The term “deep sleep” means different things to different people. Many describe deep sleep as a period of uninterrupted, refreshing sleep. Others might say that REM sleep is the deepest type of sleep, because that is when most dreaming occurs. Still others mean sleep from which it is difficult to awaken. It is the final definition that is most similar to that of sleep researchers.
Studies have shown that it is most difficult to awaken humans during slow wave sleep, also known as stage N3 NREM sleep (formerly divided into stages 3 and 4). N3 sleep is typically found in the first third of the night, and it alternates with the other lighter stages of NREM sleep (N1 and N2 sleep) as well as REM (rapid eye movement) sleep. If an individual awakens abruptly from this stage of sleep, they will often be very confused and take a minute or more to become fully aware of their environment (especially true for children).