What actually is deep sleep? It is a sleep phase characterised by very slow, the slowest, in fact, electrical brain waves that occur throughout your entire sleep period. The body's vital functions substantially slow down their activity, including both your heart rate and breathing rate. There is also a notable decrease in body temperature.On the contrary to REM sleep, this deep sleep phase also involves your muscular activities and eye movements becoming practically non-existent.When does deep sleep occur and for how long?Deep sleep is the term used to describe the third and fourth stages of light sleep which occur before the REM sleep phase begins. These two phases are respectfully known as ‘deep sleep’ and ‘slow-wave sleep’.In total, these phases account for 40% of overall sleep and occur more easily and readily at the beginning of the night than the end. We also experience a reduction in these phases as the body ages in favour of slow-wave sleep which takes place during stage 2 of the sleep cycle.What role does deep sleep play?Just like REM sleep, slow-wave sleep allows the body to more efficiently store information as memory. However, it also has other exclusive and unique functions.First of all, it allows your immune systems to strengthen as your resting cells produce more antibodies.Deep sleep also plays an essential role in child development since it allows the body to secrete growth hormones which help children to grow in the most balanced way possible.Deep sleep is crucialObviously, other hormones are also secreted during this phase, which is why it is so crucial for the human body and cell development from a very young age.It’s also worth mentioning that, generally speaking, it is more difficult for the body to wake up during these two phases. We are completely cut off from our exterior environment and we, therefore, don’t react to external stimuli as long as they aren’t too disruptive or strong.