The Stages Of Snoring
by Sharon J Borrow
Sleep Laboratory Scientist
St George's Hospital London
To understand why not all snores are the same and why some people snore more during certain parts of the night it helps to know something about the structure of sleep. Sleep consists of two distinct states that are as different from each other as each is from wakefulness. They are most often referred to as REM sleep and non-REM sleep. The terms derive from the distinguishing characteristic of rapid eye movement (REM) in the one state and its absence in the other. This characteristic, for which there is yet no proven explanation, is just one of the many physiological differences between the two states.
The State of Non-REM Sleep
This is subdivided into four stages of progressively diminishing responsiveness to environmental stimuli. The four stages form the lower part of the continuum which ranges from high alertness to deep sleep. Stage 1 is a nebulous stage that bridges the gap between definitive wakefulness and definitive sleep. Stage 2 is light sleep from which we can waken readily to environmental stimuli. Stage 3 marks the transitional phase between the light sleep of stage 2 and the deep sleep of stage 4. Stage 4 is the deepest sleep of all. We are much harder to wake from this stage and if an awakening occurs it is often accompanied by a degree of disorientation which can last a few seconds to a few minutes.
The stages are determined by changing patterns of electrical activity in the brain. If we are awake and concentrating on a problem, very fast and shallow 'waves' may be recorded via electrodes attached to the surface of the scalp. As we relax but remain awake a slightly slower, higher amplitude rhythm appears. This in turn gives way to the slower rhythm of stage 1. The trend continues and during stage 4 the brain emits very large and slow waves. There are no sharp divisions between these stages: rather they form a progression upon which science, for convenience, has imposed categories.
The State of REM Sleep
Here the situation is somewhat different. The second state of sleep can appear quite suddenly, at intervals, through the night. Brain activity looks remarkably similar to wakefulness. The 'waves' are shallow and are of mixed, but nevertheless, high frequencies. We waken in response to environmental cues less readily than from Stage 2, but more easily than from Stages 3 and 4. In the normal course of events we are well orientated on waking from this state although we may be aware of having dreamed. It is the increased tendency to report dreams on awakening from REM sleep that has earned it the secondary title of 'dreaming sleep'.
There are other stark physiological differences between REM sleep and non-REM sleep. Differences in muscle tone are particularly relevant to those with respiratory problems related to sleep. As we descend into non-REM sleep our skeletal muscles start to relax. The extent of relaxation is very variable between individuals. Some maintain a comparatively high level of tone even in deep sleep. Others quickly have very little. We all have some reduction compared with our waking level but there is no incompatibility between, for example, Stage 4 sleep and some muscle tone. By contrast a cardinal characteristic of REM sleep is an absence of tone so that our skeletal muscles are effectively paralysed.
Concomitant with the general reduction in muscle tone are subtle changes in the respiratory system whereby the airway puts up a degree of resistance to the incoming air. This resistance on its own would not compromise respiration significantly. However, in individuals where respiration may already be compromised for one or more of a host of potential reasons resistance may increase to a level that precipitates a respiratory problem.
Increasingly, 'gentle' or 'conventional' snoring is regarded as the lower end of a scale of sleep related respiratory difficulty that culminates in full obstructive sleep apnoea. Snoring may therefore be a warning sign of the propensity for a significant respiratory problem to evolve. The relationship between snoring, sleep and breathing is not straightforward, because of the differences between sleep states, stages and the qualitative differences between snores. Conventional snoring arises within sleep from a vibration of soft tissue in the pharynx as inspired air flows down a partially occluded airway.
There is no official terminology to distinguish between different sorts of snores
There is no official terminology to distinguish between different sorts of snores. Yet there are differences that are circumstantial, qualitative and - with respect to noise level - quantitative. There are, for example, those 'conventional' snores which might be emitted by almost anyone at some point, that are in effect rhythmic but somewhat noisy breaths that happen whilst the person is asleep. Such snores would not yet be regarded as sinister. At the other end of the spectrum are those extremely loud snorts that follow the ominous silence of a sleep apnoea. These snorts, whose decibel level can amount to a profound social handicap, occur by definition during wakefulness even if the individual is unaware of the wakening. They represent the reopening of the airway after its collapse and recovery breath of the post apnoeic period. These snorts are an overt sign of a serious medical condition.
