Fragmented sleep is particularly frustrating. It leaves the sufferer in bed awake but knowing the sleep period hasn’t finished and more is needed, and further, that sleep could come again at any time. Sleep fragmentation is a hallmark of sleep-maintenance insomnia, which differs from sleep on-set insomnia in that sleepers can generally get to sleep pretty fast after going to bed (low sleep latency).
Fragmented sleep means the sleep cycle typically does not include much slow-wave sleep and is unrefreshing. Total time spent in sleep (any stage) is also less than normal, although higher than many people with sleep fragmentation often believe. (People often underestimate how much sleep they actually get.) It is estimated that a third of adults have nocturnal awakenings of some sort although the range of fragmentation in this population varies greatly.
During nightly sleep the brain consolidates memories, pruning and determining which ones should go from short-term storage to long-term memory. The fragmentation of sleep undermines the smoothness of this process.
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.
Illness as a Cause
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)
There is thought that the 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. (See definition of sleep).
Apnea causes fragmented sleep of a sort – microarousals that the sufferer isn’t even aware is happening. 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 on children whose brains are still developing.
Getting older causes increases in sleep fragmentation. 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. The young and old may have the same number of awakeings during the night, but the young fall back to sleep faster and don't even remember their brief awakings. In any case, the nighttime fragmentation is frustrating and unpleasant, and perhaps more importantly leads to sleepiness during the day. This is a major and non-critical detriment to quality of life and the kind of thing we at Sleepdex are trying to combat.
Arousal or Fragmentation Index
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.
The neurology profession is taking sleep disorders more seriously, and there are calls to recognize fragmented sleep as a brain disorder.