Types of Sleep

There are basically two types of sleep:
- REM (Rapid Eye Movement) sleep
- NREM (Non-Rapid Eye Movement) sleep, which is further divided into four stages.


NREM
Almost three-fourth of a person's sleep is of NREM type. When you are entering sleep you first experience NREM sleep state which then moves from one stage to another where each consecutive stage indicates a higher level of depth in sleep and lower levels of brain activity.

NREM Stage 1
The first stage of NREM starts from the moment you fall asleep but this sleep is so slight that you can be awakened as easily as if not asleep at all. The movements of the body muscles is slowed down resulting in slow breathing and reduced eye movement. This stage is sometimes also referred to as hypnagogia. Most scientific research is done on this initial state of sleep. This is the state in which you're not deep in sleep but neither are you fully awake. You usually experience the effects of recent and present memories in this state, like falling off from a cliff or floating in water after spending a day at sea. You might also wake up with a jerk while dreaming that you were about to fall from stairs or were being attacked by a dog.

The diagram shows the four types of brain waves differing in frequency and
amplitude. During NREM our brain emanates Delta waves which decrease
 in frequency with each consecutive stage of NREM.

NREM Stage 2
After entering the second stage of NREM your eye movements almost vanish, you can still be awakened easily but you will know that you had gone in slumber. As this is a further deep state of sleep compared to stage one, therefore the body motion and functions are further slowed down; the brain waves are further lowered in frequency and increased in amplitude. Yet, there maybe sudden onset of higher frequency waves which may result in waking up abruptly.

NREM Stage 3
Once you enter this stage it may be said that you are deep in sleep and waking you might be hard. The delta waves emanating from brain at this stage reach a further lower frequency and higher amplitude - resulting in a sound sleep. However, there can still be occasional rapid waves.

NREM Stage 4
This is the stage of deep sleep where your brain waves are the slowest and longest. After waking from this sleep you will feel like you actually slept; you will feel relaxed and refreshed. Studies have shown that dreams can also occur during this stage of NREM sleep, but such dreams are hard to remember.


REM
After completing a full cycle of NREM sleep, you enter REM sleep which is the state of sleep in which most dreaming occurs. The first instance of REM sleep occurs around 90 minutes after you fall asleep which then repeats its cycle with stages of NREM sleep in between.

As the name suggests, during REM our eyes move rapidly and a person observing the eye-lids can easily notice that. The breathing during REM is comparatively faster and irregular, while the blood pressure and pulse rates also rise. During REM, the brain shuts off neurons in the spinal cord, causing temporary paralysis of the limb muscles. If something interferes with this paralysis, people will begin to physically "act out" their dreams – a rare, dangerous problem called REM sleep behavior disorder. The paralysis lasts as long as REM lasts, once you enter NREM again, you regain muscle movement.

REM sleep stimulates the brain regions used in learning. Like deep sleep, REM sleep is associated with increased production of proteins. One study found that REM sleep affects learning of certain mental skills. People taught a skill and then deprived of non-REM sleep could recall what they had learned after sleeping, while people deprived of REM sleep could not.

Studies show that newborns and infants' sleep comprise largely of REM, while we spend around 20 to 25% of our sleep in REM, infants spend around 50 to 55% of their sleep in REM, it is thought that this is necessary for normal brain development during infancy. Most mammals and birds show signs of REM sleep as well, while reptiles and other cold-blooded animals do not. Another thing worthy of note is that if REM sleep is disrupted during one night, REM sleep duration for the next few nights is lengthened in order to make up for the loss. So this actually signifies the value of REM sleep, even though if we look at it logically, the sleep which we require more is the NREM stage 3 and 4 so that we can have a more nourishing and refreshing sleep, but nature has given more importance to REM sleep - the reasons are still unknown, but it can't be denied that the significance of REM sleep has obviously much to do with the significance of dreams - since REM is associated with dreams much more than any other stage of sleep.

During NREM sleep, the brain emanates delta waves which are lowest in frequency and highest in altitude which means they are slowest of all waves and thus associate with rest and sleep, but contrarily, during REM, the activity of the brain's neurons is quite similar to that during waking hours; for this reason, the REM-sleep stage may be called paradoxical sleep. This means there are no dominating brain waves during REM sleep but rather it is a mix of alpha, beta, theta and delta.

The diagram above shows the interval between one REM cycle and another, the areas shown in yellow indicate duration and occurrence of cycles of REM sleep each night. Accordingly, we spend more than 2 hours each night dreaming.