There are five stages of sleep during the sleep cycle. Scientists categorized the stages of sleep based on the characteristics of the brain and body during sleep. Stage 1,2,3, and 4, are categorized as ‘non-REM sleep’, and the fifth stage, is REM sleep. Generally, brainwave frequencies and amplitudes from an electroencephelogram (EEG) are used to differentiate the different stages of sleep, along with other biologic rhythms including eye movements (EOG) and muscle movements (EMG).
Non-REM Sleep of the Sleep Cycle
Stage 1 Sleep
Stage 1 of the sleep cycle is the lightest stage of sleep. The EEG brain frequency is slightly slower than during wake time. There is muscle tone present in the skeletal muscles. Breathing occurs at a regular rate.
Stage 2 Sleep
Stage 2 usually follow Stage 1 and represents deeper sleep. During Stage 2 sleep, the sleeper is less able to be awakened. Stage 2 sleep is characterized by ‘saw tooth waves’ and sleep spindles.
Stage 3 and 4 Sleep – Deep Sleep
Stage 3 and Stage 4 sleep of the sleep cycle are progressively deeper stages of sleep. These stages of sleep are also called ‘Slow Wave Sleep’ (SWS), or delta sleep. During SWS, the EEG shows a much slower frequency with high amplitude signals (delta waves). A sleeper in SWS is often difficult to awaken. Some studies have demonstrated that very loud noises, sometimes over 100 decibels, will not awaken some during SWS. As humans get older they spend less time in slow wave deep sleep and more time in Stage 2 sleep.
Slow-wave sleep is generally referred to as deep sleep, and is comprised of the deepest stage of NREM. In stage three we see the greatest arousal thresholds, such as difficulty in awakening, and so on. After being awoken, the person will generally feel quite groggy, and cognitive tests that have been administered after being awoken from the third stage show that for up to half an hour or so, and when compared to awakenings from the other stages, mental performance is moderately impaired. This is a phenomenon known as sleep inertia. When sleep deprivation has occurred there’s generally a sharp rebound of slow-wave sleep, which suggests that there’s a need for slow-wave sleep. It now appears that slow-wave sleep is a highly active state, and not a brain quiescence as previously believed. In fact, brain imaging data shows that regional brain activity during non-REM sleep is influenced by the most recent waking experience
Stage 5 Sleep (REM Sleep) of the Sleep Cycle
Stage 5 of the sleep cycle, or REM sleep, is the stage of sleep associated with dreaming. It is very different physiologically from the other stages of sleep. The EEG resembles wake time. However, the skeletal muscles are atonic, or without movement. The breathing is more erratic and irregular. The heart rate often increases. It is theorized that muscle atonia evolved in order to protect the individual from injury during sleep.
K-Complexes and Sleep Spindles
Spindle activity is exclusive to NREM sleep, with most occurring at the start and end of NREM. Sleep spindles engage brain activation in the superior temporal gyri, anterior cingulate, insular cortices and the thalamus. Sleep spindles have different lengths; with slow spindles associated with an increase in activity in the area known as the superior frontal gyrus ranging between 11 and 13 Hz, and fast spindles associated with the recruitment of both the hippocampus and the mesial frontal cortex and the sensorimotor processing cortical regions ranging between 13 and 15 Hz. At this point in time it’s not clear what is meant by these sleep spindles, but it’s hoped that ongoing research will reveal their function.
K-Complexes are also exclusive to NREM sleep, and can be defined as single long delta waves lasting for only a second. Like sleep spindles, they appear automatically during the early stages of sleep, generally in stage two. However, K-Complexes can be induced at will by momentary noises, such as someone knocking on a door. Further research must be conducted on K-Complexes because their function is currently unknown.
Dreaming during NREM
During REM sleep, study participants reported both intense dream vividness and improved memory of dreams which occurred during that phase, which suggests that dreaming typically occurs REM sleep: we know that dreaming also occurs during NREM sleep; however, in comparison, these dreams appear to be more mundane. We also know that dreams that occur during the NREM stage of sleep typically occur after midnight, which happens to be the time-period with the highest rate of REM sleep. This was discovered by a study whereby subjects took naps over specific periods of time and were then forcefully woken: sleep was divided into –
- naps with only REM sleep, and
- naps with only NREM sleep using polysomnography.
The implication here is that the occurrence of REM sleep is not necessary for dreaming, but rather that the actual processes creating REM sleep result in alterations to a person’s sleep experience.
By morning, and because of these changes, there occurs a sub-cortical activation that’s comparable to the one that occurs during REM sleep. Therefore, during the morning hours in the NREM stage it’s the sub-cortical activation that causes dreaming.
Non-REM Muscle Movements
The tonic drive to the majority of the upper airway’s respiratory muscles is repressed during non-REM sleep, with the following consequences –
- Due to the intracellular calcium levels being lowered, weakened muscle contractions are caused by rhythmic innervation. Motoneurons are hyperpolarized by tonic innervation being removed.
- There’s a ‘floppiness’ in the upper airway.
The diaphragm is typically driven by the autonomous system and is therefore normally spared non-REM inhibition, and as such, the suction pressure generated stays the same. The upper airway is thus narrowed during sleep, which increases resistance and makes upper airway airflow noisy and turbulent. As we know, we determine that a person is asleep by listening to their breathing: their breathing becomes distinctly louder once they fall asleep. So, it’s not surprising that the upper airway’s increased tendency to collapse while breathing during sleep leads to snoring, which is simply the tissues vibrating in the upper airway. For overweight people, this problem is aggravated when they sleep on their back because excess fat tissue can easily push down and close the airway. This can ultimately lead to sleep apnea.
Each sleep stage serves a unique role in brain and body restoration. There are many sleep deprivation studies that have demonstrated that depriving subjects of specific sleep stages of the sleep cycle has an adverse effect on body and brain functions.