodybuilders. Similarly, the thyroid hormone, critical during a child's growth, are released mainly during the deep sleep stage.

Other medical benefits of deep sleep

Suppression of cortisol production – going to sleep after eating a heavy dinner increases the cortisol in the morning which is linked to a higher risk of coronary artery disease. During deep sleep, cortisol production is suppressed, so the risk of coronary artery disease is kept under control. However, if no deep sleep occurs, the cortisol continues to be produced. High cortisol levels in your body has another detrimental effect: it damages the body's ability to process glucose during the day.

Release of prolactin – deep sleep is extremely important if you have any injuries, mainly because prolactin is released in copious amounts, which has an important role in joint recovery. Prolactin also acts as a powerful anti-inflammatory and painkiller. Prolactin is the reason why pain seems easier to cope with during sleep.

Suppresion of the sympathetic nervous system – our stress levels are controlled by the sympathetic nervous system, so keeping it on low activity helps us fight stress. During deep sleep, the sympathetic nervous system is replaced with the parasymphatetic nervous system which is the chief controller of the recovery and recuperation mechanisms in our body. So, in other words, stress is replaced with recovery and recuperation.

The psychological benefits of deep sleep

Deep sleep is the most important sleep stage when it comes to refreshing our bodies. It is the time frame when our body regenerates the fastest and it is the main factor that makes us feel energetic after a night's sleep. As a result, waking up during deep sleep is particularly uncomfortable, a phenomenon called “sleep drunkenness”, a time when it is unsafe to drive or handle machinery. Other studies suggest that during deep sleep our brains get prepared for a new “learning day”, meaning our brain memory is partially deleted to allow the accumulation of new information for the following day.

Getting enough sleep

Deep sleep is quite elusive, as not all “sleep sessions” enter this stage. Typically, deep sleep occurs an hour after we begin sleeping and lasts for 2 to 4 hours, depending on the individual. This is why deep sleep only occurs during a full night's sleep. Also, this is why getting one hour naps doesn't make us feel rejuventated.

While most people are fine with 2 hours of deep sleep per day, athletes need more. There is a correlation between high physical activity and deep sleep needs. For instance, bodybuilders and powerlifters need more than 5 hours of deep sleep per day, which correlates to 11 hours of sleep per day. For most people, however, a full night's sleep of 7 to 9 hours will get enough deep sleep to feel fresh.

Let's take a look at how much sleep we need, depending on age:

−    newborns (0 to 3 months) - 14 to 17 hours of sleep per day
−    infants (4 to 11 months) – 12 to 15 hours of sleep
−    toddlers (1 to 2 years) – 11 to 14 hours of sleep 
−    preschoolers (3 to 4 years) – 10 to 13 hours of sleep 
−    school-age children (5 to 12 years) – 9 to 11 hours of sleep
−    teenagers (13 to 17 years) – 8 to 10 hours of sleep
−    adults (18 to 64 years) – 7 to 9 hours of sleep
−    seniors (over 65) – 7 to 8 hours of sleep

(data source:

The bottom line

We may not be aware of the state of deep sleep, but it has a critical role: it helps us recover and recuperate both physically and mentally. Similarly, it can be hard to tell what might be affecting the quality of our sleep. Unfortunately, getting enough deep sleep is increasingly becoming difficult, as many of us choose to overlook the importance of sleep. What's more, there are a lot of factors which interfere with the quality of our sleep: stress levels, aging, drugs, and sleep apnea -- that’s not even counting the all-important details of your sleep hygiene and environment. Your habits, bedding, mattress, and even your bedroom, itself, all play an important role in the quality and quantity of the sleep you get.

If you feel like you’re not getting the kind of sleep you need, consider what might be causing it. If you’re not giving yourself the amount of time you need, or if your sleep comes at irregular hours, put yourself to a more reasonable schedule, and do your best to stick to it. Also make sure that your habits aren’t keeping you awake. If you frequently watch television or keep yourself active until the moment you try to go to bed, you might be shooting yourself in the foot. Instead, try listening to relaxing music or engaging in a more relaxed hobby before it’s time to hit the mattress. If the quality of a bed or mattress is interfering with the ability to sleep well, then look in to lull mattresses or another brand that can best meet your needs. You will also want to consider curtains or a white noise machine if you find outside distractions are affecting your ability to reach deep sleep.