 The purpose of this research study is to compare the effectiveness of Zolpidem CR to that of placebo in improving sleep efficiency in people with dementia admitted to the hospital because of their symptoms. You can participate in this study if you have dementia of the Alzheimers type or vascular dementia. This study involves placebo; a placebo is a tablet that looks exactly like Zolpidem CR, the study drug, but contains no active study drug. We will use placebos to see if the study results are due to the study drug or due to other reasons. Zolpidem CR is also called Ambien CR and is widely available by prescription. Zolpidem CR is approved by the U.S. Food and Drug Administration FDA for the short-term treatment of insomnia trouble falling or staying asleep. 