Sundowning occurs in the evening hours and mostly in those with cognitive disorders. The theory is that the reduced light triggers sensory deprivations in a brain that is already functioning below the level necessary to maintain orientation and attention levels resulting in increased agitation, restlessness, and confusion. The behaviors exhibited are frantic measures for the individual to regain comfort, security, and a sense of normalcy. Recent studies I've seen are leaning towards the prospect of a fatigue effect, especially in those with AD. (Overly fatigued patients are prone to be more agitated and restless.)
Underlying medical problems and changes in the environment are also contibuting factors. UTI's, respiratory infections, electrolyte inbalance, changes in medications and doses, use of hypnotics, impaired vision and hearing can all alter an elderly individual's awarness levels. A transfer to a new facility, room change, rearrangement of furniture, holiday decorations, and so forth can cause havoc on an impaired cognition--especially at night.
It is best to keep these residents in a regular routine, assign them to primary caregivers, avoid drastic changes in the environment and routine, keep them away from large crowds and noise, avoid caffeine, make sure they get excercise, and plenty of rest intervals to avoid fatigue.
When dealing with the behaviors, it's best to remove the individual to a quiet place and offer distractions such as folding towels or washcloths, straigtening a bookshelf, or listening to soothing music. One of my residents used to be a janitor so we would let him take a push broom up and down the hall. We also make sure toileting schedules are maintained and that snacks are available as hunger and the need to toilet can add to the confusional state--especially at night when residents wake up and cannot remember where they are.