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Sundowning is a psychological phenomenon associated with increased confusion and restlessness in patients with some form of dementia. Most commonly associated with Alzheimer's disease, but also found in those with mixed dementia, the term "sundowning" was coined due to the timing of the patient's confusion. For patients with sundowning syndrome, a multitude of behavioral problems begin to occur in the evening or while the sun is setting. Sundowning seems to occur more frequently during the middle stages of Alzheimer's disease and mixed dementia. Patients are generally able to understand that this behavioral pattern is abnormal. Sundowning seems to subside with the progression of a patient's dementia. Research shows that 20–45% of Alzheimer's patients will experience some sort of sundowning confusion.
Increased general confusion as natural light begins to fade and increased shadows appear.
Agitation and mood swings. Patients may become fairly frustrated with their own confusion as well as aggravated by noise. Patients found yelling and becoming increasingly upset with their caregiver is not uncommon.
Mental and physical fatigue increase with the setting of the sun. This fatigue can play a role in the patient's irritability.
Tremors may increase and become uncontrollable.
A patient may experience an increase in their restlessness while trying to sleep. Restlessness can often lead to pacing and or wandering which can be potentially harmful for a patient in a confused state.
While the specific causes of sundowning have not been empirically proven, some evidence suggests that circadian rhythm disruption enhances sundowning behaviors.
Disturbances in circadian rhythms
It is thought that with the development of plaques and tangles associated with Alzheimer's disease there might be a disruption within the suprachiasmatic nucleus (SCN). The suprachiasmatic nucleus is associated with regulating our sleep patterns by maintaining our circadian rhythms. These rhythms are also strongly associated with external light and dark cues. A disruption within the SCN would seem to be an area that could cause the types of confusion paired with sundowning. However, finding evidence for this is difficult due to the need for an autopsy to correctly diagnose Alzheimer's in a patient. Once a patient has died, they have already surpassed the level of dementia and brain damage that would be associated with sundowning. This theory is supported by the effectiveness of melatonin, a natural hormone, to decrease behavioral symptoms associated with sundowning.
Another cause can be oral problems, like tooth decay with pain. When the time a meal is served comes close, a patient can show symptoms of Sundowning. This cause is not yet widely recognized. 
If possible, a consistent sleeping schedule and daily routine that a sufferer is comfortable with can reduce confusion and agitation. 
If the patient's condition permits, having increased daily activity incorporated into their schedule can help promote an earlier bed time and need for sleep. 
Check for over-napping. Patients may wish to take naps during the day, but unintentionally getting too much sleep will affect nighttime sleep. Physical activity is a treatment for Alzheimer's, and a way to encourage night sleep. 
Caffeine is a (fast-working) brain stimulant, but try limiting it at night if a night's sleep is needed.
Caregivers could try letting patients choose their own sleeping arrangements each night, wherever they feel most comfortable sleeping, as well as allow for a dim light to occupy room to alleviate confusion associated with an unfamiliar place. 
Some evidence supports the use of melatonin to induce sleep. 
^ abcdefghiAlzheimer's Association, "Sleeplessness and Sundowning." alz.org, 2011. Web. 14 May 2012.
^ abcde Jonghe, A, J. C Korevaar, B. C Van Munster, S. E de Rooij. "Effectiveness of melatonin treatment on circadian rhythm disturbances in dementia. Are there implications for delirium? A systematic review." International Journal of Geriatric Psychiatry. 25.12 (2010): 1201-08. Print.