Fragmented sleep is particularly frustrating. It leaves the sufferer in bed awake but knowing that it isn't time to wake up, and further, that sleep could come again at any time. Sleep fragmentation is a hallmark of sleep-maintenance insomnia, which differs from sleep onset insomnia in that sleepers can generally get to sleep pretty fast after going to bed (low sleep latency) but have a hard time staying asleep.
Fragmented sleep results in a sleep cycle that is typically short on slow-wave sleep and is often unrefreshing. Total time spent in sleep (any stage) is also less than normal, although higher than the people with sleep fragmentation often believe. (People often underestimate how much sleep they actually get.) Experts think that a third of adults have nocturnal awakenings of some sort although the range of fragmentation in this population varies greatly. Note that very brief nocturnal awakenings (less than a minute) are not what we are talking about here. Those awakenings are perfectly normal, happen in other mammals too, and may be a consequence of the way the brain controls sleep cycles.We also don't count microarousals that apnea sufferers experience. These result in unrefreshing sleep, too but they are so short the person does not remember being awake,
Some people experience unfragmented – consolidated sleep – in two periods during the night, with a large gap in between. We do not consider this fragmented sleep, but biphasic sleep, and it is very common. While a bi-phasic sleep pattern is something you can adjust to and live with, truly fragmented sleep with many short awakenings is annoying and unsatisfying.
Sleep fragmentation also makes apnea worse. Other consequences are those of excessive daytime sleepiness: reductions in measures of response time, vigilance, poor mood, etc.
Sleep quality is a slippery term, but attempts to quantify it often revolve around how continuous (unfragmented) the nighttime sleep is. When asked to elaborate on what they mean by “a good night’s sleep”, people often refer to few or no awakenings – unfragmented sleep.
Sleep disorders are a common symptom of psychiatric conditions. The mental health profession uses the word "dissociative" to refer to mixed up consciousness and memory. So-called "mind-altering drugs" produce dissociate symptoms. So does fragmented sleep.(http://pps.sagepub.com/content/7/2/159)It is hypothesized that fragmentation may make the mental illness worse, as it moves the brain between REM and waking and produces or enforces false memories and encourages ruminations on negative thoughts.
Parkinson's Disease patients often have fragmented sleep as do elderly people with dementia.
Narcolepsy also causes fragmented sleep at night. Some may find that surprising as narcolepsy is perceived as a disorder of too much daytime sleepiness. However, disturbed nocturnal sleep is a hallmark of narcolepsy, too.
Infants have highly fragmented sleep, and as they get older the sleep period becomes more consolidated. The failure of some children to consolidate sleep leads, in some part, to sleep disorders of early childhood. Because fragmentation means lower overall sleep duration, this is particularly worrisome in children whose brains are still developing.
Humans tend to have increased sleep fragmentation as they get older. Most people experience this, although why it happens is unclear. It is also a matter of some debate about whether this increased fragmentation is a normal part of aging or indicative of a subtle, underlying pathology. It may be that younger brains, being more plastic (malleable) than old brains, need to get back to sleep faster. It could be that the young and old may have the same number of awakenings during the night, but the young fall back to sleep faster and don't even remember their brief awakenings. In any case, the nighttime fragmentation is frustrating and unpleasant, and perhaps more importantly leads to sleepiness during the day.
The number of nighttime arousals correlates with the level of daytime sleepiness. Researchers have attempted to quantify fragmentation through use of a sleep fragmentation index. This is just the number of nighttime arousals (even microarousals) divided by total sleep time. EEG and polysomnography measures sleep in epochs of typically a minute or 30 seconds, so not all microarousals may be captured. Still, this seems like a good idea, even if it has not yet found wide use in normal study of sleep. A Belgian study found results were reproducible even within the limits of data quality.
Also called the arousal index, this type of index is used more in diagnosis of apnea and studies of restless legs syndrome.
Chronotherapies such as artificial bright light and melatonin supplements (or the prescription drug ramelteon) are sometimes employed. Medications can promote sleep, and choices such as daylight exposure, exercise, abstaining from evening caffeine, and good sleep hygiene can help. Getting these people to get a good night’s sleep seems to reduce some of the symptoms of their neurological conditions, and doctors place a high priority on it.
As sleep intensity increases, the amount of fragmentation decreases, so really sleepy or sleep-deprived people are less likely to have fragmented sleep.
The neurology profession is taking sleep disorders more seriously, and there are calls to recognize fragmented sleep as a brain disorder.