- Snoring associated with sleep apnoea is least likely to occur in stages 3 & 4 (deep sleep)
- Conventional snoring is most likely to occur in stages 3 & 4 (deep sleep) & also likely to occur in stages 1 & 2
- Conventional snoring is unlikely in REM sleep
- Snoring associated with sleep apnoea is most likely in REM sleep
- Sleeping pills may aggravate snoring associated with sleep apnoea by depressing the respiratory system and increasing light sleep at the expense of REM sleep.
'Conventional' snores, unassociated with the cessation of breathing, are most likely to occur during stages 3 and 4 sleep. Stages 1 and 2 are next in vulnerability. Snoring of this nature tends not to occur in REM sleep when breathing as a rule is at its shallowest. The pattern is not the same when matters progress to a situation of obstructive sleep apnoea. Rather it may be reversed. REM sleep with its loss of skeletal muscle tone is the time when those with the most seriously compromised respiratory systems are most likely to experience apnoeas. These apnoeas will splinter REM sleep with awakenings that permit the noisy recovery breaths and so start to erode the normal configuration of sleep staging across the night.
Where REM alone is affected it can lead the sufferer to be chronically but specifically REM sleep deprived with a consequent propensity to fall into this particular state of sleep during the day. To fall from wakefulness directly to REM sleep is only normal in young babies. In others it signifies a problem. Obstructive sleep apnoea specific to REM sleep is one such problem; another is narcolepsy, a disorder of REM sleep itself. It is possible, though rare, to have both problems and care must always be taken during investigation to ensure that obstructive sleep apnoea that affects REM sleep does not mask symptoms of narcolepsy.
Sleep apnoea is least likely to occur in stages 3 and 4 sleep. Basic bodily functions are somewhat protected during deep sleep. Perhaps because it is easier to snore conventionally during these particular stages, the actual cessation of breathing becomes unlikely. Only when obstructive sleep apnoea becomes very severe and uniform throughout the night will deep sleep be affected. What then tends to happen is light and REM sleep become so fragmented by apnoeas that the sufferer is not able to reach the point of descent into deep sleep, and it ceases to feature.
Ordinarily and very broadly we descend rapidly through light sleep into deep sleep where we stay for some time in the early part of the night. Approximately ninety minutes after first falling asleep and between two substantial spells of deep sleep we have our first, usually brief, REM sleep episode of the night. As the night proceeds deep sleep peters out altogether and REM sleep appears at regular intervals but for longer periods towards morning, interspersed with light sleep.
Early in any history of snoring, an individual's pattern of snoring will be in part determined by the configuration of sleep staging across the night. If respiratory weaknesses that underlie snoring develop, the individual may then cross a rubicon where the sleep pattern influences snoring less than it is influenced by the respiratory distress that gives rise to the snoring. Specifically normal sleep structure is undermined.
Given the dynamics of the relationship between respiration and sleep it follows that the impact of outside factors on snoring will have a direct effect on respiration, on sleep staging and on the complex interplay between the two systems. Alcohol, for example suppresses the central respiratory drive and muscle tone, and causes changes in blood flow that are not conducive to easier respiration. It is also known to reduce REM sleep and to encourage wakefulness in the latter part of the night. It will certainly increase respiratory impairment but paradoxically REM sleep related apneoa may be seen to diminish. Sleeping pills, conversely, are known to aggravate snoring associated with sleep apnoea. These, like alcohol, have their own depressant effect on the respiratory system. Unlike alcohol they reduce deep sleep and enhance light sleep. Deep sleep seems more resistant to apnoea and its related snores. The overall effect therefore may be to increase the serious snoring as opposed to conventional snoring.
Snoring tends to be seen as a sleep problem in itself. In reality it stems from a respiratory problem that is simply highlighted by sleep. Satisfactory solutions to snoring therefore need to be found in the sphere of respiration and not sleep itself.