Sleep is a confusing topic of study because despite every human needing it, no one really knows with 100% certainty why we do it. There is no single theory that is accepted by the entire scientific community; however, two main hypotheses seem most fitting. First, the restoration hypothesis proposes that we sleep to rest and recover, a preparation for when we wake. What is being restored remains unclear. The second hypothesis proposes that sleep is an adaptive process that keeps us out of trouble for a large chunk of the day: hiding us from predators and conserving our energy reserves. The only thing we do know about sleep is that we cannot survive without it.
The majority of our sleep (75%) is spent in non-REM (rapid-eye movement) sleep and occurs in four stages. An entire sleep cycle takes approximately 90 minutes and is classified as a form of ultradian rhythm (faster than circadian which is a 24 hour cycle).
Stage 1: The Transitional Stage
Stage 1 is known as the transitional stage as it is when our brain waves become less regular and begin to wane. If you were to look at an EEG we would pretty much look wide awake. You have probably felt the sensation of shifting between the two stages if you have ever fallen asleep in class or in front of the TV. That sense of falling is due to the slowing of our brain waves. Activity levels begin to change from alpha to theta waves; they are high amplitude but very low frequency. Due the small difference, however, stage 1 is the lightest stage of sleep and we are easily woken. Our eyes are making slow, rolling movements and the whole stage only lasts a few minutes. All in all though, it only occurs at the beginning of sleep and only lasts 5-10 minutes.
Stage 2: The First NREM Stage
Stage 2 marks the true beginning of non-REM sleep. During stage 2, our brain waves become slightly deeper and occasional variations in wave movement (oscillations) occur between 8-14 Hz occurs. These oscillations are called sleep spindles and are produced by thalamic pacemaker cells. It is proposed that sleep spindles occur because the brain is trying to inhibit processing to ease the transitional into sleep. Another characteristic stage 2 indicator is the K-complex. K-complexes are a high-amplitude, sharp wave and the largest brain event during sleep. Scientists believe that K-complexes help suppress arousal and aid in memory consolidation. Lastly, another indicator of transition from stage 1 to stage 2 sleep is that eye movements cease.
Stage 3 and 4: The Delta Rhythm Stages
During stage 3, an EEG would show large-amplitude, low rhythm delta waves. Eve movements as well as body movements will be usually be absent. Stage 3 can also be seen as a transitional stage but between light and deep sleep. As we enter into deep sleep our body and brain become increasingly less sensitive to stimuli and less susceptible to arousal. Common childhood sleep issues such as bed wetting, night terrors and sleep walking tend to occur towards the end of stage 3. The main difference between stage 3 and 4 is the amount of delta waves. When less than 50% of deep sleep is delta, we are in stage 3. When more than 50% of our brain activity is delta waves, we are in stage 4 of sleep. As delta waves correspond with very deep sleep, a person in stage 4 of sleep is the hardest to wake. This can be extremely scary if a person is sleep walking or a child is having night terror. They may seem awake but they are completely unresponsive to external stimuli. Stage 4 of sleep only happens during the first cycle as such, our sleep becomes lighter throughout the night. This is extremely helpful as it prepares our body for waking. From an evolutionary stand point this makes sense. Our ancestors certainly could not set an alarm clock, the noise of other animals and the rising sun needed to be sufficient to wake us.
REM sleep is probably the most exciting and important of all the stages of sleep. REM stands for rapid eye movement and is suitable name for this stage as an EOG would show rapid eye movement under our eyelids. During this phase we also experience dreaming. Our brain activity mimics waking, showing a myriad of different brain waves: alpha, beta and dysynchronous waves. Despite our brain activity showing an incredible change in activity, our muscles are actually paralysed. A scary albeit common phenomenon known as sleep paralysis is when we wake-up during REM sleep and our muscles remain effectively paralysed. During sleep however, this “paralysis” is known as sleep atonia. It is a beneficial process as it prevent us from acting out our dreams and putting ourselves in harms way. Certain neurons in our brain stem (specifically the tegmentum) known as REM sleep-on cells release monoamines inhibit motor neuron activity. Another curious attribute of REM sleep is the incredible recall of a person woken from it. Our dream world becomes as real and vivid as the real world. As REM mimics wakefulness, waking someone during REM means they will feel very alert. The exact reason for REM sleep is as elusive as sleep itself; however, scientists do know that like sleep, it is vital. When a person is repeatedly disturbed during REM sleep or does not get enough sleep in general, we go through a process called REM rebound. In other words, we spend the majority of our following sleep in REM. Lastly, scientists have also discovered that newborns and foetuses spend the majority of their sleep in REM. All in all, these findings suggest REM is vital to proper human functioning and development.
As we progress through the night we spend increasingly less time in stages 3 and 4. After the first cycle stage 1 of sleep is replaced by REM sleep and the amount of time spent in REM sleep increases. Even though the reason why we sleep is unclear, the change in brain activity and the determent of not sleeping is enough to say with certainty that sleep is necessary for normal human function.
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