Sleeping is a state of mind and body where all voluntary muscles stop working and mind goes from consciousness to subconscious and then to unconscious state. It might be considered same as coma if main differentiation is not pointed out because in coma one cannot reverse states of mind and body whereas in sleep it is possible to regain all senses and all voluntary muscles start working again. All mammals, reptiles, birds, and fishes must have it. Nonetheless it is a huge research subject and there many layers of it. In 1937, Alfred Lee Loomis first defined sleep stages but that was for Rapid eye Movement which is type of sleeping for mammals and birds only. Another type of it is Non-Rapid eye movement. We will see stages of sleep according to types of sleep.
Rapid Eye Movement (REM) sleep
REM is a type of sleep but it is itself a stage of sleep. We all like memorable dreaming which mostly occurs in this stage of sleep. REM is only forth part of total sleep and includes rapid eye movement & EEG (Electroencephalography). When experiments were carried out in special sleep laboratory, they resulted that in mammals, muscular Atonia is seen in descending order. REM is last stage after four stages occurring in NREM.
NREM- Non-Rapid Eye Movement sleep
Four different stages of sleep in NREM were described by Nathaniel Kleitman and William Dement in 1953 which were later altered by AASM commission in 2004. But standard sleep stages in NREM were given in R&K scoring manual in 1968 by Anthony Kales and Allan Rechtschaffen. According to this manual there are four stages in NREM type of it. 3rd and 4th sleep stages are considered as slow rate sleep and out of all wave pattern, delta waves are less than 50% in 3rd stage and more than 50% in 4th stage.
Described below are four of those levels:
N1 stage: N1 is initial stage of sleep in which subject losses consciousness about out world and muscle tone. These things occur because during this stage alpha waves of 8 to 13 Hz are converted into Theta waves of 4 to 7 Hz.
N2 Stage: Sleep spindles with 11 to 16 Hz make most of this stage and human adult loss consciousness totally during this stage which comprises almost 50% of it.
N3 Stage: Delta waves with 0.5 to 2Hz frequency are main part of this type of sleep. Night terrors and sleepwalking mostly occur during this stage.
One might wonder that there are only four stages are
described here but these four stages occur in N1-N2-N3-N2-REM pattern
which complete five stages of sleep with all of them taking around 90
minutes.